Research Bulletin
Use of Microwave Vacuum for Dehydration of Thompson Seedless Grapes
by
Carter D. Clary and Gwynn A. Sawyer Ostrom
CATI Publication #950405
© Copyright April 1995, all rights reserved

INTRODUCTION

      Dehydration offers a means of preserving foods in a stable and safe condition, and provides extended shelf life compared to fresh fruits and vegetables. Many food commodities mature and dehydrate naturally, including grains, seeds of legumes, and nuts (Decareau, 1985). Fruits do not dehydrate naturally, and specific practices are required to preserve them after harvest (Ede and Hales,1948). One option is sun drying the fruit to produce a stable and natural source of sugars. The low moisture content of dried fruit, together with high levels of sugars, tend to inhibit attack of pathogens.
      Historically, fruit has been dried using the sun as a heat source. Although sun drying is still a common practice, heated air has been used in the dehydration of about 30 percent of dried fruits in more recent times, especially when color preservation is required and sulfites are used (Nury et al., 1973). This alternative to sun drying was most likely brought about by difficulties due to inclement weather (Eissen and Muhlbauer, 1983). In either method, moisture removal is accomplished by inducing water in the product to vaporize by creating a vapor pressure gradient between the fruit and the surrounding environment. The primary factors that affect the vapor pressure gradient include the amount of water in the product, temperature, pressure, moisture in the drying environment, and the composition of the product.
      During the heated air dehydration process, water leaves the fruit tissue near the surface of the berry first, causing this region to dry more quickly and the berry tissue to collapse on itself. The exposure of the grapes to heat also induces compositional changes to the grapes. Oxidation including polyphenol oxidase activity causes a change in color from a light green to a dark purple-black or brown (Singleton et al., 1985). These changes in the shape and chemistry of grapes contribute to the distinct character of raisins with respect to color, flavor and shape.
      Although sulfites continue to be used in production of golden seedless raisins, the incidents of allergic-type reactions prompted review by regulatory agencies and may have affected public opinion related to the use of sulfites as preservatives in food products (Taylor, 1993; Sapers, 1993). These concerns and new regulations have fostered an interest in the identification of alternatives to eliminate or reduce the use of sulfites in food products.
      Freeze drying uses very low pressure to reduce the vapor pressure in the drying environment to remove water from a food product by direct sublimation (King, 1973). The reduction in ambient pressure lowers the vapor pressure of the drying environment and induces water to leave the food product more rapidly and at a lower temperature. This minimizes compositional changes during dehydration (Van Arsdel et al., 1973) and contributes to obtaining very low levels of final moisture content. The dried product maintains puffed character, flavor, and nutritional value; however, the product is powdery and has a slightly faded color because it has been frozen.


MICROWAVE VACUUM DEHYDRATION

      Use of microwave applied in a vacuum for dehydration heats the product to cause water to vaporize, without causing changes in composition. Microwave energy penetrates deeply into food products and can reduce process time by 90 percent (Decareau and Peterson, 1986). It offers opportunity to process foods in ways not possible by other means. Microwave heating offers distinct benefits in dehydration because the penetration of energy and uniform heating results in water vaporizing from throughout the product. This induces an inner pressure that maintains puffed character of the dried product and preserves color, flavor, and nutritional value.
      Application of microwave energy in a vacuum results in an increase in product temperature; however, the temperature rise is limited to the boiling point of the water at the lowered pressure. At a pressure of 3 kPa, free water boils at 22°C. This maintains a product temperature at a level below the temperature used under atmospheric conditions of 101 kPa.
      Microwave vacuum dehydration was first used for concentration of citrus juice in France (Decareau, 1985). Microwave vacuum drying of agricultural commodities has included grain (McKinney et al., 1977) and rice (Wear, 1982). This technology was adapted to grapes by McKinney et al. (1983) for production of Grape PuffsTM using zoned microwave vacuum dehydration patented by McKinney and Wear (1987) and described by Petrucci and Clary (1989).


GOALS

      Raisins occupy a major segment in the marketing of agricultural products and create a substantial demand for grapes grown in the San Joaquin Valley. Any new use of processed grapes offers potential stability to the market by broadening the need for grapes. This bulletin describes the evaluation of a method of dehydration for grapes that retains fresh fruit character including color, flavor, and puffed character without the use of preservatives. Microwave vacuum dehydration was evaluated for preserving color, flavor, and puffed character in dried grapes without the use of preservatives.


