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Food is considered spoiled when an undesirable change in the colour, flavour, odour or texture has occurred. Foreign substances in food products make foods undesirable. It is a gradual process and is a natural phenomenon; it occurs at varying rates depending on the storage temperature, kind of food involved, kind of microorganisms present, packaging materials used, food additives used and method of preservation. This discussion started in Biotalk 21 April 2003.
Preservatives continued
Preservatives permitted by the Australian New Zealand Food Standards Code are sulfur dioxide and sodium and potassium sulphites, benzoic acid and its salts, sorbic acid and its salts, propionic acid and its salts, nisin, propyl and methyl hydroxy benzoates (limited to preparations of food additives), pimaricin (natamycin) and dimethyl dicarbonate.
Preservatives can work in a variety of ways and against different microorganisms, e.g. dimethyl dicarbonate acts primarily against yeasts as does the antibiotic natamycin, nisin is also an antibiotic but its main application is to inhibit the sporulation of bacterial spores. The most widely used preservatives are the weak acids. Microorganism growth is significantly inhibited by decreasing the pH. This action is achieved by inhibiting the enzyme systems of the microbe, it has been suggested that sorbic acid does not always act as a weak acid preservative but rather as a membrane active compound. The limitations of chemical preservatives include: they are usually ineffective if the initial microbial load is high; there is a wide spectrum of resistance to preservatives by food borne microbes; resistance is known to be induced by exposure to sub-lethal concentrations; many microbes can use some preservatives as food sources.
High pressure processing (HPP)
The process uses pressure of about 300 to 700 Mpa for a few seconds to destroy non-sporing microbes. The pressure is transmitted uniformly throughout the food and there is no gradient of effect as occurs with heat processing. The effects of HPP are different to heat treatment. In HPP, proteins and enzymes are structurally changed. Enzymes can be inactivated or stimulated. The sensitivity of microbes to HPP are variable. Gram negative bacteria are more susceptible than yeasts and moulds, then comes Gram positive bacteria and finally bacterial spores that are highly resistant.
Pulsed electric fields (PEF)
There are four main variables in PEF processing. These are, strength of the electric field (typically up to 50 kilovolts/centimetre, the time of exposure (that ranges from a few microseconds to milliseconds, the number of pulses given (usually les than 100) and the pulse shape (including exponential decay, square, wave or oscillatory). Also the pulse may be monopolar or bipolar. The process causes an increase in the permeability of the microbial cell membrane, that enables the cell contents to leak and results in cell death.
Irradiation
The World Health Organisation has approved radiation doses of up to 10 kilograys as 'unconditionally safe for human consumption'. In Australia there is a food labelling requirement for any food that has been irradiated or contains any irradiated ingredients. Irradiation can be produced as gamma radiation from a radioactive isotope or by electron beam. Irradiation irreversibly damages the microbial cell DNA which leads to the inability to reproduce. Gram negative bacteria are more susceptible than Gram positive bacteria and moulds, then comes bacterial spores and yeasts and finally viruses that are highly resistant.
The discussion will continue in the next BioTalk Food Newsletter.
Sources: Spoilage of Processed Foods: Causes and Diagnosis. AIFST Inc. NSW Branch, Food Microbiology Group. Food and Spoilage and You,
George Schuler, William Hurst, Estes Reynolds and James Christian; revised by P.T. Tybor. Extension Food Science. University of Georgia College of Agricultural & Environmental Sciences.
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