Use Of High Temperature(part1)
Use Of Lo High Temperature (part1)
Heat energy is widely used for food processing and preservation. In cooking, it increases the digestibility of some nutrients, while in heat processing, it is aimed at destroying pathogenic and spoilage microorganisms. Various techniques and processes in which heat energy
is applied, in the form of high temperature nclude cooking, blanching. pasteurization and sterilization.
1. Cooking :
Cooking, as applied broadly, includes heating food in order to alter its colour, flavour and texture. It more importantly improves the digestibility of the food components. During this process, enzymes and microorganisms are also destroyed. Foods cooked at home do not keep for long time since these are always left in utensils that are not hermetically sealed and carinot prevent recontamination. Cooked foods
have a comparatively longer shelf life than the uncooked ones.
Cooking embraces different forms of heating methods such as baking, boiling, broiling, roasting, frying, stewing and steaming (Table 4.1). Broadly speaking, it may be done by direct exposure of food to a source of heat (dry cooking - grilling, roasting). Food may also be
cooked in a suitable liquid medium such as water, steam or cooking fats. In boiling, stewing and steaming, the process is carried out at boiling temperature of water or higher if pressure cooker is employed. Baking. broiling and roasting employ dry heat, while in. frying, the process is conducted in fat or oil and such as temperatures much higher than 100C are attended.
2. Blanching :
Blanching is commonly applied to plant materials to destroy some enzyme system prior to canning, cold storage, freezing or dehydration. Heat energy is applied through the medium of water or steam.
3. Pasteurization :
Pasteurization is a heat treatment designed primarily to kill veyeiaiive iorms of microbial cells in liquid foods. The object of this treatment varies from food to food, but invariably all palthogenic and non- sporing organisms are eliminated, e.g., in milk the object is to destroy
pathogenic bacteria, especially Mycobacterium tuberculosis which is responsible for tuberculosis in man. The pasteurization temperature varies with the type of food, and the length of time i! is to be exposed to that particular temperature. The heat resistance of the spoilage or pathogenic microorganisms required to be killed in the process als0
governs it. Usually pasteurized foods, which show only reasonable extended shelf life, are supplemented by other methods of food preservation such as storage at low temperature or sealing in anaerobic environment. Two types of pasteurizers, continuous and discontinuous or batch type are in use.
1. Continuous pasteurizers :
Continuous pasteurizers consist of a single metal tube or series of hollow, jacketed plates or small metal tubes through which the liquid food flows. It is heated to the desired temperature by steam or hot water that passes through a jacket. Plate pasteurizers are generally used in
preference to the tubular type, which can only be cleaned in-line. The food may be heated in these pasteurizers at low temperature for long- time or high temperature for a short time, hence the names low- temperature-long-time (LTLT) and high-temperature-short-time (HTST).
In the LTLT process, usually a temperature between 65 to 75°C is employed. Most fruit juices are held for about 20 minutes. In the HTST process, comparatively higher temperatures are used and food is held for a very short time. Milk, for example, is pasteurized at 72°C for 15 seconds and fruit juices at 82 to 91°C for a few seconds. In the ultra high temperature (UHT) process (also called flash pasteurization), the fruit juice may momentarily be subjected to 116°C and cooled to 88°C before filling, Higher temperatures for short periods, while being effective in destroying microorganisms, are less efficient in bringing about adverse changes in food components. In the LILT process, greater damage to food constituents is likely to occur than in the HTST process.
The food may also be pasteurized after being filled into bottles or cans. Normally this is a continuous process and is done mostly in hot
water bath or an atmosphere of live steam.
2. Discontinuous pasteurizers :
The discontinuous pasteurizers consist of steam jacketed kettle or tank equipped with steam coils in which food (juice, milk) may be
placed and heated to the desired temperature for the required length of time. The disadvantages in this type are that local overheating of small portions may occur. Moreover, food is exposed to oxidation during the process. Prolonged heating causes injury to colour, flavour and nutrients of the product.
4. Canning :
Canning is the method of food preservation in which food contained in a permanently sealed container is subjected to an elevated temperature for a definite period of time and then cooled. The spoilage organisms present are destroyed by heåt and the hermetic seal of the container prevents fresh contamination. All microorganisms that might
have survived the rigours of processing are unable to grow because of unfavourable conditions prevailing inside the container. In special canning techniques, heating of food is done before filling it into pre- sterilized containers, The traditional method of canning involves several distinct unit operations.
