Lesson 08: Preservation by Use of HeatPreservation by Use of Heat
Canning is the process of applying heat to food that’s sealed hermetically in a jar to destroy any microorganism that can cause food spoilage.
The food preservation process of canning originated in 1809 when French confectioner Nicolas Appert succeeded in preserving meats in glass bottles that had been kept in boiling water for varying periods of time. In the honour of inventor, canning is also known as “appertization”. Canning demonstrates that food can be preserved for quite a longer duration of time when heated and stored in anaerobic condition. Today, the method of canning is one of the most widely used methods for food preservation.
In canning the food is placed in containers, heated, and then sealed, usually under vacuum. It is used for products such as fruit juices, syrups, and sauces. Canning process is advantageous in retaining the stable vitamins and colour and flavour of food items.
Unlike pasteurization, canning of foods normally involves exposure for longer periods of time to higher temperatures created by steam under pressure in order to kill endospore-forming microorganisms. Steam under pressure (e.g. a pressure cooker) is the most effective method since it kills all vegetative cells and spores. The heat treatment generally exceeds 100°C temperature and the food is heated long enough to inactivate the most heat-resistant pathogens (disease-causing organisms) and spoilage organisms. Heating to such high temperatures is achieved by steam injection, which is followed by rapid cooling. Factors affecting the length of time the food must be heated include the kind and number of microorganisms present, acidity of foods, presence of preservatives (salt, sugar). The only dangerous spore forming bacterium which survives the treatment is Clostridium botulinum
The whole process of canning includes preparation of food, filling, exhausting, sealing, thermal processing i.e. autoclaving and cooling .
Preparation of food: Preparation of food involves processes like washing, grading, peeling, sub sizing and blanching. High pressure water sprays are most effective means of washing food items. Grading is done by passing food products through screens of various diameter sizes. Visual inspection based on weight, shape and colour may also be performed for grading. Fresh vegetables, fruits or meat of high quality are selected and made free of all dirt and undesirable parts in order to reduce bacterial load and to preserve the best portion of the product. The cleaned food is subjected to hot water or steam blanching. This treatment inactivates enzymes, removes respiratory gases, promotes shrinking, fixes the natural colour of the product, and pre-heats the product to help in vacuum formation in the can.
Filling of cans: The hot food is then filled into pre-heated cans. Cans are also kept hot in order to expand the food, remove air or gas trapped within. Containers are not completely filled with food. Headspace of not more than 10 per cent of the total container volume is maintained for expansion of food during heating. After filling, syruping or brining is done usually in case of fruits and vegetables, respectively.
Exhausting: Exhausting is removal of air from the cans before they are closed. It reduces the pressure strain on can seams and internal corrosion and also conserves quality of product.
Sealing: Cans are sealed immediately after exhausting with the help of a double seam can sealer. Precaution must be taken that the temperature of food should not fall below 74°C while sealing.
Thermal processing: After sealing cans are subjected to heat treatment in the form of steam under pressure in canning retorts or pressure canner to effect sterilization of contents. Heating medium can be boiling water or steam depending on the pH of the food.
Foods with acidic pH i.e. pH below 4.5, may require only immersion of cans in boiling water. Non-acid foods, i.e. pH above 4.5, however, need to be subjected to 15 psi pressure (116°C-121°C) for one to two hours. Meats may take a longer time. The amount of time will also depend upon the load of the retort and size of the cans. Heavily loaded autoclaves will take more time. Larger the can greater is the processing time. The retorts can be batch or continuous type. Fruits and acid vegetables are processed in open type cookers whereas low acid vegetables are processed in closed retorts under steam.
Cooling: After processing, the cans are cooled immediately to 39°C to stop cooking process and to prevent over heating/ burning. In addition, thermophilic bacteria surviving heat treatment will not be allowed to germinate. Usually cans are dipped in cold water.
Storage: After labeling, cans are kept in wooden boxes or corrugated carboard cartons and stored in cool and dry place.
Aseptic canning: Aseptic canning is a technological advancement of canning process. This technique was developed by Martin in 1950. In this method, sterilized food is aseptically filled in pre-sterilized cans which are subsequently sealed in an aseptic environment. This method avoids the typical steps like in-can thermal processing and subsequent cooling of canning. Flash pasteurization is used to sterilize the food. It involves four steps which are carried out in a sequential manner in a closed interconnected apparatus: (a) sterilization of food by appropriate flash heating, holding and cooling, (b) sterilization of cans and lids with super heated steam, (c) aseptic filling of cooled sterile food into sterile cans, and (d) aseptic sealing of cans with lids. For example- Tetra Pak system . Aseptic canning yields better quality of product in terms of colour, flavour, texture and nutrient retention and shelf life.
Hot pack: It is also known as hot fill. This method involves filling of pre-pasteurized or pre-sterilized food while still hot in clean containers but not essentially sterile containers under clean but not necessarily aseptic conditions. For example- filling of jam while still hot in clean bottles.