Executive Summary: Separation of polyphenols using proteins

Summary of Project and Key Findings:
Onion waste is produced during grading, and processing. Onion waste includes its skin, unwanted parts (tops and bottoms) and rejected bulbs. The extraction of useful components from onion waste has been widely studied. Previous studies report that there are components in onions, such as polyphenols and fibre, which can provide health benefits. The flavonoid quercetin compounds are a class of polyphenols found in onion skins which are reported to support health and wellness and are a potential ingredient for functional foods.

Research on flavonoid-protein interactions has focused on total antioxidant capacity of the flavonoid-protein complex and the preparation of bio-polymeric colloidal particles. However, the use of proteins as a means to separate and recover polyphenols has not been reported yet. The objectives of this preliminary study were to investigate the use of proteins to adsorb flavonoids from onion waste extracts, and to optimise the the protein’s flavonoid-binding capacity by changing pH and the ionic strength of sodium chloride (NaCl). The proteins evaluated were bovine serum albumin (BSA), calcium caseinate, and skimmed milk powder, which contains a mixture of milk proteins.

Fluorescence spectrometry was used as a qualitative method to investigate the binding of BSA with quercetin. The results show that the fluorescence emission intensity of quercetin solutions increases when BSA is added, indicating the the formation of protein-quercetin complexes.

The capacities of the three proteins to bind quercetin alglycone, quercetin-3,4′-O-diglucoside and quercetin-4’-O-glucoside extracted from onion waste, was quantified by using utrafiltration and HPLC techniques. The results showed that for the three proteins studied, pH and the presence of NaCl were important factors in binding of polyphenols from onion skin extract.