Process of blanching

Blanching is a thermal process most often associated with solid food commodities such as fruits and vegetables. It is defined as a heat treatment given to plant material for inactivating enzymes and killing plant tissues to prevent enzymatic and microbial deterioration.

The aim of the process is to- prevent the deterioration of products which usually starts at the time of harvest. Enzymatic activity, which continues after harvesting, breaks down the plant tissue, and it is therefore important to inactivate any enzymes present.

Most of the quality attributives are less sensitive to thermal treatments than the enzymes systems within the product. This would suggest that blanching processes at higher temperatures and shorter time would result in increased retention of the quality attributes.

Other advantage of blanching especially for vegetables canning mainly to shrink the vegetable to obtain the correct fill weight in the can and to remove the gases from the intercellular spaces which would otherwise cause oxidation of the product, corrosion of the can and low can vacuum.

Blanching is required prior to dehydration of many commodities, since temperature associated with dehydration are insufficient to inactivate enzymes with the product and enzyme activity is not controlled by reduced moisture content.
Process of blanching

Tubular heat exchangers

The simplest heat exchanger is the double pipe type which consists of two centric pipes with the cold stream flowing usually in the inner tube and the heating medium in the annulus. It operates in either co-current or counter-current flow.

The concentric tube heat exchanger is the most common type of the tubular heat exchanger. The hot liquid, or steam, flows through the outer jacket and the cold liquid flows through the inner tube.

The shell and tube heat exchangers are less expensive than other types of food heat exchangers especially when high loads are transferred. They can be operating at higher temperature and pressures (e.g. steam at 6 bar and 160 ° C) than plate heat exchangers. Shell and tube exchangers are tubular consisting of bundles of parallel tubes inside a larger cylindrical jacket (shell).

In a type of tubular exchanger known as a Joule effect heater, the tube wall is electrically heated. The length of most commercial tubular system has been standardized at six meters; therefore, tubular heat exchangers are long and thin in terms of their geometry.

Most tabular heat exchanger now use corrugations on the shell and tubes to enhance heat transfer with the heating and cooling media, typically water. Tubular heat exchangers are suitable for heating or cooling highly viscous products and where relatively high pressure must be applied.

They are therefore utilized for the bulk in-flow sterilization of products containing solid particles or for the heat treatment of cooling of tomato paste prior to aseptic packaging.

For fruit juices with fibers of up to 15 mm length and for relatively water-like foods, a multitube tubular heat exchanger is preferably used. Also fluid of moderate to high viscosity with only small particulates will flow through a multitube heat exchanger without problem.
Tubular heat exchangers

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

Factors Affecting Time of Thermal Process

Factors Affecting Time of Thermal Process
The thermal process time necessary for preservation of a chosen food depends on the rate of heat penetrating into a food.

Since every part of the food in a container or can must receive the same adequate heat treatment to prevent its spoilage, the process time work depended on the part which receives heat most slowly.

The food part near the center of the container usually receives heat most slowly. Heat penetration in food is mainly due to conduction as well as convection, which may operate simultaneously or successively.

Conduction occurs in solid particles of foods and is slow while heat transfer by convection occurs in liquids.

Some foods such as sugar syrup, thick soup and tomato juice undergo changes in their consistency during heating.

A variety of factors influence the time required for thermal processing for food. These include:

1. The material of the container – glass having a slower rate compared to that of a metal.
   
2. The size and shape of the container – smaller and slim cylindrical cans attain the desired temperature faster
   
3. Initial temperature of the food - a higher initial temperature provides the lethal range for the microorganisms for a longer time and its average temperature during heating is higher than the food in the can with a lower initial temperature. A higher temperature is necessary for foods which heat slowly, e.g. meat, pumpkin etc.
   
4. Retort temperature a higher retort temperature heats the food rapidly.
   
5. Consistency of the food – nature, sizes and shapes pieces decide the rate of heat penetration.
   
   The pieces of food may retain the identity as in peas, plums, beets, asparagus and whole grain corn with smaller pieces requiring less time.
   
   Pieces may become mushy or change into a viscous mass as in the case of pumpkin, sweet potatoes, squash and cream-style corn, which get heated slowly as conduction is the main mechanism of heat transfer.
   
   Pieces may form layers as in the case of spinach and asparagus. In addition, the consistency of the foods depends on the different types of sauce starch salt or sugar that may be present.
   
   For example tomato sauce on baked beans slows down heat penetration more compared to plain sauce. Starch concentrations up to 6% decreases heat penetration, but at higher concentration, has no additional effect.
   
   Sodium chloride does not influence heat penetration, as its concentration is usually low. On the other hand increasing of sugar decreases heat penetration. However, this effect is counteracted by the decrease in viscosity of sugar solutions with increasing temperature.
   
6. Rotation and agitation of the container during heat processing hasten heat penetration, particularly if the food is a fluid, but may cause undesirable physical damage to some solid foods.

Cooling of the canned foods also is influenced by the same factors as the heating process. Rapid, artificial cooling is usually adopted to effect a greater control on the cooling rate, because slow cooling may cause overcooking of the food and also allow the growth of thermophilic organisms.
Factors Affecting Time of Thermal Process