Grapes are one of the most popular and palatable fruits in the world. The preservation of grapes by drying is a major industry in many parts of the world where grapes are grown. Drying practices vary with geographical locality and with the variety of grapes. Drying grapes, either by open sun drying, shade drying or mechanical drying, produces raisins. Air-drying of solar energy has been demonstrated to be cost-effective and could be an effective alternative to traditional and mechanical drying systems, especially in locations with good sunshine during the harvest season. The traditional air-drying shelter of grapes has been used for thousands of years in Asia and other places around the world. The drying of grapes under sheds is common practice followed in major raisin making sites of India like Sangli, Solapur and Nashik districts of Maharashtra and Bijapur district of Karnataka.
The quality of the dried products implies that several desirable changes (physical, chemical, and biochemical) must occur during the drying process. These changes are influenced by drying conditions and physico-chemical characteristics of the targeted material. Some specific characteristics of grapes (i.e., size of the berries, sugar content, and the presence of an outer waxy cuticle) play a significant role both in the drying process itself and in post-drying operations such as washing, cleaning, and finish-drying. Prevailing temperature, wind velocity, exposure of grapes to sunlight, humidity are major environmental factors which affect the quality of dries grape. Other than these factors the berry size also affect water loss rate during drying process. The drying rate affected by kind of grapes also. The smaller berries lost water more rapidly than larger berries because of the greater relative area of skin to flesh. Water loss was not greatly affected by sugar content; however. 20 Brix berries tended to loose water more rapidly than higher maturity berries. Final water loss was greater in 20 Brix grapes because they had less soluble solids. Berry characteristics of each cultivar also affect drying rate independently.
Increasing efforts have been made on modifying and developing traditional grape drying procedures. Fruit quality parameters such as color, texture, and microbial stability can be considerably changed during the storage period, depending on storage conditions and the physicochemical specifications of grapes (i.e., moisture content, water activity (aw), skin damage, and individuality of berries). These changes may have adverse effects on processing operations reducing shelf life of processed raisins.
Pretreatments of grape bunches
Application of an oil emulsion or a dilute alkaline solution is a common practice to accelerate the drying process by reducing the resistance to moisture transfer of the surface skin of grapes, and by improving the internal moisture diffusion coefficient. Each component of the drying emulsion may interact with the berry skin, the cuticle, and underlying layers. Apart from chemical components (either “alkaline solutions with or without dipping oils” or “compounds such as ethyl oleate with or without alkaline solutions”), the pretreatments will cause an increase in the drying rate particularly at the early stage of drying process. The composition, concentration, pH and temperature of the chemicals and the pre-treating time are effective factors in microstructural changes of the skin layers. Dipping of grape bunches in solution of 15 ml ethyl oleate and 25 g potassium carbonate/l for 2- 4 minutes is common practice. Some commercial grape drying oils are available in the market and extensively in use. The dipping durations significantly affected the moisture content, colour intensity other quality attributes.
Drying of grapes varies in different parts of the world, depending on the cultivation conditions. There are three main methods that are used in raisin production; sun drying, shade drying, and mechanical drying. The sun drying method has several disadvantages including, the possibility of environmental contamination due to dust and insect infections, physical microbial deterioration caused by rain, and color deterioration due to intense solar radiation. Moreover, removal of contaminants (e.g., small stones, soil, leaves, dust, etc.) collected for the period of raisin gathering is tedious during the raisin cleaning process. Hence, using this method, raisins of low quality are produced due to uncontrolled drying conditions and environment. In recent years, new methods of indirect solar drying have been developed.
These three methods are known as traditional or conventional techniques in grape drying. More recently, the industrial application of microwave heating for the preparation of dried grapes has been reported. The microwave vacuum dryer system was originally introduced for California seedless grapes where the so-called puffy dried grapes are produced. But this system is dependent on electric energy. Considering the situation and available resources, the method adopted by raisin processors of Maharashtra and Karnataka, is much suitable. The grape drying sheds are not depend on the electricity and within 10-12 days one lot of grapes converted in to raisins.
