Expand the role of nisin.

A goal of European scientists is to expand the range of applications for nisin in assuring food safety and quality. They're using novel combinations of nisin with other biopreservatives and mild processes. As you're aware, nisin is a natural preservative with restricted applications due to its lack of activity toward gram- negative bacteria and its loss of activity above pH 4 and below 20 C.

Another problem: It is immobilized by fats and other food components. Initially, scientists focused on the identification of suitable natural compounds and preservative cotreatments that would improve the efficacy of nisin in laboratory systems. The most effective combinations of treatments are being tested in food systems. There have been some early results. Low concentrations of carvacrol and thymol in broth and solid media systematically improved nisin performance against L. monocytogenes and B. cereus. Cultures of lactic acid bacteria in addition to nisin overcame the short-term effect of nisin alone against L. monocytogenes.

Citric acid and sodium hexametaphosphate increased the sensitivity of gram-negative bacteria toward nisin and increased nisin's effectiveness in suppressing the germination of B. cereus spores. Lactic acid increased the effectiveness of nisin at low salt levels while acetic acid had the opposite effect at high salt levels. Sucrose palmitates and stearates increased nisin activity and caused complete inhibition of B. cereus and Lb. plantarum. However, they were ineffective against Salmonella spp.

Nisin and siderophores (chelates iron) acted synergistically against Ps. aeruginosa but not against S. typhimurium. A wider selection of gram-negative bacteria needs to be tested.

In the past, we've told you about efforts to replace sulphur dioxide with organic acids and nisin to cut microbial levels. The microbiological status of fresh pork sausage is influenced by the quality of the meat used and its subsequent handling and processing. When stored aerobically at refrigeration temperatures, the predominant microflora consists of gram-negative, non-spore-forming rods that spoil the product through putrefaction. In Europe, sulphur dioxide, in the form of sodium metabisulphite, is used to inhibit the microbial spoilage of fresh pork sausage.

But sulphites have been linked to the aggravation of asthmatic and other respiratory conditions, urticaria, angiodema, headache and gastrointestinal dysfunction in some individuals. In the United States, using sulphites in foods that are a source of thiamine, such as fresh red meat, is prohibited.

Scientists at University College Cork (Departments of Food Technology and Microbiology, County Cork, Ireland) examined the preservative effect of sodium lactate, sodium citrate and nisin, used individually and in combination, on fresh pork sausage. The potential of sodium citrate or sodium lactate, used singly or in combination with nisin, was assessed in sausage inoculated with S. aureus MMPR 3 and S. kentucky AT 1.

The results indicate that a combination of sodium lactate and nisin was particularly effective in reducing total bacterial counts in this product. It also appears that this combination provides increased protection against common pathogenic contaminants of fresh pork sausage-S. aureus and S. species. It appears that the use of nisin alone or in combination with sodium lactate gives better protection against S. aureus and Salmonella and a longer shelf life.

Further information. On expanding applications: E.J. Smid, Agrotechnological Research Institute (ATO-DLO), Bornsesteeg 59, NL- 6708 PD, PO Box 17, NL-6700 AA Wageningen, The Netherlands; phone: [phone]; fax: [phone]; email: [e-mail]. On the sausage project: Elke Arendt; phone: [phone]; fax: [phone]; email: [e-mail].