Abstract
The aim of this study was to evaluate the effect of water deficit on the composition of the main polysaccharides and rheological behaviour from Aloe vera (Aloe barbadensis Miller) mucilage. In particular, plants with 0 (D0), 40 (D40) and 60% (D60) water deficits were used. Water-soluble polysaccharides were isolated and subjected to carbohydrate and glycosidic linkage analysis. The steady-shear and linear oscillatory flows were studied, not only in fresh but also in reconstituted mucilages. Acemannan and pectic substances were the predominant polysaccharides in the Aloe vera mucilage, being the bioactive polymer acemannan the most affected by water deficit. Thus, increasing the water deficit by up to 60% promoted a mannose decrease of 41%, although a significant increase in its average molecular weight, from 54 to 98kDa, was detected. Interestingly, acemannan did not undergo deacetylation as a consequence of the water deficits applied. All Aloe vera mucilages, either fresh or reconstituted, exhibited a shear-thinning flow behaviour (n <1). However, water deficit affected the mechanical properties, changes being more noticeable in the reconstituted mucilages. Thus, the viscosity (η1) of reconstituted mucilages increased, from 0.12 to 0.28 Pa·s, as water deficit increased, whereas their flow index (n) decreased from 0.57 to 0.47. Further, D40 and D60 reconstituted mucilages exhibited an Ea of 17.4 and 17.6kJ/mol, respectively, whilst, for D0, Ea was 16.7kJ/mol. Interestingly, only the D40 reconstituted mucilage showed a crossover point at 7.39rad/s between viscous and elastic modulus. The understanding of the influence of water deficit on the main physico-chemical characteristics of Aloe vera polysaccharides and, in turn, of its effect on the rheology of the mucilages could be a useful tool for the design, development and control of biologically active ingredients based on the Aloe vera plant.
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