Use Of High Temperature (part2)

5. Objects Of Heat Processing :

The primary object of heat processing is to ensure that all health hazards are controlled and that, the shelf life of food products is prolonged. Additionally, heat processing improves texture, flavour and appearance of the product by cooking. In case of vegetables, meat and

other similar foods that are required to be cooked for consumption, heat processing shortens the subsequent cooking time at home.

1. Factors Affecting Heat Processing Of Foods :

The success of canning depends on how heat processing is performed, as well as the degree of vacuum inside the container and the quality of the seal. Heat processing of foods depends upon two major factors that determine the processing requirements of a particular food.

These are lethality of microorganisms and heat penetration through the container and its contents.

a. Lethality Of Microorganisms :

Nature and number of microorganisms, the environmental and the time/temperature relationship affect lethality of microorganisms during heat processing.

b. Nature And Number Of Microorganisms :

Broadly speaking, Nature pue number of microorganisms microorganisms important in food processing are yeasts, moulds and bacteria. Yeasts and moulds are easily killed, while bacteria are more heat resistant and require longer time or higher temperatures for their destruction.

                 Microorganisms live in different physiological states. They may exist as very young celis, older cells and in case of spore

formers, as spores. Young cells are easiest to destroy by heatxfollowed by older cells; spores are most resistant and hence are difficult to kill. Therefore, a food material containing mature cells or spores will require a severe heat treatment for the thermal destruction of the organisms.

              The number of microorganisms present in a particular medium for heat processing has a direct relationship with their destruction. Microorganisms multiply and die in logarithmic order. This means that a certain percentage of the population will die in a particular period and the same percentage of the surviving population will die in the next equivalent period, and so on.

Hence, greater the number of organisms, longer will be the time required for their destruction. It is, therefore, important that the raw material used for heat-processed foods is not heavily

contaminated, decomposed or over-ripe.

c. Environmental Factors :

The composition of food materials affects

heat resistance of microorganisms. Inorganic salts can increase or decrease proteinaceous substances provide some protection to the cells

and spores and thus increase their heat resistance. Sugar, their heat resistance. Fat, proteins and likewise, increases the heat tolerance of microorganisms, especially of yeasts and moulds.

                  The viability of microorganisms is influenced by the pH of the food. For most spore-forming bacteria maximum heat resistance occurs near neutral pH. Hence, bacterial spores are more easily destroyed in frult products at pH 3.0 than in meat or vegetable products at pH 5.0 or above. It has been established that all foods with a pH value of below 4.5 (fruits, fruit products) can be heat processed at 100 C, Others like chicken, meat, fish or vegetables whose pH exceeds 4.5 (medium and low acid foods) must be heat processed at 100°C or higher.

                One reason for limiting the temperature to 100 C for heat processing of acid and high acid foods is that the acid in the food retards the growth of non-aciduric bacteria and their spores that may survive heat treatment at this temperature. Secondly, even if the spores are not destroyed, these will do no harm as they

cannot germinate in such products (since one requirement for spore germination is suitable pH, above 4.5). Moreover, microorganisms do not usually thrive in heavy sugar syrups that

are commonly used in canning of fruits. On the other hand, low and medium acid foods like meat and vegetables are contaminated with heat resistant forms from animal body, intestines, soil, etc. Therefore, higher temperatures are needed to heat process them. For practical purposes, in heat processing

the food processor regards pH 4.5 as the dividing line between foods that can be heat processed at either 100°C or above.

d. Time And Temperature Relationship :

The destruction of microorganisms depends, in part, upon the length of time they are exposed to a particular temperature. Yeasts and moulds are

easily destroyed at 100°C or slightly lower, while bacteria and their spores require higher temperatures. The destructive effect of heat rapidly increases as the temperature rises, Hence, heat processing at 121 C is much quicker than at 100°C. For instance, in low acid media Clostridium botulinum spores are-

destroyed at 100°C in 330 minutes, while at 115.5°C they require only 10 and at 121°C just 2.79 minutes. It is less harmful for the food to be exposed to high temperature for a short time, than to low temperature for long time.

                  The time required to kill microorganisms at a given lethal, temperature is known as thermal death time. The relationship

between the number of survivors and time of exposure at the lethal temperature is very similar for all microorganisms. This has enabled preparation of thermal death curves for target organisms. Thus, mathematically speaking, logarithm of the number of survivors is a straight-line function of time and ite horizontal displacement varies as different organisms or diverse foods are considered.

e. Heat Penetration Through Containers And Contents :

The effecivenes of heat processing of feods largely depends upon the degree of heat penetration frott outside inwards. The healing

Characlerisics of the contents iniluence the thermal destruction OF Hiereorgantaitis Feede ae normally heal processed in hot water or live steam The faciors that affect heat penetration into the food.

2. Thermal Death Rate Curve (Left) And Thermal Death Time Curve (Right) :

a. Nature And Size Of Container :

Conventionally in canning, glass and tin are commonly used in container manufacture. Glass is a bad conductor of heat and, therefore, food packed in it will take Ionger time to heat process than in a similar sized tin can. Pouches take stit less time. Aluminium foil, plastic films and other flexible malerials are also employed for packaging. Usually such foods are first heat processed and then aseptically filled in these containers.

