CONDENSED AND DRIED MILKS

Module 9. Microbiological qualities of sweetened condensed and evaporated milks

Lesson 22
MICROBIOLOGY OF EVAPORATED MILK

22.1 Introduction

In the manufacture of Evaporated milk, the sterilization is primarily aimed at killing all bacterial spores — reduction to, say, 10^{− 8} spores/ml — and inactivating plasmin. Lipases and proteinases from psychrotrophs should be absent from the raw milk because these enzymes would be insufficiently inactivated. The most heat-resistant spores are those from Bacillus stearothermophilus . This bacterium does not grow at moderate temperatures but may do so in the tropics. At 121°C, the D value of the spores is some 4 to 7 min. The preheating suffices for a sterilizing effect almost equal to 1, whereas the sterilization gives almost equal to 3 at most and, hence, added together giving less than or equal to 4. Contamination by these spores should therefore be slight, and growth of the organism occurring in the evaporator, possibly followed by sporulation (e.g., during intermediate cold storage), should rigorously be avoided. If the sterilizing effect is adequate for B . stearothermophilus , then B . subtilis , Clostridium botulinum , and C . perfringens are also absent.

Thus, normal evaporated milk is sterile. However following observations are made:

(a) It appears that evaporated milk, as purchased in the local market is not only free from pathogenic micro organisms but may, for all practical purposes be considered sterile.

(b) From bacteriological stand point, evaporated milk may be considered a safe good for use in infant feeding.

(c) Common bacterial defects of evaporated milk are

a. Bloat caused by gaseous fermentation

b. Bacterial coagulation and

c. Off flavors due to bacterial causes of which bitter flavour is most frequently encountered.

22.2 Bloats of Evaporated Milk

Bloats caused by gaseous fermentation are commercially the most serious bacterial defect of evaporated milk. The responsible organism is commonly not always responsible. It may be due to post processing contamination of evaporated milk.

22.2.1 Characteristic organisms causing bloats

Characteristic epidemics of bloats in commercial manufacture of evaporated milk that involve the great majority of serious out-breaks of bloats are caused by microorganisms of superior heat resistance. Such fermentation is due to anaerobic bacteria belonging to the butyric acid group and in most cases, though not always the putrefactive prevail. Gas formation is very vigorous and is accompanied by putrid odour (foul odour).

The responsible organisms as reported are Plectridium foetidium. It has flagella, is spore forming bacteria and motile. It resembles to B. Tetani. At 32°C, it ferments milk in four days under anaerobic conditions. The milk first curdles and curd gradually digests, leaving a clear yellow liquid similar in appearance to butter oil. The fermentation is accompanied by the evolution of penetrating, foul, putrefactive odour suggesting H₂S. The organisms survive 15 min at 118°C. Its thermal death point lies between 118-121°C.

22.2.2 Sources of organism & prevention of defect

Plectridium foetidium belongs to a group of butyric acid group bacilli found abundantly in cultivated soil on field crops and on small grains. These organisms are more prominent in milk from stable fed cows than in milk from pasteures. They are found in cow manure. Prevention of defect thus involves improved sanitation in production of milk on the farm, or adjustment of temperature and time ratio of sterilization of the evaporated milk as will destroy these organisms if present in milk.

22.3 Coagulation of Evaporated Milk

Bacterial coagulation is probably the most common bacterial defect in evaporated milk. The coagulum found in these outbreaks has varied from the sweet curdling type to a sour curd and an intense bitter curd. Some reported curd to be sweet and no bitter taste when no acid was developed and curd was of varying firmness and complete absence of wheying off. Causative organism identified is Bacillus cereus. Bacillus coagulans was separated and identified from commercial coagulated milks. These organisms were not destroyed by a 10 min exposure to 80°C but failed to survive 112°C.

22.3.1 Bitterness in evaporated milk

Many instances of bacterial coagulum showed intense bitterness, suggesting protein breakdown and formation of peptone and other decomposition products of known bitter flavour. The coagulum was firm and striking whiteness. The content had wheyed off. This was practically clear. Acidity was normal 0.35 to 0.4%. Inoculation with sterile milk yielded pure culture of very small spore forming rods. Organism was facultative anaerobic with an optimum incubating temperature 32°C but failed to survive 110°C for 15 minutes.

22.3.2 Prevention of defect

The extreme whiteness of contents of defective cans suggested insufficient heat treatment in sterilization as the cause of defect. Non-uniformity of heat distribution while sterilization, seems the main cause which should be prevented.

22.3.3 Fishy flavour

Coagulated milk with fishy flavour development is reported by the causative organism Proteus ichthyormis in evaporated milk.