ARTICLE 56 FEED & ADDITIVE MAGAZINE January 2024 polysaccharides) or oligosaccharides of galactose, fructose or mannose that selectively stimulate the growth or activity of one or a limited number of bacterial species, already present in the gut and thus improve host health and act as “prebiotics” (promote the growth of beneficial microbes). A probiotic i.e., a live microbial feed supplement which beneficially affects the host animal by improving its intestinal balance can be directly given along with feed, however, the probiotics strains can remain dominant in the gastrointestinal tract only during the dietary treatment. Such an exogenous addition of a single probiotic will result in long-term colonisation of the gut, especially when the strains used do not belong to the normal dominant intestinal flora. In those cases, the stimulation of specific indigenous microflora by supplementing fish feed with indigestible carbohydrates that act as prebiotic is more fruitful. Indigestible carbohydrates could either be given to fish indirectly as NSP supplemented with exogenous enzymes in the feeds or included in the feed as prebiotics. NON-STARCH POLYSACCHARIDES (NSPs) Non-starch polysaccharides are long polymeric carbohydrate chains containing up to several hundred thousand monomeric units, excluding α-glucans (starch). NSPs in aquaculture feeds are present as an integrated part of the cell wall (upto 90% NSPs) of plant ingredients and also in a purified soluble form, such as guar gum, to stabilise the pellet. The most abundant plant cell wall NSPs include cellulose, hemicellulose and pectins; while fructans (inulin), glucomannans and galactomannans are not so abundant as of those aforesaid and serves as the storage polysaccharides within the plants. Mucilages, alginates, exudate gums, β-glucans and various modified polysaccharides are other constituents of the NSPs. The main difference between NSPs and starch is that, starch is composed entirely of glucose monomers, which are linked by α-glycosidic bonds while NSPs are composed of different kinds of monomers, which are linked predominantly by β-glycosidic bond. The difference in bonding structure has profound effects on digestibility, as different classes of enzymes are required for the hydrolysis of α- and β-glycosidic bond. The predominant starch digestive enzymes are α-amylase, α-glucosidase and oligo-1-6-glucosidase. In combination, these enzymes specifically hydrolyse the α-glycoside bonds of starch to yield glucose. On the other hand, the enzymes required to digest NSP, such as β-glucanase and β-xylanases, are very scarce or even absent among fish species. The physiological impact of individual NSPs is dependent on the sugar residues present and nature of the linkage present between these residues. Classification of NSPs NSPs are classified into three main groups: • Cellulose, • Non-cellulosic polymers and • Pectic polysaccharides.
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