Traditional and recent development of pretreatment and drying process of grapes during raisin production: A review of novel pretreatment and drying methods of grapes
Abstract
Raisins are highly produced value-added products that are extracted from grape processing. The most important unit operations in raisin production are pretreatment, followed by drying. Due to the presence of waxy layer, the drying cannot be used alone; however, the layer can be removed by pretreating the grapes with chemicals or physical abrasion. The food safety and environmental issue with the chemicals have forced to find alternative methods. The physical method, such as surface abrasion, is explored but showed negative effect of color. The nonthermal methods such as microwave (MW) heating, pulsed electric field, ultrasound, and ohmic heating are observed, and their outcomes are discussed in this review. Carbonic maceration, high-humidity hot air impingement blanching, and MW hydrodiffusion and gravity are the novel methods that showed promising results. In the latter part, the traditional drying process of grapes is discussed, where shade drying is found to be used remarkably in raisin production. However, the time of drying was long, and the process required large land area. Alternatively, novel methods such as hot air, MW, vacuum, and infrared drying provide higher drying rate and good-quality raisins. However, the combined drying methods such as MW-assisted air drying, pulsed vacuum drying, and MW vacuum drying have shown better results than single drying. Pretreatment and drying cause changes in physical and chemical properties of grapes, such as changes in color, size, nutrient content, and polyphenol content. Overall, the review covered the crux output with the advancement in raisin development, which could be useful for novel research on grape processing.
1 INTRODUCTION
Viticulture is the science of grapes cultivation that is practiced in many parts of the world. The top 10 grape-producing countries during the period 2016–2017 are China, Italy, United States, France, Spain, Turkey, India, South Africa, Chile, and Argentina (Khan, Fahad, Naushad, & Faisal, 2020). Grapes provide many health benefits that are probably due to plenty of phytochemicals such as phenolics, flavonoids, anthocyanins, and resveratrol. Among these, grapes mostly contain anthocyanins, which have antioxidant activity, anti-inflammatory activity, anticancer activity, and vision benefit properties that are beneficial for the human body (Adiletta, Russo, Senadeera, & Di Matteo, 2016; Colombo et al., 2019; Nasser et al., 2020; Olejar et al., 2019). Grapes are generally kept in the class of berries having plenty of juice content; however, this property makes them highly perishable in the postharvest period and marketing. This is because of the high level of moisture and sugar, which results in shorter shelf life. Therefore, after harvesting, fresh grapes become very prone to various physical damages and quality deterioration. In ambient conditions, the deterioration of fresh grapes happens at a rapid rate within 2 months; thus it is important to consume or process them into various products to reduce postharvest quantitative and qualitative losses (Khiari, Zemni, & Mihoubi, 2019).
The best and quick way to preserve fresh grapes is by removing water, that is, dehydration that lowers the water activity and thus naturally prevents the fruit from deterioration (Farias et al., 2020). The major value addition of grapes is that they are converted to raisins by drying, which is a widely used processing method in most of the grape-producing countries (Adiletta et al., 2016). Moreover, drying is considered the oldest and best method of preserving food, including grapes. The drying process significantly reduces the water content of grapes, which prevents microbial growth as well as preserves the quality of the fruit, thus allowing the ambient storage of the dried product. Further, the weight and volume of grapes reduce, which facilitates low transportation cost, convenience in handling, extra income, and ease of bulk storage (Prabhakar & Mallika, 2014). During dehydration, many desirable changes in physical, chemical, and biochemical properties have occurred, which are controlled by pretreatment and drying conditions. During drying, carbohydrate and organic compounds of fresh grapes are retained in the concentrated form in the raisins. Phenolic compounds and antioxidants are essential for boosting the health of people and, among the dried fruit categories, are generally present in huge quantity in raisins. In the presence of certain enzymes, these compounds cause the browning reaction during drying, which plays an important role in the final appearance of raisins (Dev, Padmini, Adedeji, Gariépy, & Raghavan, 2008). Thus, it is necessary to select suitable pretreatment and cost-effective drying technologies during the conversion of grapes into raisins (Khiari et al., 2019; Wang et al., 2014).
The direct drying of raw grapes is not usually practiced because of the presence of waxy skin layer, which causes difficulty in removing water in situ (Adiletta et al., 2016). One way of removing this waxy layer is to apply pretreatments before drying grapes so that water gets migrated easily, thus improving the process. This not only decreases the energy consumption but also promotes quality preservation of the final product, such as retention of antioxidant during drying (Farias et al., 2020). In commercial scale, grapes are first dipped into the solution of sodium hydroxide (NaOH) or potassium carbonate (K2CO3) before drying (Vázquez, Chenlo, Moreira, & Costoyas, 2000). This causes dissolution of the waxy layer of grapes along with the formation of microcracks on its surface. This resulted in faster drying with improved quality of dried grapes (Celik, 2019; Dev et al., 2008; Vázquez et al., 2000). Besides producing good-quality raisins, there are some environmental and health concerns related to the use of chemicals for grapes pretreatment. It was observed that these alkaline chemical residues are left in raisins even after drying, which may trigger food safety issues and affect human's health (Carranza-Concha, Benlloch, Camacho, & Martínez-Navarrete, 2012; Farias et al., 2020). Some studies found that the use of sulfite pretreatment can cause asthmatic reactions in some sensitive individuals (Deng et al., 2019; Kamiloglu et al., 2016). The wastewater produced from chemical pretreatment contains larger quantities of corrosive chemicals, high saline and organic solids, which is a severe problem for the environment (Adiletta et al., 2016; Bai, Sun, Xiao, Mujumdar, & Gao, 2013; Wang et al., 2017). In this sense, physical pretreatments producing no residue would be preferable (Huang, Wu, Wu, & Ting, 2019). The abrasion method, type of physical pretreatment, showed a significant increase in the drying rate of grapes due to the formation of microcracks on the grape peel surface (Selvi, Baskar, & Aruna, 2014). However, most of the raisins formed are dark in color when compared to chemically treated raisins (Adiletta et al., 2016; Senadeera, Adilettta, Di Matteo, & Russo, 2014). Thus, it is crucial to find other types of pretreatment methods, both physical and chemical, which can provide good drying characteristics, acceptable sensory attributes, and least nutrient degradation.
