Pretreatment grape before drying

Fresh grapes are very sensitive to microbial spoilage during storage, even under refrigerated conditions, having relatively high moisture contents (80% - 85% wet basis). Therefore, within a few weeks following harvest, they must either be consumed or processed into various products. Drying is the most common method for grape processing.

The reduction of water activity in dried grapes is of greatest importance, because the products become more stable and with storage under appropriate conditions the optimum microbiological, nutritional, and sensory properties of the finished product are achieved.

Pre-treatment is a necessary step in raisin production in order to ensure the increased rate of water removal during the drying process. A faster water removal rate decreases the rate of browning and helps to produce more desirable raisins.

The peel of the grape plays a critical role in controlling the drying process. Grape peel consists of a wax layer as a protective barrier against fungal pathogens. However, this coating reduces permeability and therefore hinders the process of water transfer.

Pre-treating helps keep light-colored fruits from darkening during drying and storage and it speeds the drying of fruits with tough and waxy skins.

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.

Due to this, prior to drying, several chemical and physical pretreatments are used to enhance permeability and increase the drying rate, while preserving the physical, chemical, nutritional and sensory qualities of the final product.

Therefore, chemical dip treatments (both hot and cold) have been used to increase the drying rate of grapes which not only reduce the drying time but also improve quality of the raisins. The hot dip treatment with dipping solution causes cracking and perforation in the waxy cuticle and thus increases the drying rate over the cold dip treatment.

The effects of pretreatment solutions on seedless grapes during drying were reported in literature.

Study shows dipping grapes in to 0.5% NaOH solution at 90 °C for 2-3 sec repeated for two to three times and then dipped in to Lye solution 0.5% + Ethyl oleate 1.0% + Potassium metabisulfite 1.0% will improve the physical and chemical properties of raisins and also reduce the time required for drying.
Pretreatment grape before drying

Pretreatment of foods

It is generally necessary for foods to undergo a treatment prior to the canning process, but these pretreatment differ depending on the foods.

The purpose of pretreatment is to help preserve color nutrients, flavor and overall quality.  Specific vegetables and fruits are high in different vitamins. It is important to know how the vitamin is destroyed and what pretreatment will stop the loss.

Some pretreatment are applied to many different foods. One of these, usually applied to vegetables, is blanching. Vegetables may be pretreated before drying, just as they are before freezing, by blanching in boiling water or steam.

Blanching is steam or water is a method of partially cooking the food, typically just to the point of inactivating the enzymes and also used to break the skins of fruits that have a waxy coating.

Steam blanching preserves more of the food’s natural vitamins and minerals than water blanching but requires slightly longer processing period.

Vegetables are first washed, usually in water and detergent, then rinsed. They are then passed over belts, where any remaining foreign matter, such as weeds or stalks, can be removed by hand.

Blanching consists of heat in steam (no pressure) or hot water (usually about 98.9 ° C) until the temperature of the food is brought up to about 82.2 C – 87.8 ° C in all parts, the cooling in water.

Some vegetables cannot be blanched at these high temperatures without adversely affecting their taste and texture. In such cases, catalase is used as the test enzyme for sufficiency of blanch.

Off flavors may result, though even in the absence of catalase, if peroxidase is present.
Pretreatment of foods

Pretreatment before Process of Drying

Pretreatment before Process of Drying
The preservation of foods by drying is probably the oldest food preservation process practiced by humans. It is believed that many foods, especially grains and fruits of high sugar content, were preserved by primitive peoples by allowing them to dry in the sun. Spices and fish, cut into thin strips, were also preserved in this manner.

There are number of different methods of drying foods for preservation. The most important are sun drying, tunnel or cabinet drying, drum drying, spray drying and freeze drying.

Foods to be dried must be washed, and some peeled and cut. Others may be pre cooked. Cut fruits are subjected to darkening through enzymes action and must be either blanched or treated with salts or sulfur dioxide. However if treated with sulfur dioxide, the product must be so labeled, because of known allergenic reactions in some consumers. Certain vegetables may be pretreated in the same manner. Sulfuring may also be required to limit non enzymatic browning (the Maillard reactions). Browning refers to the development of brown color.

Various dried eggs products are also subject to browning and also susceptible to the development of off flavors. In this case, the reaction involves a combination of small amount of glucose, which is naturally present, with proteins. Because of this, dried eggs products, especially egg whites, may be treated with glucose oxidase and catalase. The glucose oxidase converts glucose to gluconic acid (which does not combine with amino groups) and hydrogen peroxide. The purpose of the catalase is to convert the undesirable peroxide to water and oxygen. The elimination of glucose may also be done by natural fermentation using microbes. However this process can be considered unsanitary and in order to avoid product spoilage and even food poisoning, it is necessary to hold the product at 130 degree F for several hours after drying.
Pretreatment before Process of Drying