Study of the effect of technological processes on starch hydrolysis, non-starch polysaccharides solubilization and physicochemical properties of different ingredients using a two-step in vitro system

Abstract

The present study evaluated in vitro the effect of technological processes on four cereal grains: maize, wheat, barley and oats and one legume grain: green seeded pea. Processing included extrusion, cooking and grinding (0.8 and 3.0 mm opening), and the parameters evaluated were starch hydrolysis, content of soluble non-starch polysaccharides (NSP) and physicochemical properties: solubility of dry matter (DM), viscosity of the supernatant and water retention capacity (WRC). All samples were submitted to an in vitro digestion that consisted of two steps, gastric (G; pepsin; pH 2; 4 h) and small intestine (SI; pancreatin; pH 7; 4 h) simulation. Prior to incubation, particle size was evaluated in all samples by hand sieving and calculated on a weight basis. Total starch and total NSP analyses were performed on raw samples (0.8 mm). Hydrolysis of starch was evaluated by measuring glucose in the supernatant during the course of SI incubation. Soluble NSP were evaluated in the supernatant at the end of SI incubation. Physicochemical properties were evaluated after G and SI incubation. In raw ingredients (0.8 mm) the hydrolysis of starch at the end of SI incubation (ratio between glucose released in the sample and glucose released in the sample gelatinized) was lower in peas (0.45) than in cereal grains (0.69 in maize; 0.95 in wheat; 0.82 in barley and 0.92 in oats). The highest amounts of soluble NSP were found in barley (19.8 g/kg DM) and oats (13.8 g/kg DM), and were associated with higher viscosities. The WRC in raw samples was positively related to the amount of total NSP after both G ( r 2 = 0.847, P<0.001 for 0.8 mm samples) and SI incubation ( r 2 = 0.773, P<0.001). Technological processing increased starch hydrolysis and solubilization of NSP in all ingredients. The extent of the effect was dependent on the ingredient as well as on the severity of the technological process. Component monosaccharides of soluble NSP were differently affected, while glucose was increased by all the processes, arabinose and xylose required the more extreme conditions of extrusion. Contrary to raw samples, WRC of heat treated ingredients showed a negative relation with total NSP content of the sample after G incubation, which turned out to be a positive relation after SI incubation. It is concluded that technological processing of ingredients promotes higher starch hydrolysis, increases in the amount of soluble NSP and modifications in the physicochemical properties depending on the nature of the feed ingredients.