During the past research, carbonic maceration method was used as a pretreatment method of grapes, which resulted in increased drying rate along with the minimum change in quality parameters such as color and rehydration ratio (Khiari et al., 2019; Roselló-Soto et al., 2015). Some nonthermal treatments such as the pulsed electric field (PEF), microwave (MW) heating, and ohmic heating have produced an acceptable result in the pretreatment of grapes (Dev & Raghavan, 2012; Dev et al., 2008; Ruan et al., 2001). Apart from pretreatment, drying is the most important operation in raisin development. Drying time mostly defines the quality of the end product; thus, the type of drying treatment should be selected carefully. The drying methods of grapes, such as sun drying, shade drying, and hot air drying, were used since so many years and they produced acceptable results. However, at the time of rapid production, these technologies mostly failed or started producing an adverse quality of raisins. Thus, the novel drying techniques such as MW heating, vacuum drying, infrared drying, and ultrasound-assisted drying should be used as an assistance or replacement of traditional drying methods.
Before setting up of industry for raisin production, it is important to explore the potential research activities carried out in the past for both pretreatment and drying operations. However, the collective information about different pretreatment and drying processes with their positives and negatives was limited. For this reason, in this literature review, the processing of grapes during the development of raisin is discussed briefly. The various pretreatment and drying techniques along with the recent advancement in the development of these techniques are introduced in the review. The chemical and physical changes occurred during pretreatment and drying operations of grapes are shown along with the novel trends in the process application in the commercial raisin industry.
2 FLOW PROCESS OF RAISIN DEVELOPMENT
During the growth of grapes in the field, the formation of a waxy layer on the skin of berry and softening of berry take place along with the increase in sweetness and development of a specific type of color, which is based on the berry variety. The grapes are harvested at ambient or/and low temperature and transported during storage (Jin, Wu, Liu, & Liao, 2017). During storage, water comes out slowly from the stalk and rachis (holding grape bunch) but not from the grapes, probably due to the presence of white waxy layer on the skin. This results in the drying of pedicels (holding a single grape) and therefore is not able to hold the weight of berries; thus, the latter is dropped (Sharma & Adulse, 2007). The berries are then collected and stored in cool environment.
In the process technology for raisin production, the stored berries are first cleaned, washed, and sorted. The berries are then further sent for drying treatment for the development of raisins, as shown in Figure 1. The drying process comprises the pretreatment of grapes for removing the waxy layer that is hydrophobic and prevents moisture migration. This pretreatment can be done using chemicals method or physical method. The treated grapes are dried in three stages. In the first stage, maximum moisture is removed, which is up to 40–50% of its initial weight. This also decreases the surface area of berry by 28%, which results in shrinkage. At the end of this stage, the drying rate decreases due to less surface area and lower moisture gradient. After this stage, the partially dried berries are cleaned and sent again for drying. In the second and third stages, the water secretes from the interior pulp to the outermost skin of the berries and subsequently evaporates. Finally, when the berries are dried up to 13% moisture content, the drying process is stopped, which is then sent for cleaning, sorting, and packaging (Sharma & Adulse, 2007).
The dried raisins are highly hygroscopic, and when they come in contact with moisture, the mold growth, rat attack, and fermentation process begin. These damages can eventually affect the entire process. Thus, the packaging of the sorted raisins is carried out in 400-gauge low-density polyethylene film bags and stored in corrugated boxes of 5–15 kg capacity at low temperature (4°C). These boxes can bear the mechanical, climatic, biotic, and chemical stresses to which raisins may get exposed during transportation, storage, and cargo handling. This safety procedure may preserve the original color, taste, texture, and aroma of raisins (Sharma & Adulse, 2007).
3 VARIOUS PRETREATMENT METHODS OF GRAPES
The skin structure of the grape plays an important role in controlling the drying process and the quality of dried raisins. It contains epidermis and six to ten layers of small thick-walled cells. The outer epidermis is covered by nonliving layers such as cuticle, lenticels, wax, and collenchymatous hypodermal cells (Esmaiili, Sotudeh-Gharebagh, Cronin, Mousavi, & Rezazadeh, 2007). The hydrophobic nature of the cuticle or waxy layer has many advantages and disadvantages based on the requirements and applications. The waxy layer serves as a protective barrier against fungal pathogens, controls gaseous exchange between berry and surrounding, reduces transpiration water loss, and further protects against UV light as well as physical injury (Esmaiili, Sotudeh-Gharebagh, Mousavi, & Rezazadeh, 2007). The main disadvantage is the hindrance in moisture removal during drying, which is an important process in raisin development. The wax layer is made up of both amorphous and intracuticular layers. The amorphous layer is made up of a series of overlapping platelets that are hydrophobic. However, this layer can allow the migration of water, only in the vapor state.
The diffusion of moisture in the cuticle occurs at a very low rate (10−22–10−18 m2/s), which is measured using labeled cholesterol as a tracer (Esmaiili, Sotudeh-Gharebagh, Cronin, et al., 2007). The low moisture diffusivity of the cuticle results in a time-consuming drying process. However, this can speed up using higher temperature for drying, but it may cause serious damage to quality attributes of the final dried product (Adiletta et al., 2016). Thus, it has become essential to remove the waxy layer before the drying step. This can be achieved by various pretreatment techniques, which are broadly categorized into chemical and physical methods.
3.1 Chemical method
In this method, the grapes are dipped into specific chemical solutions at favorable temperature for the required time. The chemicals have the main objective to solubilize the waxy layer and quickly remove waxes from the berries. At the same time, care should be taken for preventing any possibility of occurrence of taste and aroma of chemicals inside the grapes, which otherwise could result in unpleasant quality. Various types of chemicals used for pretreatment of grapes along with its effects and side effects are discussed in Table 1. Among them, the widely used chemicals are sodium carbonate (Na2CO3), K2CO3, NaOH, and oil emulsion (e.g., ethyl oleate [EO] and olive oil). The mechanism and favorable outcomes are discussed given as follows.
