Cream is a yellowish selective milk concentrate containing a high level of milk fat globules. It can be called as that portion of milk which is rich in milk fat. It serves two important purposes in the food industry, firstly it is used as a principal ingredient for a variety of food products like ice cream, cake, etc and secondly it is used as raw material for butter making.
As per the Food Safety and Standards Regulations (FSSR), cream including sterilized cream means the product of cow or buffalo milk or combination thereof. It shall be free from starch and other ingredients foreign to milk. It may be of following three categories:
- Low fat cream: containing milk fat not less than 25% by weight
- Medium fat cream: containing milk fat not less than 40% by weight
- High fat cream: containing milk fat not less than 60% by weight
Cream not sold without any indication of milk fat content is considered as a high-fat cream.
The Microbiological quality of cream and milk are mostly the same except the difference in the proportion of various constituents like fat, protein, lactose etc. The initial microbiological quality of cream is parallel to that of milk, however in advanced stages, cream suffers more problems than milk due to longer storage and erratic distribution patterns. It is believed that keeping other things constant, the cream has longer keeping quality than milk, albeit it seems doubtful.
Since the cream is more viscous, it forms clumps which lead to lower bacterial count in the microbiological examination. It has been reported that a good quality cream has a shelf life of 6 days at 5°C. Cleaning and sanitization have remained an important aspect of quality control. Therefore, the cleanliness of the processing plant has a great impact on the microbiological quality of the cream. Samples taken from poorly cleaned plants have initial colony count at 32°C of about 104 per ml which further increased 107 per ml after five days cold storage. But after following proper cleaning and sanitization steps-initial counts were only 10 per ml which further increased to 10 per ml during storage for seven days.
Other quality factors include good quality raw milk, the temperature of heat treatment and prompt cooling thereafter, storage and transportation temperature and hygienic conditions during cream separation (if cream separators are not cleaned properly then they may serve as bacterial contamination).
A varied microflora such as bacteria, yeasts and moulds have been frequently found in cream.
Bacterial flora of milk reflects in cream followed by subsequent contaminants like spoilage causing bacteria and pathogens. The bacterial flora includes micrococci, streptococci (lactic and non-lactic), corynebacterium as well as spore formers (aerobic and anaerobic). Contaminating organisms include gram-negative rods, staphylococci, lactobacilli, salmonella and coliforms. The presence of the above microflora in fresh cream depends upon the level of hygiene and storage temperature.
THE MICROFLORA OF CREAM
The microflora of cream processed in unclean and unhygienic conditions predominantly include psychrotrophs. If the cream is not promptly cooled to 5°C or lower temperature then cream shall be contaminated by staphylococci, lactobacilli and Bacillus cereus. However, if the cream is stored for a long time at such temperature, the cream may contain psychrotrophs, psychrophlies mainly pseudomonads (gram-negative, non-sporeforming, oxidase-positive, catalase-positive rods which arise due to contamination by dirty water). The above microflora present in cream is generally proteolytic and lipolytic in nature. The presence of psychrotropic coliforms has also been reported which grow slowly at 3° to 5°C and may increase 100 to 1,000 times in a few days. Pseudomonas (fluorescent and non fluorescent), achromobacter, alcaligenes, aeromonas, flavobacterium, coli-aerogenes, coryneform and micrococci have been reported predominantly in samples of ‘market cream’ and ‘pasteurised cream’. The predominance of above species indicates that heat treatment given to cream is not efficient. Generally, spore formers are found in pasteurised, sterilised or UHT processed cream. The presence of thermoduric organisms like Streptococcus faecalis, microbacteria and micrococcus have also been reported. Among yeasts, the contamination is caused by Torula cremoris, Candida pseudotropicalis, Torulopsis sphaerica, Rohodotorula mucilaginosa and other lactose fermenting yeats. Mould contamination is caused by Mucor, Geotricum candidum, etc.
