- Research Bulletin -

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Use of Liquid Media for Dehydration of Seedless Grapes
by
Carter D. Clary

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CATI publication #960903
© copyright September 1996, all rights reserved

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ABSTRACT

This bulletin describes the first phase of preliminary research evaluating liquid media dehydration as a method for drying seedless grapes to produce Golden Seedless raisins. The grapes are submerged in heated liquid media to induce vaporization of the water from the grapes. This is done under vacuum to minimize the heat used and therefore reduce changes in composition and color of the grapes. The result is production of Golden Seedless raisins without the use of sulfur compounds.


INTRODUCTION

Although production of Golden Seedless raisins accounts for only about four percent of total California raisin production, this amounts to about 17,651 tons annually (California RAC Bulletin, 1994). Of the nine types of raisins classified by the Raisin Administrative Committee, Golden Seedless raisins represent the most distinctive product from the standpoint of color. Produced from Thompson Seedless grapes, Goldens exhibit a bright yellow color and a tangy flavor. This is attributed to the presence of sulfur compounds which are used as an antioxidant.

The heated air used to dehydrate grapes induces compositional changes including polyphenol oxidase activity which causes change in color from the light green of the fresh grapes to a dark brown to purple-black color of the dehydrated product (Singleton et al., 1985). The heated air drying process also results in changes in flavor, shape, and nutritional value. To prevent the change in color, grapes used in production of Golden Seedless raisins are treated with sulfur dioxide to preserve original color. Sulfur dioxide is applied in airtight enclosures at a liquid rate of 2 to 4 kg per metric ton of fresh fruit (4 - 8 lb/fresh ton). A sulfur concentration of about 2000 ppm in the raisin is needed to preserve color for one year. Alternatives to sulfur dioxide to preserve original color are limited. Some researchers have evaluated citric acid; however, results have been negative (Salunkhe et al., undated). Nevertheless, there is considerable interest in producing a Golden raisin product with a reduced sulfur residue, or no sulfur at all.

Drying fruits and vegetables using an inert liquid medium has been understood for some time (Webb and Webb, 1977; Webb, 1988). Several patents have been issued that describe the use of a liquid medium for heat transfer. Use of a heated medium under vacuum minimizes changes in color and shape. Often called vacuum frying, this method has been used in production of products such as apple and banana chips.

Other adaptations of this type of dehydration have included submersing a frozen food material in vegetable oil heated to 160 to 225°C (325 to 440°F) under a partial vacuum of 3.2 to 26.8 kPa (23.4 - 201.2 Torr) (Forkner, 1966). This process avoids shrinking with the goal of maintaining the general identity of the form and size of the fresh product. Forkner (1967) describes a process of applying a similar temperature range to produce dried particles having a form similar to the form of the original particles of source material, without excessive shrinkage. Other patents describe processes that use oil under vacuum to produce puffed products having structures and compositions that are unique (Lankford, 1973). Methods have also been outlined for producing puffed, low moisture fruit and vegetable particles (Webb and Webb, 1977) and for a process for producing "vacuum fried" banana slices exhibiting a crisp texture and a moisture content of 1.5 to 3.0 percent (Numata and Sugano, 1980).

Variations of vacuum frying systems have produced puffed products of low moisture content using heated oil (Sakuma and Sakuma, 1988; Webb, 1989). The desired result of this process was puffing fruits and vegetables. The intent was to produce a puffed structure that was hardened by cooling to create a crisp, low moisture product. Product moisture was cited as being so low (less than 2 percent) that specialized packaging was required to prevent loss of crunchiness.

Although the dehydration methods described above targeted preserving original shape, these methods also preserved the color of the fresh fruit. The purpose of the experiments described in this bulletin was to evaluate different process conditions to preserve color, yet produce dried grapes with the wrinkled, collapsed character of a raisin.

Clary and Petrucci (1991) and Clary (1994) first described treatments combining low temperature and high pressure which resulted in collapse of the fruit tissue into a wrinkled, raisined texture. It was suspected that temperature and pressure were the primary factors in determining the texture and final moisture content of the dried grape product, and liquid media dehydration showed potential for dehydration of grapes for production of Golden Seedless raisins without the use of sulfur compounds.


OBJECTIVES

The objective of the experiments was to develop a prediction model based on process time, temperature, pressure and other factors to optimize these factors to produce Golden Seedless raisins without the use of sulfur.


MATERIALS AND METHODS

The liquid media test bench used in the experiment consisted of a group of Erlenmeyer vacuum flasks connected in series to a vacuum pumping system (Figure 1). The flask at the end of the series contained the liquid medium and grape sample. This flask was heated on a hot plate equipped with a magnetic stirrer. Vapors evolved from the grape sample were carried from the flask through tubing to a second and third flask, and ultimately to the vacuum pump.