| Type of chemical | Level of concentration or operation | Advantage | Disadvantage | Overall acceptance | Reference |
|---|---|---|---|---|---|
| EO emulsions | EO (0–3%) | Higher porous structure and low bulk density dried raisins increase drying rate | Increase in concentration produce inferior quality attributes of the dried grapes | EO resulted in high shrinkage and higher drying rate | Gabas, Menegalli, and Telis-Romero (1999) |
| Sodium hydroxide solution | NaOH solution (1.5–30 g/L) at 85–100°C for time (2–30 s) followed by washing with water at 25°C for 5 min | Higher concentration offered shorter dipping time and improved drying rate; lower concentration for a low time provide minimum skin damage | Higher skin damage in general; low-quality dried product at higher concentration | Pretreatment resulted in lower drying time and good-quality dried raisins | Pahlavanzadeh, Basiri, and Zarrabi (2001) |
| K2CO3 | K2CO3 (7–15% w/w) + olive oil (0.4–0.8 v/v) | Drying rate increased with concentration | Lower shelf due to severe skin damages | The moisture migration become faster in drying process | Vázquez et al. (2000) |
| Combination of chemicals (oil emulsions) | 2% EO and 0.5% sodium hydroxide 4% K2CO3 + 1% olive oil | Higher drying rate due to reduction in the resistance of moisture movement | – | Slow down the browning enzymatic activity thus increases quality | Tulasidas et al. (1996) and Serratosa et al. (2008) |
| Sulfur compounds | SO2 solution (2–6% w/v) | Increased drying rate | Extremely dark-colored raisins; skin damaged; high chances of insect infestation, fungus growth | Excess amount can lead negative health effects and environment pollution | Lydakis, Fysarakis, Papadimitriou, and Kolioradakis (2003) |
| Abrasive pretreatment | Sandpaper-lined interior surface of plexiglass-based drum; rotational speed was 9 rpm for 30 min | Sugar content and tartaric content decreased slightly; no degradation of malic and citric acids; better texture properties compared to untreated raisins | – | Lower drying time and lower shrinkage of the treated raisins compared to untreated raisins | Adiletta et al. (2015) |
| Water blanching | Blanching with hot water | Softening of waxy skin of grape, which can increase moisture migration during drying | Loss of bioactive compounds | – | Khiari et al. (2019) and Cabrera and Moon (2015) |
| Ohmic heating | 14 V/cm field strength and various frequencies (30 Hz, 60 Hz, and 7.5 kHz) | Increase in permeability of skin layer and thus increase in moisture diffusion rate | Leaching of small amount of sugar solute | Formation of microcracks on the grape skin, probably due to breaking action of heating | Ruan et al. (2001) and Salengke and Sastry (2005) |
| Superficial abrasion | Creation of scratchy surface inside the stainless steel container; churning of berries for 2–3 min | Drying rate improved | Slightly sticky raisin | Produced darker colored raisin; effective method than chemical | Thakur et al. (2010) |
| Atmospheric pressure jet pretreatment | Atmospheric plasma jet generates plasma by a switch-mode power supplier; 500 W power and 25 kHz frequency consisting of a mixture of charged ions, electrons, and neutral species consisting of a mixture of charged ions, electrons, and neutral species | Good-quality dried raisin by air plasma jet; no toxic residue | – | Increased drying rate; 20% decrease in drying time | Huang et al. (2019) |
| Microwave pretreatment | 2,450 MHz microwave generator with adjustable power from 0 to 750 kW | Formation of microcracks on the surface and increased porosity | – | Increase in drying rate of grapes | Kostaropoulos and Saravacos (1995) |
| Pulsed electric field | Frequency (1 Hz), electric field (1 kV/cm) | Cell damage, softening of tissue, and enhancement of water movement | – | Highest TSSs than the chemically treated and untreated raisins | Dev and Raghavan (2012) |
| Ultrasound | – | Increase drying rate | Might effect of the porous structure of grapes | – | Laborde et al. (2018) |
| Carbonic maceration pretreatment method | – | Vapor transfer from these lenticels becomes easier | – | Higher drying rate | Roselló-Soto et al. (2015) |
| MHG | Power (400 W) for 10 min | Electromagnetic waves get penetrated to the surface and interior of the grapes | – | Quick evaporation of water from the grape cells and reduce the moisture content quickly | Carranza-Concha et al. (2012) |
| HHAIB | Air velocity (10.0 ± 0.5 m/s) blanching temperature (110°C), and blanching relative humidity (40–50%) | Increase in heat transfer coefficient | – | The formation of microcrevices on the surface of grape | Khiari et al. (2019) |
- Abbreviations: EO, ethyl oleate; HHAIB, high-humidity hot air impingement blanching; K2CO3, potassium carbonate; MHG, microwave hydrodiffusion and gravity; TSSs, total soluble solids.
3.1.1 Sodium hydroxide
Treatments of grapes with NaOH are used remarkably in commercial-scale production of raisins. The NaOH enhances the drying characteristics along with the improvement of color of dried grapes (Vázquez et al., 2000). The possible reasons are the softening of grape skin, which results in easy permeability and thus increases the drying rate (Zemni et al., 2016). The NaOH treatment is also shown to produce microcracks in the grape cuticle. The treatment of grapes with 0.25% NaOH at 82°C for 5–10 s leads to the cracks in the fruit skin, which consequently results in the reduction of drying time (Dev et al., 2008). However, the dipping of grapes in NaOH solution has also resulted in dull and pale raisins. Although the NaOH solution is capable of solubilizing the waxy layer, it makes some physical damages to the skin, which accelerates only in the first stage of drying (Tulasidas, Raghavan, & Norris, 1996).
3.1.2 Ethyl oleate
The pretreatment of grapes using EO solution shows improved moisture diffusion and good sensory appearance (Vázquez et al., 2000). The most effective dipping material to increase moisture migration was observed in the ethyl esters of fatty acid having carbon chain from C10 to C18. In terms of concentration, EO and ethyl linolenate at 2% have shown favorable results. The ability of EO to dissolve the waxy components on the skin of grapes along with the formation of micropores on the surface was supposed to be the reason for improving the internal water diffusion and drying rate (Tulasidas et al., 1996). Moreover, EO can reduce browning and other types of quality deteriorative reactions (Khiari et al., 2019). When hot water was used for making commercial emulsion of EO, the three treatments showed a strong increase in moisture diffusivities (average range was from 3.34 to 8.46 × 10−10 m2/s at 50°C). These diffusivity values were two times higher than the hot water treated grapes (Esmaiili et al., 2007).
3.1.3 Potassium carbonate
The industrial-scale raisin production pretreats the grapes with K2CO3 for faster drying as well as for producing good-quality raisin. Celik (2019) used the pretreatment solution having 5% of K2CO3 and olive oil for grapes, which showed increased lightness and lower a/b values. These attributes are generally considered desirable characteristics of good-quality raisins. Moreover, the total phenolic and flavonoid content was higher in K2CO3 solution than in the ash with olive oil and in controlled temperature. Vazquez et al. (2000) explained the mechanism of action of K2CO3 on the grapes, which increased drying rate. They mentioned that K2CO3 might remove the waxy layer on the grape skin and the fatty compounds might collapse the dry skin along with partial breakup of ester bonds present in the pectin. This results in faster removal of moisture.
