DETERIORATIVE CHANGES 355 



what different manner, bringing about a partial inhibition of the sensitive 

 portion of the bacterial flora, but yield the same end results as chilling — ■ 

 a delay in the rate of increase of the spoilage population. These sub- 

 stances are in fact usually used in association with chilling, and the 

 combined inhibitory effect is additive in nature. Antibiotics of the tetra- 

 cycline group, though very much more potent inhibitors than the other 

 bacteriostatic chemicals, also produce only a temporary remission of bac- 

 terial activities. However, chlortetracycline, oxytetracycline, and tetra- 

 cycline have been shown to yield considerable extensions of ' 'storage life" 

 of round and filleted fish at chilling temperatures when applied at levels 

 up to about 10 ppm in ice or as dips yielding residual concentrations of 

 about 7 ppm on the fish surface. 



Analyses of the bacteria present on fish stored in antibiotic ices have 

 shown that the antibiotics initially exert a highly selective effect, greatly 

 reducing the proportions of ''spoilage types" present in the early storage 

 period and maintaining the total bacterial populations at a low level. 

 However, after an extended lag and acceleration phase, the normal loga- 

 rithmic growth ensues and spoilage then becomes evident. The spoilage 

 flora which ultimately develops is similar to that present in normally iced 

 fish, except that its members are mostly insensitive to the antibiotic when 

 tested in vitro. 



Pasteurization, whether effected by heat or by high energy irradiation, 

 is also a partial inhibition process which by its selective effect on sensitive 

 bacteria slows down the development of the spoilage flora and delays, but 

 does not prevent, spoilage. 



Most modern smoking processes ensure only partial inhibition of bac- 

 teria. The preliminary brining and drying procedures may produce some 

 reduction in the numbers of bacteria and probably exert a selective anti- 

 bacterial effect. Smoking deposits phenolic and other substances on the 

 fish surface which are mildly antiseptic. The combined effect of these 

 factors is to retard the development of the spoilage flora and thus extend 

 keeping time slightly. 



In general, partial inhibition processes are most effective when applied 

 very soon after the death of the fish. Once the spoilage flora has entered 

 the period of logarithmic growth such processes are of little use. 



Complete Inhibition. Freezing, salting, and drying (including freeze 

 drying) provide good examples of complete inhibition processes. Though 

 each of these processes may produce considerable mortality among the 

 bacteria on fish, their primary preservative effect is due not to this but 

 to the production of conditions in the seafood which makes bacterial 

 growth impossible. In each of the quoted cases the effect is produced by 

 removal of available water. In the frozen product, water is unavailable 



