Curing: Preservation process of food

Postharvest technology is concerned with handling, preservation, and storage of harvested foods, and maintaining its original integrity, freshness, and quality. The methods of preservation depend on the origin of foods—particularly whether they are of plant or animal origin.

The basic concept in curing of foods like meat, fishes and vegetables is to reduce the moisture contents by osmosis process. When moisture contents in any food are much low, there are much lesser chances of getting microbial infection and subsequently growth.

Curing is also done for flavoring. It is done by adding salt, nitrates, sugar, nitrites in combinations which are capable of dehydrating the food. Higher salt used for curing also dehydrates bacteria resulting in their killing.

For crops, curing is a postharvest healing process of the outer tissues of root crops by the development of a wound periderm by application of heat.

The purposes of curing are
*to heal wounds of tubers and bulbs sustained during harvesting,
*to strengthen the skin,
*to dry superficial leaves, such as onion bulbs to prevent microbial infection during storage and distribution,
*to develop desired skin color (onion),
*to reduce water loss during storage in potatoes, sweet potatoes, cassavas, yams, onions, and garlic.

In term of meat process, curing is the treatment of meat with preservative chemicals that restrict or prevent the growth of spoilage bacteria and food poisoning bacteria. It is used together with processes that use heat, smoke or low temperatures to give the required shelf life of cured meats. Meat can be defined as the edible flesh of a number of species of mammal or bird, both wild and domesticated.

The cure ingredients can be rubbed on to the food surface, mixed into foods dry (dry curing), or dissolved in water (brine, wet, or pickle curing). In the latter processes, the food is submerged in the brine until completely covered. With large cuts of meat, brine may also be injected into the muscle.
Curing: Preservation process of food

Glass Containers

 As a container material, glass has several desirable properties. It permits visibility of the product, imparts no taste and is non corrosive. The use of glass containers precedes by centuries the use of metal cans and of course, flexible pouches. The advances in closures of glass containers probably more dramatic than the advances in the design of the containers and in fact, have forced the changes in container design.

Cork closures have been perhaps the most important. Cork is light, compressible, hermetically, inexpensive, plentiful and stable. Corks have long been in use for closing glass bottles, and still are. Ordinary cork closures (stoppers), however, are unable to withstand the buildup of internal pressure, except where the pressure is not too great and when they are used with bottle having small openings, such as in wine and champagne bottles.

The diameters of the openings of glass jars used for preserving fruit and vegetables are so large that a fitted cork would be easily pushed out. Even for small openings, it was necessary to develop holdown devices, such as clamps and wire fasteners, when high pressures were expected, as in the bottling of beer. The crimped metal crown with cork liner was an effective closure for beer and soda bottles. However, aluminum screw-caps are now used in nearly all cases.
Glass Containers

Green pigments in green vegetables

Green vegetables contain the green pigment chlorophyll which plays a key role in transferring light energy to chemical energy during growth and development of the plant by the prices of photosynthesis. Examples of such green vegetables include spinach, peas, beans, cabbage lettuce and celery.

Although chlorophyll does not appear to be useful in human physiology, foods that are high in chlorophyl are usually also high in B-carotene.

There are two types of chlorophyll and each produces a specific colour in green plants. Chlorophyll A is blue-green, whereas the more common chlorophyl B is green. Their chemical compounds differ in colour because of their different structures - chlorophyl a has a methyl group (-CH2) attached to one of the carbons on the chlorophyl molecule, whereas chlorophyl B has an aldehyde group (-CHO). Destruction of chlorophyl increases with cooking time.

Chlorophyll A is more readily converted to pheophytin than chlorophyll B. Green vegetables should be cooked for a very short time, less than minuets to retain the colour. Vegetables like peas, beans, greens are sometimes canned.

During canning chlorophyl gets converted to pheophytin due to the high temperatures used. Sometimes for stain the colour and to neutralise the acid, alkali is added.
Green pigments in green vegetables

Vegetable processing

It is necessary to process green vegetables to preserve them for a year-round food source.  The most common commercial method of preservation is canning.  Canning is regarded to be a universal and economical method in food processing.

The principles of canning were first enunciated by Nicholas Appert in 1810, after many years of experiment and are essentially unchanged. For this process, the vegetables are cleaned, grading, heat blanching, peeling and coring, cut, packed into cans, sealed and heated to sufficiently high temperature (in the order of 240 ° F) to destroy spoilage and disease causing microorganisms. It follows by cooling, labeling and packing.

It is important for the food processor to control pH of the water added to the vegetable prior to canning. The vegetable may be canned whole, diced, pureed as juice and so on.

Vegetables are usually packed in liquid. Canner’s brine, a weak solution of sugar and salt, is ordinarily used for vegetables.
Vegetable processing

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

Tomato paste processing

Tomato paste is usually considered as semifinished product because it is commonly used as an ingredient in other food products. Tomato paste and puree are important ingredients for pickles and sauces.

Its primary use is in tomato ketchup but it is also used in salsas, pizza and pasta sauce, sweet pickles sauce, tomato chutney sauces and other thick sauce. Functionally it provides flavor, color and consistency. 

Tomato Paste Processing 

Tomatoes are washed, trimmed and then heated to a ‘breaking’ temperature.

During processing, seeds are removed at the pulping and screening stages to improve flavor and color.

The tomatoes are chopped at room temperature and immediately, or within seconds of crushing, heated to a temperature range of 60 to 90° C.

To refine the juice, broken and pre-heated tomato pulps are screened by pumping to a series of extractors or cyclones using an initial screen with a sieve diameter of about 1 mm followed by a second screen with a sieve diameter of between 0.4 and 0.7 mm.

Tomato paste is made from tomato juice by evaporation to a predetermined percentage of soluble solid usually in a range of 20-37%.

During this processing step, tomato pulp is additionally exposed to elevated temperatures of 50-95% and a number of chemical reactions largely of non enzymatic hydrolytic and antioxidative origin, continue to affect its original composition.

Once the tomatoes have been turned into paste, it can be stored for up to 18 months. The manufacturer then sells the tomato paste to other companies that add spices, flavors and colors to turn the industrial tomato paste into products like ketchup or tomato sauce.
Tomato paste processing