3.1.4 Combination of chemicals (oil emulsions)
The moisture diffusion rate, as well as raisin quality, can be enhanced using a possible combination of chemicals instead of a single chemical. For example, the mixture of oil and K2CO3 slows down the browning enzymatic activity, thus increasing the quality. Tulasidas et al. (1996) found a reduction in drying times for sun or mechanical dryers with improved quality of raisins after the pretreatment of grapes with the mixture of 2% EO and 0.5% NaOH solution. The combination of NaOH and citric acid solubilizes the pectic compounds, and xyloglucans of cells cause faster moisture removal and thus decreased drying time (Femenia, Sánchez, Simal, & Rosselló, 1998). The EO emulsion containing K2CO3 was used for pretreatment of berries, which resulted in decreased drying time by about 25% at 50°C when compared to untreated samples (Serratosa, Lopez-Toledano, Medina, & Merida, 2008). The POTAS solution (4% K2CO3 + 1% olive oil) pretreatment of Sultana seedless grapes prior to drying resulted in higher drying rate due to reduction in the resistance of moisture movement. The K2CO3 solution produces lighter raisins due to saponification of fatty acids such as oleic, stearic, and oleanolic acids, usually present in grape wax (Doymaz & Altıner, 2012).
3.1.5 Sulfur compounds
The drying characteristic of grapes is reported to be enhanced by the sodium meta-bisulfite and sulfur dioxide pretreatment. These chemicals not only increase drying rate but also reduce browning reaction (both enzymatic and nonenzymatic). This reduction is due to the ability of sulfur compounds to inhibit the polyphenol oxidase (PPO) and lower the pH of materials, thus minimizing the degree of nonenzymatic Maillard reaction. As a result, the discoloration of the product becomes low during drying and storage of raisins (Femenia et al., 1998; Vázquez et al., 2000). Pretreatment of grapes by Esmaiili et al. (2007) with sulfur dioxide at 900 ppm of concentration for 6 hr resulted in light-colored raisins after drying. The increment in moisture diffusion from grapes is largely due to modification of pectic polysaccharides leading to collapse of cell-wall structure by potassium metabisulfite treatment. However, an excessive amount of sulfur dioxide may abruptly change the quality of processed raisin and can cause severe environmental problems such as air pollution and certain health problems such as asthma. Thus, some other alternatives of sulfur dioxide need to be explored.
3.2 Physical method
An excessive amount of chemicals and sometimes their residue can create food safety problems. This can be prevented by choosing other pretreatment methods, which generally have no chemical usage. Physical pretreatment is the second predrying method that can be successfully used to enhance grape drying. It encompasses the employment of mechanical force, thermal energy, and nonthermal techniques such as abrasion, water blanching, ohmic heating, MW heating, high-pressure processing, and PEF (Wang et al., 2016). A brief discussion about these physical methods is presented in Table 1, and the detailed outcomes are provided as follows.
3.2.1 Abrasion
The mechanical shearing of grapes can be carried out by a shaker having walls covered with an abrasive coating. This abrasion technique was first used by Di Matteo, Cinquanta, Galiero, and Crescitelli (2000) instead of chemical pretreatment. They observed a four-time increment of mass transfer coefficient compared to untreated samples due to an increase in porous surface structure (Selvi et al., 2014). Moreover, physically pretreated grapes showed drying behavior similar to chemically treated grapes. Senadeera et al. (2014) and Adiletta et al. (2016) have conducted a similar experiment and found a better drying characteristic and quality compared to untreated berries. However, one strange observation was noted about the color of raisins. The abrasion treatment resulted in darker raisins compared to chemically treated raisins, which might be due to the microstructure formation on the peel surface containing PPO enzyme. This causes the enzyme activation, which results in the formation of brown pigments, that is, higher rate of grape browning. On the contrary, chemical treatment inactivates the PPO enzyme and produces light-colored raisins.
3.2.2 Water blanching
Blanching with hot water changes the physical properties of the products, which enhance the drying rate and improve quality attributes (Xiao et al., 2017). This can be used for the softening of waxy skin of grapes, which can increase moisture migration during drying. The method is simple, easy, and cost-effective, which can be easily adopted in the grape-processing industry (Khiari et al., 2019). However, in the exhaustive review, very few researchers have used hot water blanching as a pretreatment of grapes before drying. It was noted that bioactive compounds of grapes reduce due to the extraction of colored pigments during hot water blanching (Cabrera & Moon, 2015).
3.2.3 Ohmic heating
The ohmic heating can be used as a novel pretreatment of grapes, in which alternative current (AC) is passed through grapes causing heating due to resistance offered by grapes (Ruan et al., 2001). The ohmic heating at 14 V/cm field strength and various frequencies (30 Hz, 60 Hz, and 7.5 kHz) results in the formation of microcracks on the grape skin, probably due to the breaking action of heating. This causes an increase in permeability of the skin layer, and thus increasing the moisture diffusion rate of grape during drying. These phenomena generally occur at a lower frequency range of heating (Salengke & Sastry, 2005). However, leaching of a small amount of sugar solute from the grape berries was also observed, which leads to the shifting of equilibrium moisture content.
3.2.4 Microwave heating
The MWs are absorbed by the moisture present inside the pores of grapes. This resulted in rapid evaporation of water and consequently partial puffing of grapes. As a result, the porosity of the grapes increased, which enhanced the drying rate nearly two times compared to untreated raisins. The sensory characteristics such as color and appearance of the MW-treated raisins were similar to the commercial raisins, perhaps due to the partial inactivation of browning enzymes by the MW energy (Kostaropoulos & Saravacos, 1995). A similar observation was mentioned by Dev et al. (2008) where the formation of microcracks on the surface and increased porosity was noted. This increased dehydration rate, and better appearance and quality of MW-treated raisins compared to chemically treated and untreated raisins.
3.2.5 Pulsed electric field
A more recent pretreatment method is the PEF, when applied to fruit material, this results in cell damage, softening of tissue, and enhancement of water movement (Dev & Raghavan, 2012). The application of PEF on the grapes has resulted in 20% reduction of drying time compared to untreated samples. The color of PEF-treated samples was similar to the chemically treated and untreated samples. In terms of losses, PEF-treated raisins resulted in higher total soluble solids (TSSs) than the chemically treated and untreated raisins. Moreover, the market acceptability of PEF-treated raisins was higher than the chemically treated and untreated raisins (Dev & Raghavan, 2012).
3.2.6 Ultrasound
The ultrasound waves can be used for pretreatment of grapes before drying. These waves induced cavitation that might affect the porous structure of grapes. As a result, the drying rate of ultrasound-treated grapes can be increased compared to untreated grapes (Laborde et al., 2018). This ultrasound assistance further helps in decreasing the calorie content of treated raisins because of the removal of more soluble solids than untreated raisins.
