Anti-nutritional factors of leguminous crops: qualitative and quantitative analysis of tannins in lentils and their processed products

The nutritional value of legumes is significantly reduced due to the presence of anti-nutritional substances in them. Lentils also contain a certain amount of antinutrients. Tannins form complexes with proteins, which causes inactivation of many digestive enzymes and reduces protein digestibility. Various traditional processing methods are used to increase the nutritional value of products. The presence of tannins was determined by qualitative and quantitative methods in lentils of two different varieties of KDC "Kermit" and "Orlovskaya Krasnozernaya", as well as their processed products (lentil extrudates, dried lentils, sprouted lentils, lentil extracts (aquafaba)). In this work, the method of precipitation of proteins with a gelatin solution was used for the qualitative determination of tannins. The presence of tannins in native lentils is most pronounced, less pronounced properties were shown in samples of dried lentils, sprouted lentils and lentil extract had a negative reaction with gelatin, which indicates the absence of tannins or their insignificant amount in these semi-finished products. To determine the group affiliation of tannins, a qualitative reaction of the obtained filtered extracts with trivalent iron salts was carried out. A qualitative reaction in a sample of native lentils showed a predominant amount of tannins of the condensed group due to a weak black-green staining. In the sample of lentil extrudate, the same reaction was slightly less, and in the samples of dried, sprouted lentils, and lentil extract, no reaction occurred, which also indicates the absence of tannins or their insignificant amount. The quantitative content of tannins in the studied samples was assessed by photoelectrocolorimetry. The results showed that the most effective method of reducing the concentration of tannins is 4.1–3.4 times when germinating lentils. The process of lentil extrusion helps to reduce tannins by 1.68–2.5 times. Upon receipt of the lentil suspension (aquafabs), the tannin content decreased by 1.5–1.93 times (in terms of an absolutely dry substance).