The microbiological quality of cream can be understood on the basis of microbial counts like total plate count, coliform counts, spore count, etc. The following table demonstrates the relationship between microbiological counts and the quality of the cream.
|Microbial counts||Interpretation of the quality of the cream|
|High total count and high coliform count||Inadequate heat treatment and/or unhygienic manufacture and/or storage at high temperatures|
|High total count but low coliform count||Good hygiene but storage at high temperature|
|Low total count but high coliform count||Poor hygienic conditions but storage at 5°C|
|Low total and coliform count but high yeasts||Good hygiene but contamination from fruit etc, directly or indirectly|
|Low total and coliform count but high moulds||Good hygiene but aerial contamination|
|Low total and coliform count but high aerobic spores||Cream made from milk having a high spore count|
|Few thermoduric||Heat-treated cream, not contaminated, refrigerated|
|Many thermoduric||Heat-treated cream, not contaminated, but not properly refrigerated|
|Few thermoduric with few non- thermoduric||Heat-treated cream, contaminated, refrigerated|
|Many thermoduric with many non- thermoduric||Heat-treated cream, contaminated, but not properly refrigerated|
REASONS FOR SPOILAGE
Microorganisms may create certain kind of defects in cream. Cream encounters more defects than milk due to methods of distribution and requirements for a longer keeping quality. Microbial off flavour defects are different from non- microbial defects as non-microbial defects can be removed by exposing the product to air or by applying vacuum pasteurisation. The common factors which are responsible for the proliferation of spoilage causing microorganisms are discussed in the following paragraph.
Unhealthy udder or unhealthy milch animal give rise to high microbial load in raw milk. Further, unhygienic production of milk also gives
rise to infected milk. Therefore, clean milk production is very important not only for cream but for all dairy products. Cream separators are the next source of contamination. If not properly cleaned than leads to high microbial growth in milk/cream. Proper heat treatment (preheating and pasteurisation both) is mandatory to ensure production of good quality of the cream. Unhygienic handling of cream after heat treatment can also spoil the cream.
METHODS TO PREVENT SPOILAGE
The common microbial taints occurring in the cream are sour or high acidic flavour, bitterness, rancidity, fruity taint, cheesy or putrid flavour, yeasty flavour and discolouration. There are varied responsible microbes such as pseudomonads, aerobic spore formers, gram-negative rods, lactic acid producers, coliforms, yeasts and moulds. The cold storage of pasteurised cream (i.e. at 7.2°C to 10°C) results in rapid multiplication of psychrotrophs, gram-negative bacteria such as coliforms along with certain yeasts and moulds. Post pasteurisation contamination include mesophilic lactic streptococci, micrococci, aerogenes and coliforms. The following table discusses the relation between various microbial defects, their causative agents and remedies.
|Bitterness||Gram-negative rods like Pseudomonas, Achromobacter, Proteus, aerobic spore formers like Bacillus subtilis, yeasts like Rohodotorula mucilaginosa.||
|Rancidity||Pseudomonas species like Pseudomonas fluorescent and Pseudomonas fragi, lipolytic psychrotrophic bacteria, moulds like Geotricum candidum (Oidium lactis), yeasts like Candida lipolytica.||
|Slimy/ropy texture||Alcaligenes viscolactis||
|Mouldy or musty flavour||Geotricum candidum||
|Yeasty flavour||Lactose fermenting yeasts, Torulopsis sphaerica, Candida pseudotropicalis, etc||
|Cheesy flavour||Pseudomonas putrrfaciens||
|Fruity flavour (apple-like)||Pseudomonas fragi||
|Malty flavour||Streptococcus lactis subsp. maltigenes||Adequate cooling of cream.|
- J.S. Yadav; Sunita Grover; V. K. Batish 1993 A Comprehensive Dairy Microbiology
- Richard K. Robinson 2002 Dairy Microbiology Handbook: The Microbiology of Milk and Milk Products
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