3.3 Recent advances in technology for the pretreatment of grapes
There are some limitations in both physical and chemical methods of the pretreatment of grapes, such as residue interaction with food quality during chemical treatment and production of darker raisins by physical methods. Thus, some advanced techniques have been introduced by many researchers who can reduce the problems associated with the above-mentioned techniques. A brief description of the advanced pretreatment technologies is provided in Table 1.
3.3.1 Carbonic maceration pretreatment method
The carbonic maceration process was invented by Flanzy et al. (1987) to produce wine from grapes. Further, this technique was used for producing cabernet, grape juice, and sugar (Wang et al., 2014). However, this has become a novel pretreatment process for grapes before drying, which can enhance their drying characteristics. The technology involved the placement of grape clusters inside the closed tank filled with high concentration of carbon dioxide. The grapes are held for optimized time so that the carbon dioxide gets soaked and an intracellular fermentation begins. This results in the loosening of tissues, keeping the grape structure intact and increasing the lentil size, where water is generally being stored in the grapes. As a result, the vapor transfer from these lenticels becomes easier, which facilitates higher drying rate (Roselló-Soto et al., 2015). The carbon dioxide rich environment decreases the pH of cytoplasm with a decrease in cell structure. This causes the conversion of high polymers into low polymers and an increase in the cell-wall size and membrane permeability; thus, water releases kinetics that increases and improves the drying rate as well as the quality of raisins (Khiari et al., 2019). Compared to untreated raisins, the drying time was reduced by about 31%; total phenolic content and rehydration ratio were increased by about 28.43% and 32.24%, respectively. The change in color was the lowest and rehydration ratio was the highest for carbonic maceration treated raisins compared to the alkaline emulsion of EO (AEEO) solution treated and AEEO with freeze-drying treated raisins (Wang et al., 2014).
3.3.2 Microwave hydrodiffusion and gravity
The microwave hydrodiffusion and gravity (MHG) is a novel technique that was invented by Vian, Fernandez, Visinoni, and Chemat (2008) for the extraction of oils from mint and pennyroyal. Later, this was used effectively as a pretreatment before the drying of onion, seaweed, and pomegranate peels. In case of grapes, MHG reduced 50% of its moisture content just in 10 min, whereas other methods such as oven drying and freeze drying consumed 17.83 and 149.9 hr, respectively. Thus, MHG reduced the overall drying time of grapes (Farias et al., 2020). The possible reason is the electromagnetic waves get penetrated into the surface and interiors of the grapes, which causes the heating of water inside the grapes. This resulted in quick evaporation of water from the grape cells and rapid reduction of moisture content (Carranza-Concha et al., 2012; Khan, Ansar, Nazir, & Maan, 2016). During the diffusion of water, some soluble constituents rich in nutrients, antioxidants, polyphenols, etc. secrete from grapes and are collected by gravity at the bottom of the equipment. That is why the process is called MHG (Ferreira, Passos, Cardoso, Wessel, & Coimbra, 2018).
In addition, the MHG process preserves high amount of anthocyanin, phenolic compounds, and antioxidants in a very less processing time, that is, only 10 min. This makes the process economically viable for grape processing. Moreover, the process is a promising alternative to be used for large-scale production of predried grapes and grape extracts rich in polyphenols. Moreover, no residue is generated during the manufacture of raisin and grape extracts, thus promoting circular chain production (Farias et al., 2020).
3.3.3 High-humidity hot air impingement blanching
High-humidity hot air impingement blanching (HHAIB) is another novel technology for grape processing, which uses the principle of thermal techniques. The technology is a combination of high-humidity air blanching and impingement technology, which generally result in quick, uniform, minimum nutrient loss and energy-efficient process (Liu et al., 2019). In HHAIB processing of grapes, hot air with high humidity was applied at high velocity on the surface of grapes. This resulted in an increase in heat transfer coefficient, probably due to the formation of microcrevices on the surface of grapes. These crevices might reduce the resistance of water to diffuse from the center to the peel of grapes during drying (Khiari et al., 2019). Therefore, HHAIB-treated grapes showed a higher drying rate compared to the untreated grapes. Moreover, the PPO activity was reduced by HHAIB treatment, thus preventing browning (Bai et al., 2013). Some theories of Wang et al. (2018) stated about the depolymerization and degradation of the cell wall, softening of the structure, changes in the texture profile and water status, as well as distribution of treated berries, which could be the possible causes of the increase in drying rate, and was also supported by atomic force microscopy and low field nuclear magnetic resonance technologies.
4 VARIOUS DRYING TECHNIQUES OF GRAPES FOR RAISIN PRODUCTION
Grape drying is considered the main process because it results in a new product with a new name "raisin." This word is frequently used instead of "dried grapes." Usually, drying increases the shelf life of a product because of the reduction in moisture content to the level where the microorganisms such as fungi, bacteria, and virus are not active. Apart from higher shelf life, drying offers economical transportation of food products such as raisins. However, drying methods depend on the grape variety and growing conditions. Different regions have different methods of grape drying, but the main objective of drying is to increase the shelf life, to produce high-quality raisins, and to diminish postharvest losses.
4.1 Traditional drying techniques of grapes for raisin production
The widely used and newly researched methods for grape drying are solar drying, shade drying, oven drying, MW drying, vacuum drying, and infrared. A brief discussion of these drying techniques is provided in Table 2.
| Most efficient combination of pretreatment, drying operation and drying time | |||||
|---|---|---|---|---|---|
| Types of drying | Pretreatment | Drying operation | Drying time | Overall acceptance | Reference |
| Sun drying | Combine mixture of K2 CO3 (2.5%) and olive oil (0.5%) for 1 min |
|
176 hr |
|
Doymaz et al. (2012) |
| Solar drying | Boiling water containing 0.4% olive oil and 0.3% NaOH for 60 s |
|
8 hr |
|
El-Sebaii et al. (2002) |
| Oven drying | 0.5% olive oil + 6% K2CO3 solution at 50°C for 2 min | Convective oven drying at temperature of 50°C and air velocity of 0.5 m/s | 5 days |
|
Zemni et al. (2017) |
| MW drying | Not mentioned |
|
4 hr | Good-quality of raisins and lower drying time and energy efficient with a low specific energy requirement | Tulasidas et al. (1997) |
| PVD | Solutions of NaCl (170 g/L) |
|
12.1 hr |
|
Wang et al. (2017) |
| MW oven and hot air cabinet drying |
|
|
6.8 hr |
|
Kassem et al. (2011) |
| MWVD | No pretreatment |
|
1.2 hr |
|
Clary et al. (2007) |
- Abbreviations: LDPE, low-density polyethylene; MW, microwave; MWVD, microwave vacuum drying; PVD, pulsed vacuum drying.