1. Preparation Of Raw Materials :
All raw materials require some sort of preparation to make them fit for human consumption. The food processor ensures that these are properly handled, sorted, and inedible parts removed through peeling, pitting, coring, stemming, trimming, shelling, dressing, etc. The size is reduced to suit the consumer demand, market requirements and heat processing parameters.
Fruits and vegetables are washed, normally peeled (leafy vegetables stemmed), and seeds and other inedible parts removed.
Some fruits may be halved (guava, pears); others are sliced or diced (mango, banana, pineapple). Animals are slaughtered, bled, skinned and carcass cut into small pieces. Birds are slaughtered, defeathered and evis cerated., Fish is eviscerated and cut into small pieces (tuna) or packed whole (sardines).
Some fruits and most vegetables are blanched to inactivate food enzyme systems (see Chapter 2). These may also be treated with
sulphites or other chemicals. No such treatment is required for beef or mutton. The birds (chicken, duck) may be passed over a flame to singe off the tiny hairs that remain after removal of feathers.
2. Filling :
The cans or glass jars are normally washed by subjecting them to steam jets prior to filling. This process ensures removal of adhering
contaminants.
Filling is done manually or by automated machines. At home, all products are filled into the containers manually, while the industry employs both manual and automated filling techniques depending upon the raw material and grade of the product. Fancy grades are still filled manually, while automated machines are employed for most fruits and
small vegetables. Liquid products (milk, carbonated beverages, fruit juices, squashes) are invariably filled by automated machines in the industry.
3. Syruping And Bringing :
The spaces in between the pieces of food are filled by a suitable liquid to facilitate further processing. In case of fruits, sugar syrup is
used, while in vegetables and meat, brine is added. Fish is usually canned in brine and oil or tomato sauce. Apart from facilitating heat transfer during processing, this liquid improves the taste of the canned product and fills up the intér-spaces between food pieces, driving the air out. It can also be used as a carrier of additives such as colour and flavour.
The syrup or brine is filled in the containers at a temperature of 79 to 82°C, because the hot liquid helps to drive out some of the entrapped air. Suitable headspace, 0.3 to 0.5 cm (1/8 to 3/16 inch) is left in the can or jar so that when the closure is finally. accomplished, this space will help in further processing (create vacuum and reduce strain
on the container during heat processing).. The syrup or brine is usually filled into the cans or jars by automated machines, while it can be done manually at home.
Syrup for use in fruit canning is prepared by dissolving sugar in hot water. Unrefined sugar should be avoided, since it contains sulphur that forms hydrogen sulphide and results in a black deposit of metallic sulphide in the cans, This also hastens corrosion of the cans. Concentrations of sugar syrup that vary from product to product and
grade to grade, usually range from 20 to 40%-higher concentration is used for expensive grades and lower for cheaper grades of the product. Sometimes in canning of fruit pieces, e.g., pineapple slices or mango cheeks, the processor may prefer to add some fruit juice (obtained as a by-product) to reduce the cost of sugar and to add extra natural flavour to the product.
Concentration of sugar in the syrup is measured using different instruments and scales. Brix and Balling hydrometers give percent sugar in syrup. While Brix scale is calibrated to read percent by weight of sucrose in a solution at 20°C, Balling measures the same at 17.5°C. The Baume hydrometer has a scale ranging from 0 to 70 degrees. The relation between Brix and Baume scales and specific gravity is given in Table 4.2. Different types of refractometers are also in use that employ the principle of refraction to measure sugar concentration in solutions. The Abbe refractometer is the most widely used in canneries. The hand refractometers give sugar concentration at room temperature, while table
models can be adjusted to any desired temperature.
Relationship between brix, baume and specific gravity of sugar syrups.
In canning of vegetables, brine or common salt solution is added. The salt should be at least 99 % pure NaCl. Iron compounds in the salt cause discolouration of brine and result In blackening of the product.
Calcium salts give a white precipitate. The brine is tested by a hydrometer commonly called Salometer. A brine solution containıing 26 % salt will gives a Salometer reading of 100 units. The concentration of common salt varies from 1 to 3% depending upon the product. In some products such as corn and peas, the brine may contain a small amount of sugar, usually between 3 to 10%. Sugar improves the taste by its meliowing action on salt.
