EFFECT OF ENDOGENOUS TANNINS ON LIPID OXIDATION IN OIL-INWATER EMULSIONS STABILIZED BY SORGHUM PROTEIN INGREDIENTS

International audience

English. Plant-based foods are highlighted nowadays driven by the perceived health and environmental benefits, in line with the “clean label” tendency. Sorghum is a versatile grain and has the ability to adapt to dry and hot climates, being a good natural alternative for use in foods. Some sorghum cultivars present tannins in their composition and, although these molecules can bind proteins, making them considered antinutritional factors, their presence could inhibit or delay the oxidation of polyunsaturated lipids, a major deterioration process in multiphase food systems. The oxidative stability of model systems such as oil-in-water (O/W) emulsions is thus a research topic of great importance, but the relationship between lipid oxidation and the effect of endogenous polyphenols present in plant protein extracts used to stabilize such emulsions remains unclear. In this context, this work aimed to identify and quantify the flavanols present in sorghum proteins extracted from tannin-rich cultivars and their effect on lipid oxidation in O/W emulsions in comparison to tannin-free ones. Simple polyphenols were extracted from freeze-dried sorghum protein extracts using acidified methanol, and analyzed by HPLC. Quantification of total flavanols, including condensed tannins (i.e. procyanidin oligomers and polymers), and the calculation of their average degree of polymerization (DPn) were performed after acidolysis in the presence of phloroglucinol and analysis of the reaction media by UHPLC. The formation of primary (conjugated dienes and hydroperoxides) and secondary (thiobarbituric reactive substances, including malondialdehyde) oxidation products, and the tryptophan fluorescence were monitored during incubation of emulsions at 40 °C in the presence of iron. Total polyphenol concentrations in sorghum protein extracts amounted to 1317 and 2011 mg/100 g of dry powder for tannin-rich samples BRS305(T+) and SC782(T+), respectively. In contrast, polyphenols were almost totally absent in tannin-free samples BR501(T-) and BRS310(T-). Flavanol monomers (i.e. (+)-catechin and (–)-epicatechin) and some procyanidin dimers were clearly detected in BRS305(T+) and SC782(T+) samples. However, the main polyphenolic constituents in these samples were procyanidins oligomers and polymers presenting a DPn of 13.0 for BRS305(T+) and a lower DPn value (10.6) for SC782(T+), the later showing higher concentrations of procyanidins and flavanols. For both samples, (+)-catechin and (–)-epicatechin were observed as terminal and extension subunits, in which (–)-epicatechin was the most abundant form as extension subunit, whereas (+)-catechin was the most abundant form as terminal subunit. The oxidative stability of the emulsions formulated with protein extracts rich in tannins (BRS305(T+) and SC782(T+)) was largely superior compared to that of emulsions made with the tannin-free ones, showing that the presence of endogenous tannins greatly prevents lipid oxidation. These results reveal a strong relationship between the oxidative stability of O/W emulsions and the presence of endogenous polyphenols in sorghum protein extracts. Such results highlight the great potential of tannins as natural antioxidants, even when complexed with proteins