Some Investigations Into the Batch Process of Milk Sterilization

Abstract

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.

639. Some bacteriological aspects of commercially sterilized milk: I. Incidence and nature of spoilage

“…(3) Processing the milk Burton et al (4) noted that the operation of milk-sterilizing pressure vessels could usually be considerably improved. They indicated the difficulties of producing bacteriologically sterile milk without undesirable brownness.…”

L'origine des spores de thermophiles dans le lait stérilisé en bouteilles

“…C'est ainsi qu'en 1953, BURTONet al [1], au cours de l'étude d'une chaîne industrielle de stérilisation du lait en bouteilles, ont montré que le nombre de spores de lthermophiles semblait plus élevé en aval qu'en amont de la remplisseuse et ont suggéré que celle-ci représentait une source de contaminations en spores de thermophiles. Les observations effectuées par PEYLET en 1958 [11] indiquent que cette dernière hypothèse est la plus vraisemblable.…”

“…Des tubes de 10 ml de lait UHT, préalablement portés à la température de croissance étudiée, sont inoculés avec 0,05 ml d'une préculture de cellules végétatives ou d'une suspension de (1) Cette souche nous a été obligeamment fournie par le Dr Fuchs, Alpura, Berne (Suisse).…”

“…IÉMOIRES ORIGINAUX (1) L'examen bactériologique des bouteilles de lait stérilisé comporte la recherche des germes sporulés mésophiles après une incubation à 300 C ainsi que des germes sporulés thermophiles après une incubation à 55°C. L'ensemble des contrôles bactériologiques effec- (1) Reproduction interdite sans indication de source tués pendant ces dernières années en France par les laboratoires officiels [14] et industriels [11] montre que le.…”

“…L'ensemble des contrôles bactériologiques effec- (1) Reproduction interdite sans indication de source tués pendant ces dernières années en France par les laboratoires officiels [14] et industriels [11] montre que le. lait stérilisé en bouteilles est stérile après incubation à 30°C, mais ne l'est pas après incubation à 550 C ; la proportion des échantillons non-stériles dans ce dernier cas varie de moins de 1 p. 100 à plus de 50 p. 100 suivant l'origine et la date de fabrication.…”