The Potential of Tamarind (Tamarindus indica L.) Kernels as a Substitute Source of Pectic Substances

Abstract

The present study was designed to investigate the physicochemical properties, of extracts from tamarind (Tamarindus indica) kernels and to explore their potential of being a promising substitute for pectin. Kernels from two samples, Light Brown (LB) and Dark Brown (DB), were used for extraction. The proximate composition of tamarind kernel including moisture, protein, oil, ash, fiber and carbohydrate was initially determined. Carbohydrate was found to be the major component comprising 59.30% and 57.33% in LB and DB kernel investigated, respectively. The aquous extract yielded 34% and 29%, referred to as a polysaccharide, from LB and DB kernels respectively. The physical properties of the extracts were determined and compared to those of commercial pectin. The colour of the LB sample was light creamy resembling the colour of pectin, while that of the DB sample was dark creamy. The polysaccharides from both samples were completely soluble in hot water similar to pectin. The pH-value of LB and DB kernel polysaccharides were 5.48 and 5.49 respectively while that of commercial pectin was 3.50. Thus the pH values suited the gel formation requirements. The intrinsic viscosities of LB and DB samples were found to be 4.62 dL/g and 4.42 dl/g respectively while that of commercial pectin was 3.80 dL/g. The molecular weights and equivalent weights of LB and DB extracts were similar for each, but higher than those for commercial pectin. Similar values of 1.334 as refractive indices were obtained for both kernels extracts and commercial pectin. The polysaccharides were found to be the major components in the extracts as 88.85% for LB, 85.21% for DB and 92.43% in the commercail pectin. The protein levels in LB and DB extracts were higher (3.90% and 6.50%, respectively) than that of commercial pectin (1.24%). A significant difference (P≥0.05) was observed in moisture, protein, ash and carbohydrate levels between the LB kernel and DB kernel extracts. Insignificant difference was observed in the oil and fiber contents of the two sample extracts. The functional xii properties of both extracts as gelling agent indicated that both samples contained residual amount of uronic acid, without galacturonic acid and acetyl group with small amount of methoxyl group, and high value of degree of estrification. The commercial pectin, however, contained a high level of uronic acid and relatively low degree of estrification. Individual sugars analyized by HPLC showed that LB and DB kernel extracts contained pentose sugar:xylose (16.0%, 16.6%) and arbinose (1.5%, 1.3%) and hexose sugars glucose (50.0%, 49.9%) and galactose (28.0%, 33.2%), respectively. The molar ratio of these sugars was found to be (2:1:3:1) in both extracts. The commercial pectin contains similar sugars but with fructose instead of xylose. The tamarind kernel extracts were able to form gels over a wide pH range in the presence of sucrose (with or without acid and base), while commercial pectin form gels over a narrow pH range (acidic) in the presence of sucrose. The maximum gel strength was obtained using concentration of 0.75% tamarind kernel extracts and commercial pectin, 65-70% sucrose concentration and 0.80% citric acid at pH range from 2.50-2.75. A significant difference (P≥0.05) was observed in the gel setting rate of gels made from the three gelling agents. The organoleptic evaluation of the gels showed that there were insignificant differences in appearance, texture, taste, and flavour under all conditions investigated for LB tamarind polysaccharide and commercial pectin. However, the appearance, taste, and flavour of the gels formed by DB tamarind polysaccharide under the base condition were significantly different. Although the gels made from commercial pectin obtained the higher panaleists score (9.836) compared to that made from LB and DB polysaccharide (9.756, 9.701, respectively), the statistical analysis showed insignificant difference. Due to the good gelling properties of both extracts compared to commercial pectin they were used as gelling agents for the preparation of pumpkin jam. The chemical compositions, microbiological analysis, and sensory evaluation for jams prepared by both tamarind kernel polysaccharides were compared to jam made from pectin. The results indicated that the pumpkin jams xiii made by the addition of tamarind kernel polysaccharides instead of pectin were highly acceptable and met the specification of Sudanese Standard for jams by SSMO. It was concluded that tamarind kernels, considered as a waste, can be converted into a useful by-product, which can be used as a promising substitute source of pectin.