4. Lidding And Clinching :
After filling syrup or brine into the can, the closing end or lid is placed on the open end of the can without sealing it to rest of the can body. This process is known as lidding (Fig. 4.3). However, during the subsequent exhausting operation, there is a danger that the lid may fall
off and can contents spilled over. To avoid this, some canners prefer to clinch the lid to the can body. In this operation the curl of the can end is
engaged with the flange of the can (Fig 4.3) by action of the first set of rollers in a double seamer.
5. Exhausting And Vacuuming :
The can contents contain air in the spaces between food
particles and the tissues of the product. The air in the can causes corrosion of the tin plate; aids oxidation processes in food and encourages germination of bacterial spores that survive commercial sterilization. It is, therefore, undesirable to leave air, When air is removed
either by exhausting or by mechanical means, a vacuum is created inside the can and the ends remain concave, This gives the consumer an
indication of soundness of the contents since production of gas and bulging of the can ends accompany some kinds of spoilages of heated can ned foods. Vacuum also prevents undue strain on the can during subsequent heat processing.
Different equipment are used for exhausting. A simple exhaust box consists of a narrow, shallow, rectangular metal box though which passes a steel cable that carries the cans (Fig 4.2). This type is known as the cable exhaust box, while a modification of this has the cable replaced by a chain conveyor. The speed of the cable or chaino is regulated depending upon the size of the container and type of the
product. The cans are carried through an atmosphere of live steam that is allowed into the exhaust box through openings at various points in the equipment. Another type is the disc exhaust box that consists of rectangular metal box in which several rows of large metal discs are fitted. Curved iron rods above the discs quide the cans through the equipment that travel down one row of discs and back the next. The number of rows and the length of exhaust box vary according to the capacity desired and the nature of product to be exhausted.
A simple exhaust box with can sealing machine.
The usual exhausting temperature for fruits is 82 to 96°C inside the exhaust box. When the can- centre temperature reaches 77 to 82°C.
cans would have been exhausted. The length of time depends upon type of product and type and size of the container.
In many modern canneries, steam flow closure or vacuum closing machines have replaced the exhaust box. Both these help to
create vacuum in the can.
At home, two different arrangements can be made to remove air from the food. The prepared material (apple slices or mango pieces) is placed in a vessel and syrup added. This is heated to the desired temperature (82 to 96°C) and filled hot in the cans or jars. These are
then sealed immediately. The steam that appears at the surface leaves a vacuum after condensing. In the second arrangement, food is placed in the container and syrup or brine is added (headspace must be left). The containers are placed in water bath, the level of water being about 2.5 çm below the top of the jar or.can. The water is heated to the desired temperature for specific time and the containers are sealed airtight. Only acid and high acid foods should be cạnned or bottled at home and processing of low and medium acid foods be left to the experts.
6. Sealing :
As soon as cans emerge out of the exhaust box, these are hermetically sealed with the help of a can-sealing 'machine, called the double seamer. A hermetic seal consists of a double seam, In the first seaming operation, the curl of the can end is engaged with the flange of the can. A good first operation seam has the body hook approximately parallel to the cover hook. The second operation is designed to compress and smooth out the first operation. During both operations, considerable pressure is exerted on the can end, the can body and the sealing compound. This pressure forces the sealing compound into the voids of the seam. This forms a strong mechanical structure that is air-
and water-tight. When the products are packed in glass container, the cap may be screwed on. Some jars are sealed under vacuum-the vacuum inside the jar retains the cap in such cases.
7. Heat Processing :
Heat processing, as applied in canning, means heating food at a
certain temperature for sufficient length cf time to render the contents
stable against pathogenic and spoilage microorganisms. In food
processing industry moist heat is employed for the destruction of
microorganisms. However, absolute sterilization is difficult to achieve
with foods since the temperature/time conditions which bring about this,
grossly damage food texture and nutrients.
8. Steps In The Formation Of A Double Steam :
The food processor aims at maintaining a balance at which- maximum numbers of microorganisms are destroyed and minimum
nutrients are adversely affected. This is referred to as commercial sterilization or heat processing. This may be defined as a treatment
aimed at destroying all pathogenic and most spoilage organisms at the cost of minimum damage to food texture and nutrients. Additionally, microbial spores that are likely to thrive under the subsequent handling and storage conditions must also be eliminated. Microorganisms that survive the heat processing operation are unable to cause any trouble, since other treatments that have been given to the canned food render them dormant.
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