A bulletin for the Australian Food Industry August 1998
Contents: Protozoan parasites in water and food | Infective dose of Salmonella in ice cream | Microbiological status of beef in Australia | Hygiene monitoring by ATP luminometry | Botulism associated with home preserved mushrooms | Quality system guidelines | Spoilage of low salt margarine
Spoilage of low salt margarine
Margarine, like butter, is a water in oil emulsion in which the water is dispersed in tiny droplets generally a few micro-metres only in diameter. The microbiological stability of these products depends on proper moisture distribution. Most contaminating microorganisms are restricted in growth by either space limitation within a small water droplet or by exhaustion of nutrients available in the droplet in which it is present (ICMSF, 1998).
While lipolytic yeasts occasionally cause spoilage problems in margarine, moulds are usually the cause of any off flavour problems which develop as a result of microbial growth. Unlike yeasts and bacteria, moulds can grow through the fat matrix of margarine (or butter). The composition of the aqueous phase is seldom such that it will in itself inhibit mould growth. This is particularly true of low salt margarines.
Low salt margarines containing approximately 1% salt are susceptible to mould spoilage because their water activity (aw) is higher than that of margarine containing the regular amount of salt - approximately 2 per cent. These low salt margarines have a water activity in the range 0.915 - 0.93 compared to 0.865 - 0.89 for regular salt products. They are thus susceptible to spoilage by a wider range of moulds.
Spoilage of margarine by moulds is usually only detected by microbiological investigation undertaken after off-flavour complaints are received. Such an instance was reported recently by Hocking and co-workers from this laboratory (Journal of Food Mycology 1 (1) 1998 23-30).
Samples of low salt margarine were submitted to the laboratory following consumer complaints of a 'crushed ant' flavour in the margarine. A pure culture of the mould Penicillium solitum was isolated from the affected margarine and a range of aliphatic ketones were identified as the cause of the unpleasant odour and flavour associated with the spoiled product.
Attempts to reproduce the off-flavour problem under laboratory conditions by the inoculation of low salt margarine with P. solitum and incubation at 10°C and 15°C were unsuccessful. There was no discernible mould growth in the margarine after six weeks incubation.
However a similar range of odour compounds could be produced by inoculating P. solitum into laboratory media supplemented with margarine. Two conclusions can be drawn from this work. P. solitum is best known as a post-harvest pathogen of pome fruits although it has been reported as a spoilage species in packaged cheese (International Journal of Food Microbiology 16 1992 123-130). Changing the formulation of standard food products in such important characteristics as water activity and pH will almost certainly introduce the risk of spoilage by a different range of microorganisms from those normally associated with the product. Secondly, the spoiled margarine had been stored at around 10°C for four to six months. While margarines, including polyunsaturated margarine, are relatively robust with regard to chemical change when stored up to 10°C (Lebensmittel Wissenschaft und Technologie 16 1983 198-199), if mould contamination of the margarine has occurred, storage at such a high temperature will increase the risk of the product becoming unacceptable before its nominated use-by date has expired.
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