OBJECTIVES

      Evaluation of the effect of microwave applied to grapes in a vacuum was conducted using a laboratory batch system. The intent of the experiments was to determine the optimum level of microwave energy to dry grapes, and to preserve color, flavor, and puffed character in the dried product without these use of preservatives. The objectives of the experiments included these:




MATERIALS AND METHODS

      The laboratory microwave vacuum unit(Figure 1) used for the dehydration experiments consisted of a microwave power supply, microwave control, vacuum vessel equipped with a turn table, vacuum pump and vacuum control, and system controls and instrumentation. A continuous wave output from the microwave power supply was adjustable from zero to a full power level of 3,000 W. Since thermocouples and other methods of direct temperature measurement were not compatible with the microwave field, the temperature of the grape samples was monitored remotely by an infrared detector. An emissivity1 setting of 88 was used to adjust the gain of the detector.
1Emissivity =   total radiant energy emitted by the grapes
----------------------------------
  total radiant energy emitted by a blackbody

fig1

      Fresh Thompson Seedless grapes were removed from the cluster stem; sugar content and initial moisture content determined; and a sample weighing 1814 g was placed on the turntable inside the laboratory unit. The vessel was sealed, evacuated to 3 kPa, and the microwave treatment was applied. When each experiment was completed, the dried sample was removed from the unit and evaluated for final moisture content and physical character including color, puffed character, and texture. Sugar content was measured using a refractometer and moisture content was determined by vacuum oven (AOAC, 1980).
      The treatments consisted of applying 3,000 W microwave power to the sample of grapes until the temperature of the sample rose to the treatment temperature indicated by the infrared detector. Five temperature levels from 54 to 77°C were used. In each experiment, the microwave system was operated at 3,000 W until the grape sample temperature approached the treatment temperature. The power level was subsequently decreased to 1,500 W to prevent the grape sample temperature from exceeding the treatment temperature. Microwave power was reduced again to 200 to 500 W until the grape sample temperature increased beyond the specified treatment level. If the temperature started to decrease, microwave power was increased. Each temperature treatment was replicated three times.
      Specific energy,2 time, fresh fruit sugar content, initial moisture content, and treatment temperature were evaluated using multiple linear regression analysis to develop a model for predicting final moisture content and puffed character of the dried product. Regression analysis also provided a decomposition of the regression sum of squares value for each independent variable. Regression surface plots were used to describe the interaction of two independent variables on final moisture content and puffed character.

RESULTS

      The r2-value of the multiple linear regression analysis of the effect of temperature, process time, specific energy, and fresh fruit sugar and initial moisture content on the final moisture content of the dried grapes was 0.942 (Table 1). Decomposition of regression analysis indicated temperature and fresh fruit sugar content accounted for 64.5 and 21.5 percent of the variation due to treatment, respectively. Treatment temperature was achieved by microwave heating applied for sufficient time to heat the grape sample to the treatment temperature. The final moisture content of the grapes ranged from 3.5 to 9.8 percent. Within this range, the portion of each sample exhibiting puffed character ranged from 0 to 80.3 percent.

fig2
fig4
      A regression surface plot of the effect of time and total specific energy is shown in Figure 2. Based on this analysis, the optimum total specific energy was 0.86 W-h/g applied over a period of 70 to 75 min. The optimum treatment temperature was 73°C. This treatment regime produced a dried grape sample with a final moisture content of about 4 percent, and 80 percent of the sample exhibited a puffed, crunchy character.

      Multiple linear regression analysis of the effect of treatment temperature, time, specific energy, fresh fruit sugar, and moisture content on the amount of dried grapes exhibiting a puffed character indicated an r2- value of 0.985 (Table 2). Decomposition of regression analysis indicated that temperature had the most significant effect on the final character of the grapes dried in these experiments. The optimum levels of these variables are shown in Figure 2.

fig3

      A total specific energy of 0.84 to 0.88 W-h/g fresh grapes was found to be the optimum range to dehydrate the grapes into Grape PuffsTM. Equally important, the power levels used to achieve this total specific energy were applied in stages sufficient to heat the grapes to between 70 and 80°C, and maintain this temperature until the grape sample dried to about 5 percent final moisture content. This produced dried grapes that were 95 percent puffed.


