Mechanical drying of raisins

Grapes intended for dehydration are hand harvested and transported to the drying facility in pallet bins. In artificial drying, grapes are first dipped n 0.25 – 1 % lye (sodium hydroxide) solution at 200 – 212 degree F for 2 – 5 sec to remove a natural wax that impedes drying and to produce tiny cracks in the skin of the fruit in order to speed drying.

The fruit is then placed on trays and exposed to gaseous sulfur dioxide. This is to prevent enzymatic and nonenzymatic browning during drying. Hence instead of having the dark brown color of raisins that normally be expected, the raisins will be of a light yellow color when dried. The purpose of the pretreatment is to increase the permeability of the grape skin to moisture.

There are three main methods that are used for fruit drying: sun drying, shade drying and mechanical drying. Mechanical drying which is safe, rapid, and controllable is attractive to dry fruit production, especially when high throughput is needed.

Raisins are dried at temperatures not exceeding 165 degree F and at a low relative humidity. After the moisture content has been lowered to about 16 – 18%, the raisins are packaged in containers of various sizes to be sold at retail or for use by the food industry.
Mechanical drying of raisins

Tempering the eggs

Once milk/cream mixture have been flavored, it’s time to add the eggs. Tempering is the process of slowly adding a hot liquid to eggs in a recipe, in order to gradually raise the temperature thus slowly exposing eggs to heat without the danger of coagulation.

In this manner, the eggs do not curdle and produce an unacceptable consistency. If the eggs are not properly tempered the hot liquid will cook them, making final product uncomfortably reminiscent of scrambled eggs.

Overheating eggs in sauces, puddings and soft custards can be avoided by tempering the eggs. For sauces and puddings, cook the starch first - it needs extra time to thicken and lose its raw taste – then add the eggs.

Egg also can be tempered with hot liquid milk, is carefully streamed into them, bringing the eggs up to a very high temperature without cooking them.
Tempering the eggs

Use of High Temperatures for Food Preservation

Use of High Temperatures for Food Preservation
Preservation of food by the use of heat finds very wide applications compared to other methods.

Heat may be used either for processing or conversion of foods or simply as a means or preserving the food.

In heat processing or conversion the application of heat is used primarily to effect chemical changes in food.

Cooking, frying and baking involve both processing and preservation operations.

Cooking makes food palatable and tender and also destroys a large proportion of microorganisms and natural enzymes.

Cooked foods can be stored for several days provided they are protected from recontamination.

Refrigeration cooked food is a normal household practice to prolong the storage time.

However, cooking will not sterilize a product.

Cooking also destroys the toxin formed by Clostridium botulinum during a ten minute exposure of the food to moist heat at 100 degree C.

Thus cooking provides a final measure of protection for consumer form food borne diseases.

The killing of microorganisms by heat is due to thermal denaturation of protein and enzymes of the microorganism required for its metabolic activity and growth.

The heat treatment necessary to kill the organisms or spores varies with the kind of organism, its state and the environment during heating.

The type of heat treatment will depend on the kind of organism to be killed, other preservative methods to be employed and the effect of heat on the food.

The use of heat also affects the food adversely and hence it is necessary to use only mild heat treatment that ensures freedom from pathogens and enzyme activity and enhance the self life of the food.
Use of High Temperatures for Food Preservation

Food Preservation by Canning

Food Preservation by Canning
Using high temperatures to kill microorganisms in food is one of the principal processes implemented on the preservation of food. The canning industry uses this method most extensively.

Acidity greatly reduces the time and temperature necessary to preserve foods by heat, even though that may contain resistant spores. High acid food such as grapes, pineapples, oranges and tomatoes, for examples, require only a few minutes at 100 degree C to preserve them; foods with acidity, such as spinach, corn, carrots, and beans, require much longer periods at high temperature – depending on the solidity, size of the packages and the amount of preheating.

There are three primary thermal processing methods used today; the conventional retort, the hot-fill and the high temperature short time methods (HTST).

The conventional retort method includes filling the can with the food product, heating the can sufficiently to kill microorganisms, sealing and slowly cooling the can. Whether the slow cooling period damages the food, whether the food is overcooked, and the type of food to be preserved must all be considered when this method is chosen.

The hot fill method is used primarily for high acid foods. Then a comparatively low temperature is sufficient to reduce the microbial; population to an insignificant level due to the pH level of these foods. The food is simply heated near boiling in a method similar to pasteurization poured into a sterile can while hot, and sealed.

Another commonly used method in the heat processing of food is the HTST method. Here the food is poured into the can and exposed to a high temperature for a comparatively brief time. Sealing is accomplished at the maximum temperatures, followed by rapid cooling. This rapid cooling, usually by immersion in cold water, prevents physical damage and over cooking of the product. One major problem with this process is that any microscopic leaks in the can allow the partial vacuum caused by the contraction of cooling to draw any microorganisms in the cooling water into the can.
Food Preservation by Canning