Introduction
The action of performing a series of mechanical or chemical operations on food in order to change or preserve it.Food processing takes clean, harvested or slaughtered and butchered components and uses them to produce marketable food products.
Unless natural foods are processed and preserved, they will deteriorate.
Objectives of Food Processing
The main objectives of food processing are:1.Removal of unwanted matter from the food.
Unwanted matter may be inedible, indigestible or harmful to health. Unwanted matter is removed by appropriately designed gadgets. The processes may include shelling, destoning, milling, peeling etc.
2.Making food safe for consumption.
Some foods contain natural toxins which need to be inactivated. Fungal growth on food items, infected portion of food are removed by visual examination. Heat is used to destroy microorganisms and their toxins.
3. Increasing Digestibility
Most foods are difficult to digest unless they are cooked. Cooking softens fiber, gelatinizes starch, denatures protein and makes food easier to digest. With exceptions to fruits and some vegetables, all other foods need some kind of processing to make them more digestible.
4. Enhance flavor, colour and taste
The acceptability of food depends to a large extent on its sensory qualities. Processing techniques enhance the appearance of food and many techniques make food more appetizing and tasteful.
5. Improving texture and consistency
Processes such as emulsification, aeration, gel formation are aimed at improving the texture and consistency of ready-to-eat, cook-chill and cook-freeze operations.
6. Minimizing nutrient loss
Nutrients are better retained by controlled processing conditions. Nutrients lost during processing are generally compensated by adding synthetic vitamins.
7. Extending the shelf life
Processing extends the shelf life because apart from removing unwanted, spoilt and harmful matter and subjecting the food to temperatures outside the danger zone, all processes such as dehydration, cold storage, canning and pasteurization are aimed at preservation of food.
8. Increasing acceptability through fabricated foods
New products of uniform size and shape are being introduced in the market. They are made from low grade commodities which are plentiful or good for health.
So to summarize,
- Foods are processed for the following reasons:
- All the raw food materials are processed to improve their palatability, nutritional value and shelf-life.
- Preservation for later consumption or sale to fetch better price
- Removal of inedible portions
- Destruction or removal of harmful substances
- To boost the shelf life of food articles.
- To prevent contamination of food.
- For transportation and food storage.
- To turn food products into the ones that appeal to customers.
Food Spoilage and Contamination
Food can spoil at any stage in its preparation or storage. In fact, spoilage in food begins as soon as
Vegetables and fruits are harvested.Eggs are laid.
Fish is caught
Animals are slaughtered for meat.
Milk is drawn from milch animals.
This spoilage continues till food is consumed.
Food Contamination:
The term contaminated is used for those foods which are not fit to be eaten for sanitary reasons. Although contaminated food may look, smell and taste good, it may contain harmful chemicals, non-food matter and bacteria. Contamination of food results in its spoilage.
Food Spoilage:
It is the decomposition or damage causing undesirable changes in food. It is caused due to various agents making food unsuitable for consumption. Spoiled food looks, smells and tastes bad.
Causes of Food Spoilage
Food spoils because of any one or more of the following reasons.- Growth and activity of bacteria, yeast and mold – Micro-organisms can cause visible changes in the food.
- Insect infestation – Insects such as worms, weevils and moths infest cereal grains and make grains unfit for consumption.
- Enzymatic changes – Natural enzymatic changes also cause food spoilage.
- Chemical action – Rancidity.
- Physical Changes.
Methods of Food Preservation
Use of Low Temperatures
Low temperatures preserve food by retarding chemical reactions, enzymatic action, growth and activity of microorganisms. The lower the temperature , the better the food will be preserved. Microorganisms are not killed at low temperatures.The various food preservation techniques using low temperature are as follows:
- Refrigeration / Chilling
- Cook – Chill
- Freezing
- Cook – Freeze
- Vacuum Packing
Refrigeration / Chilling
Temperature of 1 to 40C prevents food from spoiling for short periods.Frozen foods kept at -180C preserves food for a year.
Frozen food kept at -280C preserves food for a two years.
Chilling temperatures retard microbial growth and biochemical changes which affect colour, texture, flavor and nutritive value.
Food can be kept at chilling temperatures for a limited period and have to consumed by the ‘best before date’.
Relative humidity needs to be controlled when fruits and vegetables are stored in refrigerators. Low RH means loss of moisture and shrinkage in them and high RH favours surface spoilage by microorganisms.
Cool - Chill
In this system, food is cooked in the kitchen in advance and rapidly chilled and stored at 0 – 30C.
It is reheated just before it is served.