DISCUSSION

      In the course of developing the treatment ranges for the experiments, it was observed that the temperature setting required to effectively dry grape samples was above the temperature at which grape sugars would normally burn at about 57°C. It is possible that the emissivity setting of infrared detector was set too low. Even though the infrared readings shifted upward, they were consistent and served as a good indicator for performing repeatable tests.
      Although the temperature monitored by the infrared detector was as high as 75°C, there was no evidence of burned sugar. The estimated upward shift from the actual temperature of the grape sample and the temperature indicated by the infrared detector was about 10 to 20°C. This upward shift may be explained by the emissivity setting used in the experiments. Nevertheless, the infrared detector was a useful tool in controlling the process. It is suggested that further evaluation of infrared temperature detection be conducted to determine a more accurate emissivity setting and to confirm the validity of the regression analysis in predicting final moisture content based on the process variables.
      Microwave power applied at levels to heat and maintain the grapes to between 70 and 80°C as indicated by the infrared detector resulted in drying the grape samples to a final moisture content of 5 percent or less. The dried product maintained puffed character, color, and flavor of the fresh grapes, but with the water removed. The product exhibited a crunchy texture and, when sealed in an airtight package, stored a minimum of 1 year. Most samples maintained original color, flavor, and texture for more than 1 year without the use of preservatives or refrigeration.


CONCLUSIONS

1. The experiments defined a relationship between microwave energy and time of exposure and their effect on product temperature and final moisture content of grapes. The optimum total specific energy was 0.84 to 0.88 W-h/g for 70 to 75 min in an infrared temperature range of 70 to 80°C.

2. Within this range of specific energy based on time, 95 percent of the grapes exhibited the integrity, puffed character, and color of the fresh grape.


REFERENCES

AOAC. 13th Ed. 1980. Moisture in dried fruits. In Official Method of Analysis of the Association of Official Analytical Chemists, ed. W. Horowitz, 22.013(7). Washington, DC.

Decareau, Robert V. and R. A. Peterson. 1986. Microwave Processing and Engineering. Chichester, England: Ellis Horwood Ltd.

Decareau, Robert V. 1985. Microwaves in the Food Processing Industry. New York: Academic Press.

Ede, A. J. and K. C. Hales. 1948. The physics of drying in heated air with particular reference to fruit and vegetables. Dept. of Scientific Research and Industrial Research Ð Food Investigation Report No. 53. London: His Majesty's Stationery Office.

Eissen, W. and W. Muhlbauer. 1983. Development of low-cost solar grape dryers. In Proc. Int. Workshop on Solar and Rural Development, Bordeaux, France, May.

King, C. J. 1973. Freeze drying. In Food Dehydration, ed. W. B. Van Arsdel, M. J. Copley, A. I. Morgan, Jr., 161-200. Westport, CN: AVI Publishing Co.

McKinney, H. F., N. L. Higginbotham and D. Q. Durant. 1977. Seed drying process and apparatus. U.S. Patent 4,015,341.

McKinney, H. F., F. C. Wear, H. L. Sandy, V. E. Petrucci and C. D. Clary. 1983. Process of making hollow dried grape. U.S. Patent 4,418,083. 23 November.

McKinney, H. F. and F. C. Wear. 1987. Zoned microwave drying apparatus and process. U.S. Patent 4,640,020. 4 February.

Nury, F. S., J. E. Brekke and H. R. Bolin. 1973. Fruits. In Food dehydration, ed. W. B. Van Arsdel, M. J. Copley, A. I. Morgan, Jr., 158-198. Westport, CN: AVI Publishing.

Petrucci, V. E. and C. D. Clary. 1989. Microwave Vacuum Drying of Food Products. EPRI Report CU-6247. Electric Power Research Institute, Inc., 4312 Hillview Avenue, Palo Alto, CA 94304: EPRI.

Sapers, G. M. 1993. Browning of Foods: Control by sulfites, antioxidants, and other means. Food Technology 47(10):75-84.

Singleton, V. L., E. Trousdale and J. Zaya. 1985. One reason sun-dried raisins brown so much. American Journal of Enology and Viticulture 36:111-113.

Taylor, S. L. 1993. Why sulfite alternatives? Food Technology 47(10):14.

Van Arsdel, W. B., M. J. Copley, A. I. Morgan, Jr. 1973. Food Dehydration, Vol. 1 and 2, Westport, CN: AVI Publishing Co.

Wear, F. C. 1982. Apparatus and process for drying granular products. U.S. Patent 4,347,670.

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Copyright © 1995. All rights reserved.
CALIFORNIA AGRICULTURAL TECHNOLOGY INSTITUTE - CATI
College of Agricultural Sciences and Technology
California State University, Fresno