Modern Industrial Microbiology and Biotechnology

224 Modern Industrial Microbiology and BiotechnologyTable 11.1Temperature of air after compressionFinal pressure (p.s.i.g.) Temp. ( o C)20 7840 11760 14080 169100 189150 229200 261much higher pressure than that at which it will be used) and for fairly long holdingperiods. Although not a very practical method, compression could reduce the microbialpopulation of air.Other methods which have been explored include direct or indirect heating with thegases and also with electrical heating. In each case while the procedure was effective, itwas too expensive.Moist heat: Moist heat can be employed in industry to kill microorganisms during boiling,tyndallization, and autoclaving.Tyndallization consists of boiling the material for one half hour on three consecutive days.Vegetative cells are killed on the first day’s boiling. Spores are not but they germinate.During the second day’s boiling, the vegetative spores resulting from the spores not killedon the first day, are killed. Any spore still surviving after the second day will be killedduring boiling on the third day as the spores would have germinated. After the thirdday’s boiling the medium is expected to be sterile. It is a method which can be used forsterilizing heat-labile media where filtration is not possible for whatever reason,including that the medium is too viscous for filtration.Pasteurization is very widely used in the food industry. It is used for treating beer andwine. It consists of exposing the food or material to a temperature for a sufficiently longperiod to destroy pathogenic or spoilage organisms. Pasteurization can either be batch orcontinuous. The low temperature long time (LTLT) technique usually involves heating atabout 60°C for one half hour and is used in batch pasteurization whereas the hightemperature short time (HTST) of flash method involves heating at about 70°C for about15 seconds. The flash method is employed in continuous pasteurizing.When batch pasteurization is used on a large scale the final temperature ofpasteurization is attained by gradual increases. Similarly, the temperature is loweredgradually to cool it; for 600 ml bottles in many breweries batch pasteurization time is atotal of about 90 minutes divided equally between raising the temperature, holding at thepasteurization temperature, and cooling. This prolonged time during which the materialis exposed to high temperature and which may give rise to a ‘burnt’ odor is the majordeficiency of batch in comparison with continuous pasteurization.Steam under pressure: Steam is useful as a sterilizer for the following reasons:(i) It has a high heat content and hence a high sterilizing ability per unit weight orvolume; this heat is rapidly released.