Identification and evaluation of the prebiotic effect of water-soluble polysaccharides extracted from green almond hull

Today, the incidence of non-communicable and emerging diseases is increasing due to lifestyle changes, reduced mobility and changing dietary patterns. Some clinical evidences in simulated samples and real cases show that some compounds and plant extracts have a significant effect on the prevention and even treatment of these diseases. On the other hand, due to the structural and functional diversity of plant polysaccharides, there is a great tendency among researchers to find new polysaccharides in different sources with new functional and bioactive properties. Despite extensive studies in this field, no study has been done on the extraction of polysaccharide compounds with prebiotic properties from green almond hull as one of the agricultural wastes. Therefore, the purpose of this study was to introduce a new type of synbiotic compound to balance clone microbiota and promote consumer health. 

In this study, after extraction of water-soluble almond hull polysaccharides (AHP) by hot water extraction and precipitation with alcohol, the chemical analysis was done. To investigate the chemical composition of AHP, phenol sulfuric acid test was used to measure total sugar and Bradford test was used to measure protein. The amount of fat and ash in the sample was measured using standard methods (AOAC, 2005) and (AOAC, 2000), respectively. The amount of uronic acid of AHP was measured by calorimetry using metahydroxyphenyl at a wavelength of 520 nm. The content of AHP phenolic compounds was investigated by Folin Siocalcu calorimetric method. Fourier transform infrared (FT-IR) was also used to identify the functional groups and the anomeric status of AHP components. The prebiotic effect of this compound was also tested by digestion resistance and also by growth stimulation of the probiotic strain of Lactobacillus casei ATCC 393 in vitro for the first time. 

Chemical analysis showed that AHP is a heteropolysaccharide consisting of 86.30% w/w of total sugar, 5.10% w/w protein and 3.21% w/w uronic acid. FT-IR analysis also confirmed the chemical structure of AHP as a heteropolysaccharide. The results of digestion resistance showed that 91.24% of AHP can remain stable and undecomposed after the stages of gastrointestinal digestion, while this rate was 74.94% for inulin as a commercial prebiotic. The second prebiotic property of AHP investigated in this study was the stimulation the growth of Lactobacillus casei ATCC 393 as probiotic in sugar-free MRS-based culture media and the results showed that AHP compared to inulin significantly increased the survival of Lactobacillus casei ATCC 393 (p <0.01). The proliferation index in media containing AHP and inulin showed a significant difference and AHP stimulated the growth of Lactobacillus casei ATCC393 significantly more than inulin (p <0.01). Therefore, to design a synbiotic product, if AHP is used as a prebiotic, the probiotic strain of Lactobacillus casei ATCC 393 would be a good choice.Considering the annual production of thousands of tons of almond green hull waste in Iran and the concerns related to environmental problems caused by its accumulation, the introduction of industrially feasible and economically justified methods to produce value-added products from this agricultural waste seems essential. In the present study, polysaccharides extracted from almond green hull by hot water extraction and alcohol precipitation, which is an economically feasible method and can be implemented on an industrial scale, were introduced as a useful compound. In vitro studies also used culture medium containing AHP as a commercial prebiotic in comparison with culture medium containing inulin. The results showed that this compound has a good resistance to digestive conditions in the gastrointestinal tract compared to inulin. The compound was also able to stimulate the growth of the probiotic Lactobacillus casei ATCC 393 in culture medium. In general, in this study, a new synbiotic compound including Lactobacillus casei ATCC 393 and AHP was introduced as a health beneficial food additive.