Physical Preservation of Food
Asepsis is the process of prevention of growth of microorganisms and their contaminants or both. The covering or wrapping, prevents primarily contamination during handling, protect the process foods from microbial contamination.
The killing of microorganisms at high temperature is supposed to be caused by the denaturation of the proteins and especially by the inactivation of enzymes required for metabolism. The heat treatment also kills microorganisms or their spores.
The heat treatment selected will depend on the kinds of organisms to be killed. Certain factors are known to affect the heat resistance of cells of spores and must be kept in mind when microorganisms for the destruction of an organisms considered.
The temperature-time relationship, initial concentration of spores (or cells), previous history of the vegetative cells or spores, composition of substrate in which cells or spores are heated. Moisture (moist heat is a much more effective killing agent than dry heat), and Hydrogen-ion concentration (pH) are the other factors which play role in killing of microbes.
At neutral pH many spores or cells are more heat resistant. An increase in acidity or alkalinity hastens killing by heat, but change towards the acid side is more effective than a corresponding increase in alkalinity.
The heat resistance of microorganisms usually is expressed in terms of their thermal death time (TDT) which is defined as the time it takes at a certain temperature to kill a stated number of cells or spores under specific conditions. This sometimes referred to the absolute thermal death time. Whereas, the thermal death point is the temperature necessary to kill all the organisms in 10 min.
Pasteurization is another method in which microorganisms are killed by heat treatment usually involves the application of temperature below 100° C. The heating may be generated by hot water, dry heat, or electric currents and products are cooled promptly after the heat treatment.
The high- temperature for a short time (HTST), whereas the low-temperature-long-time (LTH) are given in pasteurization. For e.g. the minimal heat of market milk is at 62.8°C for 30 min in the holding method; at 71.7°C for at least 15 sec. in the HT5T methods; and at 137.8°C for at least 2 sec in the unpasteurized methods.
Canning is defined as the preservation of foods in sealed containers and usually implies heat treatment as the major factor in the preservation of spoilage. Spallanzani (1765) preserved food by heating in a sealed container. Nicholas Appert, who has been called the “father of canning”, performed heating of foods in sealed containers and also published direction of canning.
Foods contain moisture, hence drying of the food is necessary by removal of water. Various methods of drying such as sun- drying, artificially produced heat etc. are used for desiccation.
It is a process in which the replacement of air by CO2 or by an inert gas may bring anaerobic conditions. For example spores of some of the aerobic spore forming organisms are specially resistant to heat and may survive in canned food but are unable to germinate due to lack of oxygen.
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