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Researchers highlight the potential of Nisin as a preservative in fruit juice processing

29 July 2015

Historically the best method in fruit juice processing to remove the risk of bacterial contamination and spoilage has been through heat treatment; however the drawback of this has always been that, although effective, the sensory impact, experience and taste may be diminished in comparison to the fresher form. This is due to the compounds responsible for such aspects as flavour, aroma and taste also being heat sensitive and prone to alteration under stress. This dilemma has convinced the food industry to explore alternatives and innovations to preserve the sensory experience in the longer term of fresh food as well as preserving the food product from spoilage, without the need for aggressive heat treatment. 

It has been long known that alternatives do exist and a recent study by researchers in Brazil (Universidade Federal de Sergipe, São Cristóvão, Sergipe, Brazil) has showed that bacteriocins, (proteinaceous toxins produced by bacteria to inhibit the growth of similar or closely related bacteria) can have enough antimicrobial activity against microorganisms to prevent spoilage and pathogenic microorganism growth whilst maintaining the integrity of the beneficial compounds which confer the desired flavour, aroma and texture of the food products. 

For the purposes of the Brazilian study the bacteriocin in question is Nisin, a polycyclic antibacterial peptide produced by the bacterium Lactococcus lactis. The study aimed to verify the stability, effect on physico-chemical properties, and the antimicrobial activity of Nisin in a variety of fruit juices. It was hoped this would show promise for its use as a food additive/preservative. 

The study results showed that Nisin remained stable in fruit juices (cashew, soursop, peach, mango, passion fruit, orange, guava, and cupuassu) for at least 30 days at room or refrigerated temperature and did not cause any significant alterations in the physico-chemical characteristics of the juices tested. This is important for being considered a viable preservative, as a preservative without longevity of function is of limited use no matter how great the antimicrobial effect. For example a stability of 2 days may wipe out 99% of a bacterial population, but a resistant population may survive and create spoilage weeks down the line. The specific spoiling agents tested and being discussed in this case were the organisms Alicyclobacillus acidoterrestris, Bacillus cereus, Staphylococcus aureus and Listeria monocytogenes which were tested in cashew, soursop, peach, mango juices. 

The analysis yielded positive results in many cases as cashew juice with added Nisin caused a rapid (within 8 hrs) decrease in the viability of A. acidoterrestris, S. aureus, and B. cereus. After 24 hrs of incubation, the antimicrobial activity of Nisin for B. cereus and S. aureus was lower in mango juice, whereas the reverse was observed for L. monocytogenes. This varying heterogeneous effect of antimicrobial activity in different fruit juices has been reported in previous studies although the exact factors were not completely defined.

The most sensitive microorganism to Nisin was A. acidoterrestris and the least sensitive was L. monocytogenes, but still up to 90% of observed cells in peach and mango juices inoculated with L. monocytogenes were inactive after 24 hrs of incubation. It was clear that when used appropriately under the right conditions bacteriocins or perhaps a combination could be a potent tool in the fight against spoilage. 

It can be concluded that although the antimicrobial activity of Nisin was heterogeneous and depended on both the type of juice and microorganism, the application of bacteriocins as preservatives in fruit juices could be a viable option, despite being limited by their properties such as peptide solubility, interaction with the juice components, pH, and inactivation by proteases. Nisin could be a more viable and widespread additive in future with widespread benefits, albeit with some necessary considerations.

The stability of the antimicrobial activity of any bacteriocins is an essential characteristic in their function compared to heat treatment. This is dependent on a multitude of factors as earlier mentioned, and hence a cocktail of these peptides could be the way forward for use as a preservative process i.e. Nisin favoured the preservation of vitamin C content in juices, while favour preserving a different characteristic. Nisin favours A. acidoterrestris reduction, there may well be bacteriocins for groups of spoilage bacteria that can be combined.

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