Methods of Assessing the Psychrotrophic Bacterial Content of Milk

Abstract

Introduction . Development of standard methods . (a) Incubationat 10" . (b) Incubation a t 5" .(c) Incubation a t 7" . Standard reference method . . Technique of examination . (a) Age of raw milk on examination . (b) Age of pasteurized milk on examination . (c) Effect of sublethal heat treatment .(d) Diluent and preparation of dilutions . (e) Temperature of agar medium a t pouring .(f) Temperature variation in refrigerator incubators . (a) Violet red-bile agar . (b) Crystal violet agar (c) Penicillin agar . (d) Other selective media . (e) Oxidase positive colony count Selective media for Gram negative bacteria . Indicator agar media . Preliminary incubation of plates . Surface plating methods . Total colony count of refrigerated milk The Moseley method . Screening and quick methods . (a) Frost little plate method. . -. (b) Microcolonies in the drop plate method . (c) Microcolonies on membrane filters . Conclusions . Acknowledgments . References . PAQE 270 271 272 272 272 273 274 2 74 275 2 76 276 277 277 277 278 278 280 281 281 283 2 84 285 286 288 289 2 89 290 290 290 291 291 SUMMARY. The vaiious methods used for the determination of the psychrotrophic bacterial content of refrigerated milk are reviewed and assessed. A standard reference method, involving incubation of poured plates at 7" for 10 days, is recommended for the determination of psychrotrophic colony counts for investigational purposes. Many ingenious techniques, based on media selective for Gram negative bacteria, with incubation a t higher temperatures and shorter times are described and appraised. A simple procedure which would provide a more rapid assessment of the number of active milk spoilage types of psychrotrophs is needed for routine purposes, and i t is suggested that the enumeration of microcolonies by the drop plate or membrane filter methods with incubation at 7" for 72 h would provide a suitable technique for this purpose.Incubation of plates at 25" for 5 days is recommended as a standard reference method for determining the total colony count of bacteria in refrigerated milk, but for routine purposes where large numbers of milk samples are being examined incubation at 28-30" for 3 4 days would provide a sound enough estimate.

Influence of Production Conditions on the Bacteriological Quality of Refrigerated Farm Bulk Tank Milk—A Review

“…psychrotrophs and coliforms, in refrigerated bulk collected milk is a good index of production conditions and bacteriological quality (Thomas, 1969 ;Blankenagel & Okello-Uma, 1969;Blankenagel, 1970). psychrotrophs and coliforms, in refrigerated bulk collected milk is a good index of production conditions and bacteriological quality (Thomas, 1969 ;Blankenagel & Okello-Uma, 1969;Blankenagel, 1970).…”

Selective Media in Air Sampling: A Review

“…those which would form colonies after 7 days a t 8". (For selective methods and sources see Thomas, 1969;Druce & Thomas, 1970. ) It is obvious from the tables of Cowan & Steel (1965) that further use could be made; thus Listeria monocytogenes can form colonies at 5" and many of the lactobacilli a t 45".…”

“…I n looking for Gram negative rods, 10-50 mg/l of methicillin or cloxacillin should eliminate virtually all the Gram positive organisms except perhaps Strep. faecalis; penicillin agar has been used for some years by milk bacteriologists (see Thomas, 1969).…”

Rapid plate counting techniques for enumeration of psychrotrophic bacteria in pasteurized double cream

“…Although plate and sample preparations are standardized many conditions for the subsequent incubation of plates have been advocated for the enumeration of psychrotrophs in dairy products (Baumann & Reinbold, 1963; Thomas, 1969;Luck, 1972). Most of the methods currently employed involve incubation at low temperatures (about 6OC) for periods up to 14 days.…”