The cook – chill system is used for almost all kinds of foods and is popular in airlines and institutional catering.
Proper cooking procedures and hygienic standards need to be followed to ensure safety and quality.
Precautions for Cook – Chill Foods
Food should be properly cooked to destroy the microorganisms and the cooking process should be complete.
Cooked food should be portioned and chilled in a blast chiller unit within 30 minutes of cooking.
Chilling temperature of 30C must be reached within 11/2 hours after cooking.
Chilled storage temperature should be between 00C to 30C
While distributing cook-chill foods, temperature should be as low as possible.
During reheating the temperature must reach 700C.
Food should be reheated just before it is consumed and must be consumed after 2 hours of reheating.
If the temperature exceeds 100C during storage or distribution the food must be discarded.
Recipes should be suitably modified.
The storage life of cook-chill meals should not exceed 5 days.
Freezing
Food is preserved for long periods by reducing the temperature to -180C or lower.
At this temperature, water gets converted into ice and microbial growth stops.Freezing retains colour, flavor and nutritive value.
If properly stored, frozen food has a self life of 3 – 12 months.
Food is quickly frozen by using any one of the following equipments.
Blast Freezers – In this cold air at -180C to -340C is vigorously circulated over food while it passes through an insulated tunnel.
Plate Freezers – Food to be frozen is placed in contact with a metal surface which is cooled by a refrigerant. It is used for ice creams, juices, etc., both packaged and unpackaged foods.
Immersion freezers – packaged or unpackaged food is frozen by immersing it or spraying it with a freezing agent. It is used for freezing poultry.
Spray freezers – This is the quickest freezing method in which liquid nitrogen or carbon dioxide is used. It is called cryogenic freezing. The food to be frozen is placed on conveyor belt which passes through an insulated freezing tunnel. Liquid nitrogen or carbon dioxide is injected into the tunnel through a spray which changes its state and vaporizes, resulting in instant freezing.
Freeze flow – In this system, food freezes but does not harden. A wide variety of frozen foods, both cooked and uncooked are available in the market. A cryoprotectant is used. Frozen foods once thawed should not be refrozen as when it is thawed its temperature is in the danger zone where bacteria present in the food multiply rapidly.
Cook – Freeze – It is a specialized method of a food processing system in which food is cooked and immediately blast frozen at -18oC or below and stored at this temperature till it has to be served. It has a shelf life of 3 – 6 months.
Vacuum Cooking
Vacuum cooking is a form of cook – chill using a combination of processes. The food is first sealed in a plastic pouch. It is then cooked by steam and quickly chilled in ice water.
Although this system has many advantages, its main disadvantages are uneven cooking if size of cut food varies, longer cooking time, and extra cost of vacuum pouches and packing equipments.Vacuum packed food does not get oxidized and once packed.
Packs must be stored at appropriate storage temperature, well labelled and FIFO must be followed strictly.
Flow of Freezing Process for Fruit Based Products
Flow of Freezing Process for Veg. Based Products
Use of High Temperatures
High temperatures destroy microorganisms by denaturation of cell proteins and inactivation of enzymes needed by them for their metabolism. At temperatures above 630C, bacteria stop multiplying and as the temperature increases, they are gradually destroyed.The Thermal Death Time (TDT) is the time needed at a given temperature to kill a number of microbes. The heat used to destroy microbes maybe in the form of wet heat or dry heat.
Uses of High Temperatures
WET HEAT
This is more commonly used in the food industry. If carefully administered, it is a useful method of controlling microorganisms. The food processing techniques using wet heat are as follows:
1.Blanching: Foods which are to be frozen, dried or canned are immersed in hot boiling water for a few minutes prior to processing. Balancing helps in removal of peel, inactivation of enzymes that oxidize vitamin C, removal of gas from tissue spaces and wilting of tissues. The enzymes which bring about discoloration or browning seen in apples, pears and potatoes are also inactivated.
2. Pasteurization – This method is used to control microorganisms in milk, fruit juices and wines. Food may be pasteurized by any one of the three methods:
Low temperature holding (LTH) method at 620C for 30 minutes
High temperature short time (HTST) or flash method at 720C for 15 seconds.
Ultra high temperature sterilization (UHTS) at temperatures above 1350C for 2 seconds. This method makes food commercially sterile. Such foods are packed under aseptic conditions and can be stored at room temperature for three to six months.
3. Canning - In this process, temperatures used are above 1000C. All microorganisms that could spoil food under normal conditions of storage are destroyed by heating the food in an autoclave at temperatures between 1150C and 1250C.
- The exact temperature and time required for canning depends on the type of food to be canned.