                    Since heat has to travel from outside to reach all parts, larger centainers require longer time for heating than smaller ones the distance from the surface of a large container to the centre is greater than in a small one. This

problem posed by the container size is, in part, eliminated by the agitalion provided in agitation type retorts.

b. Kind And Consistency Of Food :

Liquid (condensed milk, fruit juices), sem-sold (eaby foods) or solids suspended in syrup or brine canned in the industry. Heat processing of each varies according to heat conductivity of the foods. Compactly packed solid food requires longer processing time at the same temperature than the same food packed loosely in some liquid, liquid foods need much less time to attain particular temperature under similar processing conditions. Tables 4.3 and 4.4 provide recommended heat processing parameters for fruits and vegetables in still retort.

c. Types Of Processing Equipments :

Since heat has to reach all parts of the can contents, the type of processing equipment would affect the heat penetration. Agitating retorts are more efficient than still types in achieving the same goal.

Exhausting temperature and heat processing requirements for some selected fruits. 

3. Processing Methods And Equipments :

Basically, there are two methods for obtaining commercially sterile foods: aseptic canning and conventional canning. In aseptic canning, food is first heat processed and then filled or package aseptically into suitable sterile containers. It is also employed in HTST method, in prpcesses designated as aseptic canning an pasteurization and sterilization utilizing microwave heating. The use of this principle is seen in the processing of fruit based beverages, fluid milk and most fluid foods.

Exhausting temperature and heat processing requirements for some selected vegetables.

The commercial thermal processing isxconventional canning process in which food is filled into the container, which is sealed It may be accomplished in any of the many available heat exchange devices: hermetically and then subjected to high temperature.

a.Open cookers.

b.Retorts.

c.Hydrostatic cooker and cooler.

d.Direct flame sterilizers.

The open cookers are used for heat processing of acid and high acıd foods by the small and home-scale processors as well as some large units. The equipment consists of open metallic pans or tanks D! appropriate size filled with water. The water temperature is maintained at

Joiling by suitable means such as firewood, coal, gas, electricity, steam Coils or open steam jets. Heat transfer is more efficient in the agitating types than in the still cookers.

                 Retorts are pressure cookers in which foods are processed in water at higher temperatures than attainable at atmospheric pressure. Different retorts are in use, primarily for processing low and medium acid foods packed in metal and glass containers. A conventional still retort is basically a discontinuous, non-agitating type of vertical or horizontal vessel that is used for heat processing of canned foods under pressure.

                     The efficiency of retorts has been greatly increased by providing a gitating movements inside the equipment. For example, heat processing of evaporated milk requires 18 minutes at 115.6°C in a still retort, while in the agitating type 2.25 minutes are needed to achieve the same results at 93.3°C. The modern retorts are fitted with several gauges and controls (air vent, temperature and pressure gauges, etc.). All air from inside the retort must be removed, since it is a bad conductor

of heat and will act as an insulator during the heat transfer operation. Moreover, air in the presence of moisture at high temperatures, causes steel cans to rust.

                  The hydrostatic cooker and cooler is basically a still retort operating at a constant steam temperature through which the food

containers are transported for the required process time by conveyer system. The equipment comprises of a feed and discharge station, in- feed section, feed leg, steam dome, and discharge leg. The water heads in the feed and discharge legs provide the pressure necessary to balance the constant steam pressure in the steam dome. This type of

Cooker is efficient but expensive to operate.

                 Direct flame sterilization of foods in tin. cans has been successfully achieved with small sizes only. The cans are conveyed

across a bank of gas burners operating at about 1093°C. Applications of this system are somewhat limited.

4. Cooling :

During heat processing, the temperature of food inside the can reaches 100°C or higher. If the cans are held at this temperature longer

than necessary, the heat energy will cause damage to the nutrients. To avoid unnecessary nutrient loss and over-cooking of the food, the cans must be cooled to around 43°C. This temperature ensures that after Cooling operation the cans will be dried by their own internal heat, thereby preventing can rust as a result of moisture adhering to their bodies.

                   In most modern retorts, arrangements exist for subsequent

cooling after heat processing. Glass containers, if processed in retorts, are also cooled in them. On a small scale, cooling may be accomplished

by sprays of cold water or passing the containers through a tank of running cold water or even by exposing them (and especialiy glass

containers) to properly circulating air.

A vertical still retort for heat processing of foods at above 100C courtesy. Dixie canner equipment Co. Athens, USA.

6. Labelling :

Amost all modern cans come with a printed can body. However, some factories prefer to use labels printed on a paper. For home-scaie

processing and in small factories, printed labels are widely used. After the cans have been cooled and dried, they are labelled. The commercial food processor uses a printed label and prefers special labelling machines to do the job. The home-scale processor writes the label

indicating the product and date, and pastes it to the container with suitable glue.

7. Storing And Packing :

Although proper heat processing of canned food ensures safety against spoilage, yet some chemical reactions, and in some cases microbial cack, can occur in the product if

is stored at high temperatures. Microbiologicaliy speaking, commercially sterile cans may contain some viable bacterial spores that may constitute a source of spoilage if held at a temperature of 37°C or above. This information is particularly important in Pakistan where warehouse temperatures are often quite high during summer months. It is, therefore, recommended that canned foods be stored in a cool, dry place, whether inside the house or in the warehouse. If storage temperatures in excess of 37°C are expected in the environment, then heat processing shouid be morexdrastic to ensure the destruction of all organisms capable of activity at these high temperatures. This will provide a longer shelf life to the product even though slightly more nutrients will be damaged during such a heat treatment.

                  For shipping to far off places, several cans are packed into suitable sized cartons. In large processing plants this job is also done by

machines, while a small-scale processor can utilize manual labour.