4.1.1 Solar drying
Solar drying is the most traditional way of drying agricultural products using solar energy. The natural open sun drying consists of spreading the grapes in direct sunlight until the required dehydration is achieved (Prakash & Kumar, 2013). The advantages of solar drying of grapes are simplicity, feasibility, low costs, and low-temperature drying. However, some of its common drawbacks are high drying time, weather dependency, the possibility of mold growth and insect contamination, unable to control conditions, and attacks of birds, insects, and mice (Pangavhane & Sawhney, 2002). In grape-drying operations, solar drying is characterized into direct type, indirect type, and mixed type (Wang et al., 2016). In direct type, longer infrared wavelengths of sun rays are trapped by the glass above the grape bunches, which results in warming and consequently in drying of grapes (Barnwal & Tiwari, 2008). However, poor quality such as discoloration and aroma loss of grapes is obtained from direct sun drying due to light sensitivity of ascorbic acid and polyphenol (Gee & Webb, 1980). In indirect type, the air is first heated by sunlight, which is then transmitted to the chamber containing grape bunches; thus, convection is the main principle of drying. The study of El-Sebaii et al. (2002) showed the ability of designed indirect-type solar dryer dehydrating 10 kg of chemically pretreated grapes within 20 hr of sunshine. In mixed mode, the combination of direct and indirect types is used for the drying of grapes. The grapes are free to absorb sunlight along with the passage of hot air from a solar air collector (Mustayen, Mekhilef, & Saidur, 2014).
4.1.2 Shade drying
The shade-drying process was performed by placing the grape bunches in a dark open place away from sun exposure in a proper airflow condition usually maintained by the surrounding environment. It is an alternative indirect method of sun drying using solar energy without direct sun rays on the grapes. The main source of drying in this method is the ambient air, which is why shade drying is recognized as a natural rack dryer. It is widely popular in China, Australia, and India (Pangavhane & Sawhney, 2002). Shade drying of grapes has resulted in a better color quality of raisins than sun drying, mainly because of no direct effect of sun rays on the grapes. However, long drying time, dependency on weather, high labor requirement, possibility of the attacks of birds, insects, mice, etc. are the challenges of shade drying, which are similar to sun drying, thus making the processes not commercially sustainable (Wang et al., 2016).
4.1.3 Hot air drying
Hot air drying is also known as tray drying, which uses hot air as a medium to make temperature gradient between grapes and air with simultaneous removal of moisture. The temperature of air decides not only the drying rate but also the quality of the end product (Adiletta et al., 2016). Generally, the drying time of hot air drying is higher because of the decrease in diffusivity due to the decrease in moisture content of the grapes. During hot air drying, the moisture is generally diffuse in liquid phase in capillary vessels of varying width. In these capillary vessels, the vapor formed on the water surface moves toward the surface of fruits and is removed by the moving air. However, the part closer to the fruit surface dries up earlier, and the interior part is still left with some moisture. Thus, the thermal energy consumes more time to reach that part and the vapor generated in the process also takes more time to reach the surface, resulting in higher drying time (Çağlar, Toğrul, & Toğrul, 2009). Hot air drying took 180 min to remove 65% of moisture from grapes (Lokhande, Ranveer, & Sahoo, 2017). Hot air dehydration took 72 hr to dry 3.3 kg of Sultana grapes, which reduced to 1 kg, having the final moisture content of 18% dry basis (Margaris & Ghiaus, 2007). The quality characteristics of grapes, such phenolic content, anthocyanins, and antioxidant activity, also reduced during hot air drying treatment compared to nonconventional treatment. However, hot air is considered to be a good alternative compared to sun drying of grapes, due to higher retention of phenolic content, anthocyanins, antioxidant activity, and other quality attributes along with less drying time (Coklar & Akbulut, 2017).
4.1.4 Microwave drying
The electromagnetic waves are generated form magnetron and then passed on to grapes, which results in the vibration of water molecules, and thus generating thermal energy. This energy helps evaporate the moisture present in the grapes and results in quick drying (Michailidis & Krokida, 2014). The study of MW drying of grapes using MW power from 350 to 1,000 W at 2,450 MHz frequency reveals the lower drying time (20 min to remove 60% of moisture from grapes) than the conventional hot air tray drying (took 180 min) carried out at 60°C. Moreover, MW drying (750 W of MW power) retained highest ascorbic acid content, highest rehydration ratio, and good color properties (golden yellow color) compared to tray drying. Higher rehydration ratio showed lower shrinkage of raisins due to the generation of internal pressure gradient, which transfers the moisture from inside to outside, thus resisting shrinkage better than hot air tray dried grapes (Lokhande et al., 2017).
4.1.5 Vacuum drying
The drying process, which removes the air surrounding the product inside a closed chamber by a vacuum pump, and results in lower pressure for artificially increasing the water vapor pressure difference between the product and surrounding, is called vacuum drying. The unbound moisture can be easily removed from the heat-sensitive food product using vacuum drying (Parikh, 2015). The same technology has been successfully used in raisin processing where the grapes are dried under subatmospheric pressure. The vacuum increases the mass transfer between food and surrounding, which facilitates lower heat requirement and thus better quality products (Khiari et al., 2019). A company, "Odlicno," has prepared good-quality vacuum-dried grapes, where it was claimed that the temperature had never exceeded 45°C (Odlicno, 2018).
4.1.6 Infrared drying
Infrared heating or drying of food involves the transfer of heat from a hot element to the food having ambient temperature with the help of radiation. The wavelength of electromagnetic waves ranging between 0.78 and 1,000 μm will be called infrared radiation. These wavelengths depend on the temperature of the heating element. When the infrared waves fall into the highly moist food such as grapes, some parts of energy get absorbed, reflected, and only a fraction of the infrared spectrum passes into the material (Krishnamurthy et al., 2008). First, the surface of the grape is heated through infrared waves and then due to temperature difference, the heat is transferred to the interior of the grapes (Hebbar & Rastogi, 2001; Trivittayasil, Tanaka, & Uchino, 2011). The intense heating decreases temperature gradient within a short interval of time, without the heating of air surrounding the grapes. The heating resulted in grape drying, mainly by moisture migration in the vapor phase (Sharma, Verma, & Pathare, 2005; Toğrul, 2005). As a result, the total diffusivity values were not changed with the decrease in grape moisture, which becomes progressively low with the conventional hot air drying (Çağlar et al., 2009). Experimental results showed 2.63 W as the optimal power of drying of one grape using infrared radiation. Extending beyond this power causes the burning of the skin of berries (Utgikar, Shete, & Aknurwar, 2013).