The procedure involved selecting a sample of single Thompson Seedless grapes for treatment and determination of their initial moisture and sugar content. The treatment sample was placed in the first flask filled with test media heated to treatment temperature. The flask was sealed from the atmosphere and opened to the vacuum system. Based on earlier research, Durkex 500 was selected as the best media material.

Pressure was adjusted to the treatment level and monitored by a Leybold vacuum gauge (Model DIN 180 63) reading in mm mercury absolute pressure in a range of 0 - 100 Torr (0 to 13 kPa). Media temperature was measured by a type K thermocouple and digital thermometer and the hot plate adjusted as needed (+ 5°C). Drying time among the treatments ranged from two to three hours.

When evolution of vapor slowed to a rate of a few bubbles per second, the flask was removed from the hot plate and carefully tilted to drain the media through the tubing into the second flask. In this way, the dried grapes were isolated from most of the media before venting the system to atmosphere. The intent of this procedure was to minimize infusion of media into the grapes when the system was vented. Following removal from the test flask, grape appearance including color, shape, and texture was noted, the sample was weighed, and final moisture content determined by vacuum oven.

Grape samples were dehydrated using a combination of four levels of pressure and three levels of temperature, with each treatment replicated three times. Pressure treatments ranged from 1.3 to 12 kPa, and temperature treatments ranged from 54 to 82°C, applied in 36 bench tests.


RESULTS AND DISCUSSION

At less than 68°C (150°F) and 12 kPa (90 Torr), the dried product exhibited the collapsed, wrinkled characteristics of a Golden Seedless raisin with a moisture content of about 16 percent (wet basis). Treatments applying high temperature and low pressure produced a puffed crispy grape product.

Multiple regression analysis indicated a r² value of 0.910 (Table 1). Decomposition of the regression sum of squares (SSR) of the five predictors used in this experiment set showed pressure and temperature contributed to 95 percent of the variation. Regression analysis of pressure and temperature as the only predictors of final moisture content indicated an r² value of 0.865. Decomposition of the SSR for the remainder of the treatment variables and the coefficient of mean response are also shown in Table 1.

A regression surface plot describing the effect of pressure and temperature on final moisture content is shown in Figure 2. Decomposition showed thatpressure, as a single factor, accounted for 49.3 percent of the variation in final moisture content due to treatment. Independent of the temperature used in the experiments, treatment at high pressure produced dried grapes with a final moisture content of 4 to 13.5 percent. At lower pressures, final moisture content was about 2 to 6 percent. Temperature as a single factor contributed to 45.7 percent of the variation in final moisture content due to treatment.

The multi-colinearity of pressure and temperature accounted for 95.0 percent of the variation in final moisture content due to treatment. Treatments at low temperature and high pressure produced a raisined product with a soft pliable texture and a final moisture content of 13.5 percent. Treatments using high temperature and low pressure resulted in a puffed, crispy product with a final moisture content of 1 to 2 percent. In all cases, the dried product exhibited a bright golden color.

The results of these experiments have proven the concept of producing Golden Seedless raisins using liquid media. Future work will address more detailed character of the dried raisin product, the issues related to the residue of Durkex 500, shelf life stability, scale-up from the laboratory bench configuration, and economics.


CONCLUSIONS

Temperature and pressure were the primary factors in determining the texture and final moisture content of the dried grape product. Liquid media dehydration showed potential for dehydration of grapes for production of Golden Seedless raisins.


REFERENCES

California Raisin Administrative Committee. 1994. Marketing policy for the 1993-94 marketing season. RAC, P.O. Box 231, Fresno, CA 93708.

Clary, C. D., and V. E. Petrucci. 1991. Method and apparatus for dehydrating matter. Patent Application Serial No. 07/793,471.

Clary, C. D. 1994. Microwave Vacuum and Liquid Media Dehydration of Grapes. A dissertation. Michigan State University, East Lansing, MI.

Forkner, J. H. 1966. Method for dehydration of moisture containing materials of cellular structure. U.S. Patent 3,261,694.

Forkner, J. H. 1967. Method of dehydrating moist materials. U.S. Patent 3,335,015.

Numata, F. and K. Sugano. 1980. Process for producing fried banana slices. U.S. Patent 4,242,365. 30 December.

Sakuma, M. and K. Sakuma. 1988. Device for producing fried food. U.S. Patent 4,732,081. 22 March.

Salunkhe, D. K., J. Y. Do and H. R. Bolin. Undated. Developments in technology and nutritive value of dehydrated fruits, vegetables, and other products. MSU Library TX 609.S36 c.2.

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.

Webb, W. A. 1988. Process for dehydrating and puffing food particles. U.S. Patent 4,769,249. 6 September.

Webb, W. A. 1989. Method of dehydrating and puffing food particles. U.S. Patent 4,857,347. 15 August.

Webb, W. A. and W. R. Webb. 1977. Method and apparatus for evaporation of moisture from fruit and vegetable particles. U.S. Patent 4,006,260. 1 February.


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