Post-drying (Processing) of Dried Grapes
Following production of dried grapes, either by sun drying or other drying techniques, they must be delivered to an appropriate processing unit. The post-drying operations may vary depending on the drying method. The operating conditions may affect the physical and hygienic characteristics of the dried product. The cleaning involves individualizing the dried fruit, removal of stems and foreign materials, and removal of off-grade raisins. Since water is used to eliminate foreign materials such as dust and sandy soil, through multiple washing, this may lead to further rehydration. The rehydration, leakage of some of the dissolved sugars of the dried fruit into the washing water, and extension of micro-cracks and skin-wounds are the main phenomena taking place during the post-drying practices. There is a lack of research and development on rehydration phenomenon during the washing step. Because of an increases in the moisture content of the dried product during the washing, especially when the grapes have been partially dried or washed off before packaging, one additional step, finish-drying, is needed in order to control the amount of the moisture content. The efficiency and quality of the post-drying operations are significantly influenced by biomechanical and physical properties of the dried grapes.
Dried Grape Quality
The quality of dried grapes as a semi-processed product and raisin (the final product) are evaluated in terms of the appearance, texture, free-flow (having nonsticky surface), cleanliness (for dried grapes that easily could be processed with minimum damage), flavor, and nutritional value. Apart from varieties and preharvest conditions, the quality of dried grapes/raisins largely depends on operating, pretreatment, drying, processing, and storage conditions. The quality of different varieties of grapes after drying in similar operating conditions had not been identical due to differences in texture and composition. Fruit maturity has a direct influence on the appearance, texture, flavor and food value so that raisins produced from low-maturity fruit are skinny, coarse-wrinkled, hard, light in weight, and tend toward a lighter, more reddish color. The texture of fruit is influenced by pre-harvest factors such as environmental, cultural, physiological, and genetic factors. Both color and texture greatly influence the marketability of the product.
Color of the final product could be influenced by the state of the fresh fruit. Sunlight exposure affects grape composition, especially the phenolic compounds in the skin, which play a significant role in browning of grapes on drying. Several times colour parameters chosen as an index to evaluate the quality of dried grapes. It is well known that chemical pretreatment of grapes leads to the production of light-coloured dried grapes.
Sulfur dioxide (SO2) and its derivatives have long been used in foods as general food preservatives. They act as antioxidant, bleaching and reducing agents, and as such inhibit non-enzymatic browning, enzyme catalyzed reactions, and control microorganisms. The sulfiting process is generally performed by gas fumigation or by immersion in solutions of SO2 generating compounds.
Storage of raisins
Raisins are very hygroscopic. Contact to moisture may result in mold, rot and fermentation and if a fermentation process is initiated, it may eventually affect the entire lot. Under appropriate temperature and humidity conditions, there is a risk of infestation by maggots, mites, cockroaches, moths, beetles, rats, mice and ants. Mite infestation may be determined by examination with a magnifying glass: mites may be distinguished from crystallized glucose because they are whitish, slow moving dots. At temperatures > 25°C and on exposure to mechanical pressure, there is a risk of candying, agglomeration, syrup formation and fermentation. Heat generally causes the risk of discoloration and hardening and the product should thus be stored away from heat sources. At temperatures < 10°C, mite growth is usually inhibited.
Storage of raisins at ambient conditions also deteriorates the quality in terms of colour, mouth feel, aroma etc. To maintain the quality of raisins and increasing shelflife, storage at low temperature is always advised. During storage period, the product turns sticky and hard due to exudates syrup and moisture loss. To overcome this problem the application of edible coating suggests being of proper assistance. This coating can affect the respiration and moisture loss. The performance of pectin film was found better than gum and starch coatings. The results of sensory evaluation showed that the colour and texture of Thompson Seedless variety coated with pectin were the best, while in the term of flavour, samples covered with gum proved to be the best and microbial count was also decreased. Raisin storage in darkness is found to maintain their quality. In view of the above, the final packing of produce is done in 400 gauge LDPE film bags and stored in corrugated boxes of 5 to 15 kg capacity at low temperature (4°C) to withstand the mechanical, climatic, biotic and chemical stresses to which raisins may be exposed during transport, storage and cargo handling and preserve the original raisin color and prevent the attack of pests.
Thus there is a big potential of raisin industry in India in terms of the marketing of this product in domestic and international market for import substitution and better utilization. Besides, there is possibility of diversification of raisin industry by promoting the production of flavoured and coloured raisins and promoting raisins as neutraceuticals in public health care.
Ajay Kumar Sharma
National Research Centre for Grapes, Pune