4.2 Recent advances in technology for raisin production
Drying is the most important unit operation in grape processing for raisin development. This unit operation accounts for maintaining the processing time, cost of operation, labor requirement, and quality of the final dried product, that is, raisins. Thus, nowadays the selection of the drying method has become an important decision due to the availability of a variety of options and the increasing market competition. Each traditional process of drying of grapes has advantages on one hand and disadvantages on the other hand. For example, sun drying and hot air drying are low-cost drying processes but the quality loss and drying time are maximum, alternatively, MW drying and infrared drying provide less drying time with minimum quality losses. However, the ability to dry a bulk number of grape bunches is still a question. A similar problem arises for the vacuum drying, in which ability to dry grapes at low temperature is much favored, but the commercial scale of processing using this technique needs to be explored yet.
These problems could be solved by the combination of different drying processes in order to take advantages of each other's favorable outcomes, such as hot air assisted MW drying, MW vacuum dehydration, and ultrasound MW-assisted drying. Moreover, novel drying methods such as pulsed vacuum drying (PVD) can be explored for raisin development. These advanced processes are described briefly in Table 2 and are detailed as follows.
4.2.1 Pulsed vacuum drying
Recently, PVD has emerged as a novel technique for drying of food products, which uses a sequential change in drying chamber pressure between atmospheric pressure and vacuum for enhancing the moisture transfer (Xie et al., 2017). The pulsating changes of the pressure resulted in the tunneling effect of enlargement and interconnection of micropores, which regenerate the porous and structure of fissure in the peel surface. Moreover, the vacuum used in the drying makes an oxygen-deficient environment around the grapes. Consequently, the combined effect of pulsation and vacuum enhances the drying rate and lowers the biochemical reactions (Zhang et al., 2018). Studies on the ripening of the grapes using PVD technology showed an increase in the drying rate and effective moisture diffusivity with the increase in ripeness. The lowest drying time was recorded as 12.1 hr (Wang et al., 2017). The vacuum in the PVD process prevents the browning reactions and oxidation deterioration, which results in good color and texture quality of raisins (Khiari et al., 2019).
4.2.2 Microwave-assisted air drying
The microwave-assisted air drying (MWD) has been explored by many researchers and has proved to decrease the drying time and produce an acceptable quality of raisins (Khiari et al., 2019). This drying uses the quick drying of MWs in the hot air drying method for increasing the drying rate as well as preventing quality deterioration. In the MWD process of grapes, the MW heating is initially applied, which results in quick heating of the interior of the grapes. The rapid release of vapor causes puffing, which generates porous structure, thus facilitating higher moisture transfer. The hot air passing through the grapes quickly removes the moisture or water vapor secreting from the grapes under drying. At the final stage of drying, the moisture is left mainly near the center of the grape, which can be easily removed using MW heating (Lokhande et al., 2017; Rattanadecho & Makul, 2016). The combined effect of MW and hot air drying of grapes resulted in shorter drying time along with lower energy requirement compared to conventional hot air convective drying (Tulasidas, 1995). Moreover, MW-assisted drying reduced or almost eliminated the use of alkaline pretreatment and sulfur dioxide fumigation during raisin production (Tulasidas, Raghavan, & Norris, 1993). Thompson seedless grapes were dried by combining MW oven and hot air cabinet dryer, which showed less drying time compared to hot air cabinet dryer. Drying of grapes using MW oven followed by hot air cabinet dryer was found to be the best combination compared to the other process, namely hot air cabinet dryer followed by MW and hot air cabinet (Kassem, Shokr, El-Mahdy, Aboukarima, & Hamed, 2011).
4.2.3 Microwave vacuum dehydration
Microwave vacuum drying (MWVD) can be used as a promising technology for the drying of grapes. The combination of quick heating of moisture by the MWs and the high partial pressure gradient by the vacuum can be used for achieving a higher drying rate along with greater quality of dried product. Clary, Mejia-Meza, Wang, and Petrucci (2007) performed an excellent research on the MWVD of grapes and found better retention of nutritional composition and quality attributes than sun-dried raisins. The proportional–integral–derivative temperature control the MW power and never allowed an increase in temperature higher than 66°C. As a result, vitamin A, vitamin C, thiamine, and riboflavin were found higher than sun-dried raisins.
5 PHYSICAL AND CHEMICAL CHANGES DURING THE PRETREATMENT AND DRYING PROCESS OF GRAPES
The pretreatment method for grapes is used for removing the waxy layer on the skin. This waxy layer is the main cause of slow drying of grapes, which if not removed can cause significant quality and sensory loss during drying. These waxes are made from the complex mixture of alcohols, alkanes, aldehydes, ketones, and esters made from long-chain fatty acids (Bingol, Roberts, Balaban, & Devres, 2012). Apart from this, the PPO enzymes are present on the skin surface, which causes browning reaction and alters the color of the developed raisins. As already observed, both types of pretreatment methods, whether chemical or physical, mainly target the skin of grapes. The objective of chemical pretreatment is to dissolve the waxes from the grape skin, whereas the goal of physical treatment is to make microcracks on the grape skin to facilitate moisture migration. During the pretreatment process, chemical, as well as physical changes, occurred in the grapes, which may or may not be visible, depending on the intensity and type of pretreatment.
In the case of chemical pretreatment, inactivation of PPO enzymes is mostly seen, which resulted in the prevention of browning of grapes (Khiari et al., 2019). The pH of the grapes also increased, which reduced the occurrence of nonenzymatic Maillard reaction (Vázquez et al., 2000). As a result, the chemically pretreated grapes showed an increase in lightness and decrease in a/b values (Celik, 2019). Regarding waxes, some chemicals such as K2CO3 solution caused saponification of fatty acids such as oleic, stearic, and oleanolic acids. This led to the production of lighter raisins after drying (Doymaz & Altıner, 2012).
The microcrack formation on the grape skin from the physical pretreatment exposes the PPO enzyme more easily to the environment (Senadeera et al., 2014). As a result, the PPO enzyme gets activated during drying and produces a higher degree of grape browning. This leads to the production of dark-colored raisins in comparison to chemically treated raisins (Adiletta et al., 2016). The cracks on the grape skin also cause the leaching of a small amount of sugar solute from the grape berries (Salengke & Sastry, 2005). The physical treatment also causes significant losses of ascorbic acid and changes in total soluble solids compared to untreated grapes (Thakur, Saharan, & Gupta, 2010).
