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Nisin in Foods Application of Alternative Food-Preservation Technologies 85 asparagine in nisin Z [14]. The structural modification has no effect on the antimicrobial activity, but it gives nisin Z higher solubility and diffusion characteristics compared with nisin A, which are important characteristics for food applications [15, 16]. The thioether bonds give nisin two rigid ring systems, a N-terminally and a C-terminally located; a hinge region (residues 20-22) separates the ring systems. Due to the ring structures, the nisin molecule is maintained in a screw-like conformation that possesses amphipathic characteristics in two ways: the N-terminal half of nisin is more hydrophobic than the C-terminal one; and the hydrophobic residues are located at the opposite side of the hydrophilic residues throughout the screw-like structure of the nisin molecule. Nisin production is affected by several cultural factors such as producer strain, nutrient composition of media, pH, temperature, agitation and aeration, as also by other factors, for example, substrate and product inhibition, adsorption of nisin onto the producer cells and enzymatic degradation [17]. ANTIMICROBIAL EFFECT AND MODE OF ACTION OF NISIN Nisin is a natural, toxicologically safe, antibacterial food preservative. It was shown to have antimicrobial activity against a wide range of Gram positive bacteria, including L. monocytogenes, together with different strains or species of streptococci, staphylococci, lactobacilli, micrococci and most spore-forming species of Clostridium, Bacillus and Alicyclobacillus, but not against Gram negative bacteria, yeasts or fungi. It can also act against Gram negative bacteria, such as Escherichia coli or Salmonella species, in conjunction with chemically induced damage of the outer membrane. Moreover, several works showed the antimicrobial activity of nisin against a number of food pathogens including, E. coli, Campylobacter jejuni, Cl. difficile, Helicobacter pylori, B. cereus, as well as Shigella and Enterococcus species [15, 18, 19]. The potent activity of nisin against a broad range of gastrointestinal pathogens indicates that the peptide could have therapeutic potential in the treatment of gastrointestinal infections. There are many hypotheses about the mechanism of action against spores and vegetative cells [20-22]. Initially, the antimicrobial activity of nisin was thought to be caused by reacting with sulfhydryl groups of enzymes via the dehydro residues [11], by inhibition of cell wall synthesis [23, 24] or by the strong adhesion to cells, causing leakage of cellular material and subsequent lysis, as a cationic surface-active detergent [25]. However, experiments with intact bacterial cells and isolated plasma membrane vesicles have shown that treatment of nisin resulted in rapid efflux of small cytoplasmic compounds [26, 27]. The mode of action of nisin is shown to involve interactions with the membrane-bound cell wall precursor lipid II (undecaprenylpyrophosphoryl- MurNAc-(pentapeptide)-GlcNac), concomitant with pore formation in the cytoplasmic membrane of the target organism [28, 29]. In particular, the C-terminal region of nisin binds to the cytoplasmic membrane of vegetative cells and penetrates into the lipid phase of the membrane [30], forming pores which allow the efflux of potassium ions, ATP, and amino acids [31-36], resulting in the dissipation of the proton motive force and eventually cell death [28, 37, 38]. It is now believed that the depletion of the proton motive force is the common mechanistic action of bacteriocins from lactic acid bacteria. Therefore, it is generally accepted that the bacterial plasma membrane is the target for nisin, and that nisin kills the cells by pore formation. Nisin’s effectiveness is concentration-dependent, in terms of both the amount of nisin added and the number of spores or vegetative cells that need to be inhibited or killed. Nevertheless, the effectiveness of nisin depends also on growth and exposure conditions, such as the temperature [33, 39, 40-42] and the pH [33, 39, 41]. In general, nisin is more active at lower pH values, whereas the influence of temperature on its effectiveness is controversial. Its action against vegetative cells can be either bactericidal or bacteriostatic, depending on a number of factors including nisin concentration, bacterial population size, physiological state of the bacteria and the condition of growth. The insensitivity of Gram negative bacteria to nisin could be due to the large size (1.8–4.6 kDa) of nisin, which restricts its passage across the outer membrane of Gram negative bacteria [43, 44]. The outer membrane, covering the cytoplasmic membrane and peptidoglycan layer of Gram negative bacteria, is composed of lipopolysaccharide (LPS) molecules in its outer leaflet and glycerophospholipids in the inner leaflet [45].
92 Application of Alternative Food-Preservation Technologies to Enhance Food Safety & Stability, 2010, 92-113 Chitosan: a Polysaccharide with Antimicrobial Action Daniela Campaniello* and Maria Rosaria Corbo Department of Food Science, Faculty of Agricultural Science, University of Foggia, Italy Antonio Bevilacqua, Maria Rosaria Corbo and Milena Sinigaglia (Eds) All rights reserved - © 2010 Bentham Science Publishers Ltd. CHAPTER 7 Abstract: At present, discards from the world’s fisheries exceed 20 million tons. Traditionally, fisheries wastes are used in the production of fertilizers, fish silage or pet foods; nowadays with advances in bioprocess engineering technologies and novel enzymatic and microbial hydrolysis methods, processing wastes may serve as cheap raw materials for the generation of high-value bioactive compounds and novel environmental and ecological material derived from marine wastes. In particular, shellfish waste is the main source of biomass for chitin (and its derivatives) production. In this contest chitosan, thanks to its versatility, has found numerous applications as antimicrobial agents, antioxidants, additives, enzyme immobilization and use in the encapsulation of nutraceuticals. In addition, chitosan possesses a film-forming properties for use as edible films or coating. Several researchers studied chitosan, its chemical and physical characteristics and its applications. This chapter is an attempt to summarize these works focused on the following questions: what is chitosan? How does it act against microorganisms and what is its impact on food properties? Key-concepts: What is chitosan, How it acts against microorganisms to enhance shelf life of foods, What is its impact on food properties. BIOCHEMICAL CHARACTERISTICS, PREPARATION AND USE Chitosan is a natural polysaccharide, prepared by the alkaline deacetylation of chitin (found in fungi, arthropods and marine invertebrate); commercially, it is produced from exoskeletons of crustacean such as crab, shrimp and crawfish. Structurally chitin is a straight-chain polymer composed of β-1,4-N-acetylglucosamine (Fig. 1); chitosan, is also a straight-chain polymer composed of N-acetylglucosamine and glucosamine [1] and like the cellulose is one the most abundant natural polysaccharide on the earth [2]. Hydroxyl‐ group Amino‐ group Acetyl‐ group Figure 1: Structure of chitin, chitosan and cellulose A variety of processes have been proposed for the preparation of chitosan: generally, this biopolymer is prepared by N-deacetylation of chitin. In particular, the procedure consists of four basic steps: deproteinization (DP), deminarilization (DM), decolouration (DC) and deacetylation (DA) (Fig. 2), where DP and DM are *Address correspondence to this author Daniela Campaniello at: Department of Food Science, Faculty of Agricultural Science, University of Foggia, Italy; E-mail: d.campaniello@unifg.it
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