Methods of Assessing the Sporicidal Efficiency of an Ultra‐high‐temperature Milk Sterilizing Plant. Ii. Experiments With Suspensions of Spores in Milk

Abstract

SUMMARY: The sporicidal efficiency of an ultra‐high‐temperature (UHT) milk processing plant has been tested using spores of a strain of Bacillus subtilis in milk. With the inoculum and volume of milk adjusted to obtain a countable number of survivors by a conventional dilution counting method, a temperature of 130·5° with the minimum time setting of the plant was found necessary to give a destruction of 99·99999%. This temperature was lower than that found previously (135°) for spores suspended in water and evidence is produced to support the suggestion that UHT milk may be inhibitory to the germination and/or subsequent growth of heated spores.

Spore resistance and ultra heat treatment processes

“…No explanation was given for the non-linearity but some of the deviation could have been due to the chamber gradually achieving equilibrium temperature over a longer time period than that calculated by the investigators, Other indirect heating methods include pilot scale systems which have been used principally to confirm process times and temperatures for factory scale equipment. Examples include tubular (Galesloot 1956;Cuncliffe et al 1979;Wadsworth and Bassette 1985) and plate (Williams et al 1957;Franklin et al 1958a) heat exchangers. Galesloot (1956) reported that increasing residence times by 16.5 s at 120-135°C increased the sterilizing effect of the system by approximately two [sterilizing effect = log (initial concentration/final concentration)].…”

(Symposium on Bacterial Spores: Paper XV). Spores in Milk: Problems Associated with UHT Processing

“…While it is necessary to achieve destruction of the more resistant thermophilic spores in milk marketed in tropical countries, in temperate regions where thermophiles are unlikely to be able to grow and thus spoil the milk less severe treatments can be used. The test strains were BaciUus subtilis 786 and B. stearothemtophilus TH24 and their respective heat resistance characteristics have been described by Franklin, Williams & Clegg (1958) and Franklin, Williams, Burton, Chapman & Clegg (1959). Their Ql0 values for heat destruction in milk were calculated 30 and 11.5, respectively, indicating a considerable difference in heat resistance.…”

“…With survivors at a level of < l/ml and sometimes as low as 1/1 or more, the use of colony count methods is obviously not feasible and dilution counts of the UHT milk have to be used. However, UHT milk sometimes inhibits the germination and/or outgrowth of some bacterial spores (Franklin, Williams, Chapman & Clegg, 1958 ;Franklin, Williams, Burton, Chapman & Clegg, 1959;Segner, Frazier & Calbert, 1963;Busta, 1966). This phenomenon, as would be expected, affects dilution counts of spores in UHT milk (Table l).…”

“…With some foods it is acceptable for a product, although not sterile, to be free of pathogens and of proliferating bacteria capable of causing spoilage. Residual bacteria may not be able to grow for a variety of reasons related to the nature of the food, and the recommended sterilizing procedures may take this into (Franklin et al, 1959). account. It is unsound practice however, to rely on inhibition as an integral part of the sterilizing process for milk, when it is evident that some spores are less susceptible than others and when occasional batches of UHT milk occur in which the inhibitory effect is absent (Franklin, Williams & Clegg, 1958).…”

METHODS OF ASSESSING THE SPORICIDAL EFFICIENCY OF AN ULTRA‐HIGH‐TEMPERATURE MILK STERILIZING PLANT. III. LABORATORY DETERMINATIONS OF THE HEAT RESISTANCE OF SPORES OF BACILLUS SUBTILIS IN WATER AND IN MILK

“…The calculated values agree well with the results obtained in the plant by Franklin et al (1958). EARLIER PAPERS in this series (Williams et al 1957;Franklin et al 1958) have dealt with the destruction of Bacillus subtilis spores in an ultra-high-temperature (UHT) milk sterilizing plant. It seemed desirable to investigate the effect of heat on spores by laboratory methods to assist in the interpretation of earlier results, and also to determine if it were possible to predict the sterilizing efficiency of a UHT milk sterilizing plant without resorting to the bacteriological experiments described in the previous papers.…”