The drying is the most important process that converts the grapes into its dehydration form, that is, raisins. Many physical, as well as chemical changes occurred during the drying of pretreated grapes. The major physical change that occurred is shrinkage of grapes, which is most prominently due to the removal of moisture from the interior of grapes to the surrounding. This shrinkage creates negative consequences for the quality of the dehydrated product (Adiletta et al., 2015). The quality parameters related to shrinkage is the bulk density of dried grapes. Higher the bulk density, higher will be the shrinkage, and vice versa. The most favorable quality for raisins is having a lower value of bulk density, as it is an indication of lighter porous quality raisins that promote soft texture due to higher porosity (Dev et al., 2008). The size of berry affects the rate of water loss during drying. The smaller berries lost water more rapidly than larger berries due to the higher relative area of skin to the flesh (Sharma, 2013). The presence of light-sensitive compounds such as ascorbic acid and polyphenol causes discoloration and aroma loss of grapes during drying done at open environments, such as sun drying. Therefore, it is mostly recommended to dry the grapes in a shaded area (Karadeniz, Durst, & Wrolstad, 2000; Wang et al., 2016). However, the resultant color of the raisins is mostly due to the pigments formed by the effect of the enzymatic and nonenzymatic reactions during the drying process (Karadeniz et al., 2000). The long heating time sometimes results in the loss of antioxidants and decreases in total phenolics (Dev et al., 2008). The drying process such as MW and sun drying significantly reduces the nutritional proprieties such as potassium, carbohydrate, calories, ash, fiber as well as minerals (Clary, Mejia-Meza, Wang, & Petrucci, 2007). The drying causes loss of phytochemicals, however, it also results in an increase in glucose and fructose levels due to the removal of moisture, as these finally lead to the concentration of raisins (Khiari et al., 2018)
6 NOVEL TRENDS OF RAISIN PRODUCTION IN THE INDUSTRY
The most commonly used method for pretreatment of the grapes in the industry is to apply sulfiting technique (fumigation or immersion). This is because sulfur dioxide causes improvement of the color of grapes and conserves quality (Khiari et al., 2019). Later, the pretreated grapes are mostly preferred to get dried through sun drying or shade drying rather than other drying methods (Çoklar & Akbulut, 2017). However, these techniques need to be changed to the novel technologies due to the higher demand from consumers, need for good-quality raisins, and environmental concern over using sulfate techniques for pretreatment of grapes. The industry is focusing on using infrared drying technologies because of diminishing drying time, producing an acceptable final dried product and greater energy saving ability (Ahmad, Marhaban, & SOH, 2015). The infrared drying uses infrared radiation, which is advantageous for processing thick products. This drying is cost-effective compared to MW drying and vacuum drying (Khiari et al., 2019). In relation to pretreatments, the discussed novel pretreatment technologies such as carbonic maceration and MHG have the potential to be used in the raisin industry. The carbonic maceration has successfully improved the drying characteristics of the grape, without producing any residue. Even the process has resulted in a better quality of raisins than chemically pretreated grapes in terms of color and rehydration properties (Wang et al., 2014). On the other hand, MHG process has the ability to predry the grapes, which reduces the overall drying time (Farias et al., 2020). In addition, the process produces soluble by-product constituents rich in nutrients, antioxidants, and polyphenols, which can be later used in a food industry (Ferreira et al., 2018). Both processes have been industrially used for products such as wine, oils, etc. The carbonic maceration process is used for producing wine from grapes (Flanzy et al., 1987) and MHG is used for the extraction of oils from mint and pennyroyal (Vian et al., 2008). Thus, these technologies can be easily adapted for the commercial level pretreatments of grapes.
7 FUTURE TRENDS
The pretreatment and drying processes of grapes are the two most important processes for the production of raisins. As already seen, pretreatment of raisins loosens the waxy layer and improves drying; however, many pretreatment processes have different advantages and drawbacks.
Chemical methods of pretreatment provide excellent improvement in the drying rate and the chemicals are available readily in the market at low cost. Thus, the process is commercially viable in both economic and production point of view. However, the residues left in the raisin can cause serious health problems, and the leftover chemicals after processing can pollute the surrounding (Wang et al., 2016). Thus, it is better to use physical method of pretreatment, which has no residue and no harmful effect on the environment. From the customer point of view, the good-quality attributes and low cost are the two most important considerations during the purchase of grapes. However, compared to chemical methods, the physical methods such as abrasion and water blanching have not successfully achieved the lower level of drying rate. Thus, new technologies such as HHAIB, carbonic maceration, and MHG should be explored commercially because these technologies have successfully shown their effectiveness in improving the drying rate and preserving the quality of drying grapes. The future research in the pretreatment of grapes should be focused on the designing, optimizing and extension of these technologies in large-scale units.
The second most important process is the drying of pretreated grapes. Solar drying, shade drying, and hot air convective drying have been used for so many years in the raisin development. However, possible drawbacks such as low drying time, poor sensory quality, less chance of innovative improvement, high labor requirement, etc. have made the researcher to think of navigating newer drying processes such as MW drying, vacuum drying, infrared drying, and freeze drying. These processes have shown an improved drying rate, however with the availability of suitable conditions such as low feed input, applicable for a specific range of moisture, higher cost of processing, and working for a specific part of grapes. The solutions include the use of combined drying process, which can control the temperature of drying, with the maintenance of higher moisture diffusivity throughout the drying process. The advanced drying technologies such as PVD, MWD, and microwave vacuum dehydration have shown acceptable outcomes. Thus, more future research on drying of grapes should focus on the possible combination of novel drying processes by keeping in mind the objective of commercialization.
8 CONCLUSION
Raisins have several advantages such as higher shelf life, sweet taste, commonly used as dry fruits, ease in convenience, value-added product, etc. The process required during the development of this product must be carefully defined in order to get higher market values and good-quality product at a lower processing cost. However, most of the review literature has summarized the important pretreatment and drying process in just one or two headings, though these are primarily important unit operations defining the cost and quality of the end product. Thus, this review has focused on providing detailed knowledge about traditional, commercial, and advanced processes of the pretreatment and drying unit operations required during raisin development. Each traditional process has been well explained in a topic. The new development in pretreatment and drying process has been described in detail for the future research in further innovation of raisin processing. The effect of pretreatment and drying process on the chemical and physical properties of grapes is discussed for providing useful information while selecting the technology. Thus, the review shows the potential engineering and research development done on the grape-processing field. However, the area still requires further research in the processing of grape for raisins development.
ACKNOWLEDGMENT
All the authors have contributed equally in writing, framing, and revising the review paper.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest.
