Effects of different drying methods on the physicochemical properties and antioxidant activities of isolated acorn polysaccharides

Abstract

Our earlier studies showed that the Acorn Polysaccharides (AP), as a forest byproduct, have a good prebiotic properties and antioxidant activity, hence can be used as an ingredient to produce functional foods. Three drying methods (freeze, hot air and vacuum drying) in different temperatures were comparatively studied on the physicochemical properties (solubility, water and oil-holding capacity [OHC/WHC]), bioactivity (resistance to acidic and enzymatic digestions, effect on a probiotic strain growth) and antioxidant activity of AP along with the structural changes. Results suggest that the drying methods in combinations of temperatures and time of drying process affect physicochemical properties, antioxidant activity and bioactivities of AP. Freeze dried AP exhibited the highest solubility, WHC, OHC and antioxidant activity, digestibility with simulated gastrointestinal juices and fermentable by a Lactobacillus plantarum. Whereas, hot air dried (80 °C) exhibited second highest antioxidant and functional activities like solubility, WHC, OHC and fermentation. FTIR analysis showed that the changes caused by varying drying methods of AP starch are related to its amorphous or crystallinity structure and differences in functional group. Overall, these results suggest that freeze drying and hot air drying at 80 °C can be appropriately use to obtain a functional polysaccharide from acorn, as a prebiotic (resistant starch).

Keywords: Acorn; Drying; Functional properties; Polysaccharide; Prebiotic.

Fig. 1.

The solubility of acorn polysaccharide dried with different drying methods. Values presented in bars are the average of triplicate experiments and error bars represent SD. The column not followed by the same letter are significantly different with p < 0.05 level of significance, according to LSD Test.

Fig. 2.

Water holding capacity (WHC) of acorn polysaccharide dried with different drying methods. Values presented in bars are the average of triplicate experiments and error bars represent SD. The column not followed by the same letter are significantly different with p < 0.05 level of significance, according to LSD Test.

Fig. 3.

Oil holding capacity (OHC) of acorn polysaccharide dried with different drying methods. Values presented in bars are the average of triplicate experiments and error bars represent SD. The column not followed by the same letter are significantly different with p < 0.05 level of significance, according to LSD Test.

Fig. 4.

a FTIR spectrum of acorn polysaccharide dried with different drying methods: Before drying; AP-F: Freeze drying; AP-H 100 °C: Air drying 100 °C; AP-H 80 °C: Air drying 80 °C; AP-H 60 °C: Air drying 60 °C; AP-H room tempt: Air drying in room temperature; AP-V 40 °C: Vacuum drying 40 °C; AP-V 60 °C: Vacuum drying 60 °C. b. FTIR-ATR spectra of APP samples dried in different condition: A) Freeze dried, B) Air dried sample in 100 °C, C) Air dried sample in 80 °C, D) Air dried sample in 60 °C, E) Air dried sample in room temperature, F) Vacuum dried sample in 60 °C, G) Vacuum dried sample in 40 °C. Data have been offset for simplicity. c. Curve fit with band combination for the second-order FTIR spectra of freeze dried sample.

Fig. 5.

Scanning electron micrographs of AP dried by different methods. (A) Freeze drying: A1: × 125; A2: × 250; A3: × 500. (B) Air drying in room temperature: B1: × 65; B2: × 125; B3: × 250. (C) Air drying 60 °C: C1: × 62; C2: × 125; C3: × 250. (D) Air drying 80 °C: D1: × 65; D2: × 125; D3: × 250. (E) Air drying 100 °C: E1: × 31; E2: × 62; E3: × 250. (F) Vacuum drying 40 °C: F1: × 32; F2: × 62; F3: × 250. (G) Vacuum drying 60 °C: G1; × 65; G2: × 125; G3: × 125.

Fig. 6.

Antioxidant activity of acorn polysaccharide dried with different drying methods compared to vitamin C as a positive control. Values presented in bars are the average of triplicate experiments and error bars represent SD. The column not followed by the same letter are significantly different with p < 0.05 level of significance, according to LSD Test.

Fig. 7.

Hydrolysis degree of acorn polysaccharide dried with different methods to simulated gastrointestinal digestion conditions in comparison to Inulin (a commercial prebiotic) as a positive control. Step 1: Simulated gastric fluid, pH 1.2 (SGF). Step 2: Mixture of simulated gastric and intestinal fluid, pH 4.5 (SMF). Step 3: Simulated intestinal fluid, pH 7.4 (SIF) and α-amylase. Values presented in bars are the average of triplicate experiments and error bars represent SD. The column not followed by the same letter are significantly different in the same digestion step with p < 0.05 level of significance, according to LSD Test.