SOME INVESTIGATIONS INTO THE BATCH PROCESS OF MILK STERILIZATION

Summary Bacteriological aspects of the batch‐sterilizing process as practised in one dairy, based on meso‐phile and thermophile spore counts and an analysis of subsequent spoilage, have been examined. The results from the commercial process have been compared with those from processes on a pilot scale giving treatments of 100°C. for 90 min. and 113°C. for 35 min. The treatment at 100°C. gave unsatisfactory bacteriological results: 20% of the treated bottles showed mesophile spoilage within 7 days, while almost all the bottles gave thermophile spoilage. The 113°C. treatment gave milk which was free from mesophile spoilage after 10 weeks, while the thermophile spoilage was reduced to 29%. The colour of the milk was affected considerably by this treatment. After treatment by the normal process in one of the dairy sterilizers, no spoilage occurred due to growth of thermophiles during the long cooling period. Slow cooling appeared to encourage subsequent mesophile growth slightly. Mesophile spoilage was 12 times as great for bottles treated at the bottom of the sterilizer than for those treated at the top. This was due to temperature variations throughout the sterilizer, caused by large amounts of air in the steam. The effect of efficient air removal on the temperature distribution is shown, and the influence of sterilizer operation on potential spoilage is discussed. The number of mesophile spores in the milk as received was normally below 5 per 100 ml. The number of thermophile spores in this milk was of the same order, but increased as the milk passed through the plant, particularly towards the end of the day's run. The filler was found to be the main source of this thermophile contamination. Preliminary results are given on the browning of whole milk during sterilization. At normal processing temperatures it is difficult to produce effectively sterile milk without undesirable brownness.