- Acidic foods such as fruits are heated to 1000C only because acid also helps in preventing microbial growth.
- A vacuum is created inside the can or the air in the headspace may be replaced by nitrogen gas to prevent growth of aerobic bacteria.
- This is the most common method of food processing . It is used to preserve fruits, vegetables, fish, meat, poultry etc.
- In this process no preservatives are added to the food which is sealed in air tight containers. These are then heated to sterilize the food.
Canning Process
The following basic steps need to be followed while canning or bottling food:- Cleaning and Preparing – The food to be canned is cleaned thoroughly and prepared for canning.
- Filling – Raw prepared food is filled into cans or bottles either mechanically or by hand. Filling should be carefully controlled to ensure that the headspace is neither too little nor too large.
- Exhausting – A partial vacuum is created in the can by removing part of the air. As oxygen is reduced, bacterial spoilage is retarded. Exhausting prevents the ends of the can from bulging during heating.
- Sealing – Cans and bottles are sealed with airtight lids by sealing machines.
- Processing - The sealed containers are heated at a controlled temperature for a specified length of time. The time and temperature depends on the food being processed and the size of the container.
- Cooling – As soon as processing is over, cans are cooled immediately to stop further cooking. Cans are cooled by dipping them in cold water or even air cooling. He cans are then labelled and packed in cartoons to be marketed.
Advantages of Wet Heating
- Convenience
- Long Shelf life
- Needs little preparation
- No chemical preservatives
- Variety of food is available.
Disadvantages of Wet Heating
- Heat labile nutrients are lost.
- Heat required for processing affects the texture, colour and flavor of the product.
- Cost of canned food is high in India.
- Once opened, canned food should be treated like fresh food and consumed within stipulated period.
Cooking (boiling, steaming, stewing and poaching)
In these methods of cooking, wet or moist heat is used. The temperature attained is 1000C.At this temperature, most microorganism are destroyed but spores survive. Foods cooked by these methods cannot be stored for long periods under normal conditions.
Methods of Food preservation
DRY HEAT is used in the following methods.Sun drying, Smoking and Freeze drying – In these methods, dry heat is used to control microorganisms. Dry heat brings about dehydration of the foods or the surface of the food. It destroys molds, yeast and most bacteria and spores.
Cooking (baking, roasting, grilling) – In these methods of cooking, food is cooked by dry heat. The temperature reached on the surface is approx. 1150C. Most bacteria are destroyed. Internal temperature of food is generally lower.
If food is to be kept for sometime, it should be cooked
thoroughly. Foods cooked by dry heat methods do not spoil as fast
as moist heat methods as they have a lower moisture content.
Removal of moisture from food
Microorganisms need moisture for their growth. If foods are dried or dehydrated (removal of moisture), they will not be spoilt by bacteria, yeast or molds.
Moisture can be removed by sun drying, mechanical dryers and freeze drying.
Sun Drying
It is used for certain fruits and vegetables. The dried items should be turned during drying. Light coloured fruits are sulphured to prevent enzymatic browning. It can be used in hot dry climates only.
Use of mechanical dryers
The food to be dried is passed on conveyor belts through hot air with controlled relative humidity, or hot air is passed through the food. Liquid food such as milk is dried by passing it over heated rollers. Dried food should b packed in airtight containers immediately.
Freeze Drying
In this process, the food to be dried is first frozen in a cabinet. A vacuum is created and by the process of sublimation the ice turns into vapours.
Freeze dried food needs no further refrigeration or preservation. It is light in weight and retains its size and shape , and when soaked in water regains its original size and flavor, It is used to dry fruits, vegetables, meat, poultry and seafood.
Advantages of Drying
Dried food is easy to transport and store.
It has a long shelf life provided it remains dry.
It occupies less storage space.
It is a cheap method of preservation and easy to use.
Effect of Heat, acid and Alkali on Food Constituents
HeatHigh temperatures destroy harmful micro-organisms due to coagulation of cellular proteins and inactivation of enzymes required
For the metabolism of these organisms. Heat also causes dehydration which makes water unavailable for microorganisms to survive in.
Acids (Acetic Acid, Citric Acid)
Acids reduce the ph. level of foods below the growth range and also Inhibit metabolic activity.
Effect of Heat, acid and Alkali on Food Constituents
Alkali
It causes high osmotic pressure causing plasmolysis (breakdown of cell walls) of cells. It dehydrates food by drawing out water and dehydrates microbial Cells.
It ionizes to yield chlorine which is harmful to microorganisms. It reduces solubility of oxygen in the moisture .
It interferes with the activity of proteolysis enzymes
