Quantification of “cholesterol lowering” soyasaponins I and βg in raw and cooked legumes by an in vitro digestion model

Legumes represent an important component of human diet in several areas of the world, especially in the developing countries where they complement the lack of proteins from cereals [1]. They are rich in protein, low in saturated fat, and high in complex carbohydrates and fiber. They are also a good source of several micronutrients and phytochemicals [2]. In fact, legumes are a primary dietary source of food saponins, bioactive compounds that have been demonstrated to possess multiple health-promoting properties such as lowering of cholesterol level. Although the mechanism was not completely clarified, a proposed theory was based on the formation of complexes between SSs and cholesterol in the gut lumen, and thus the inhibition of the molecule absorption. Legumes mainly contain soyasaponin I (soyasaponin βb) and soyasaponin βg (also called soyasaponin VI), both belonging to the B group soyasaponins. The aim of this work was the quantification of SS I and SS βg in raw and cooked legumes of difference provenience and variety (60 samples) by a SPE-HPLC-MS method developed in our lab [3]. Results of this study showed that the 60 raw legume samples analyzed had a content of soyasaponin I that ranged from 636 to 907 mg kg-1, while that of soyasaponin βg ranged from 647 to 1807 mg kg-1. Soybean contains the highest level of soyasaponins, and also pea and chickpea could be considered a great source of “cholesterol lowering” soyasaponins. Additionally, it is important also to understand absorption and metabolic process of soyasaponins in the body. In fact, legumes normally used in human nutrition need to be processed prior to consumption to reduce their levels of anti-nutritional factors. So, six legume samples were chosen for cooking studies for the investigation of the amount of soyasaponins that persists after cooking. Changes in the soyasaponins content were investigated for the first time in chickpea, lentil, bean, pea and chuckling after the seeds were soaked and cooked in distilled water. Cooking did not modify the total soyasaponins content but soyasaponin βg was partially degraded during cooking to soyasaponin I. Moreover, a small amount of soyasaponins leached into the cooking solution, and the percentage was depending on the legumes analyzed. Moreover, a study on the bioaccessibility of SS I and βg contained in lentils was determined by using an in vitro digestion model in such a way to understand the amount that can reach the duodenum and colon and that can explicate the cholesterol lowering activity.