The Effect of Alcohol Washing on the Flavour Profiles, Functionality and Digestibility of Dry Processed Pea Protein Fractions

Abstract

In this research, the potential of aqueous-alcoholic solvent washing on removing off-flavours in air-classified pea protein enriched flour (PPEF) was investigated. PPEF was treated with aqueous ethanol or aqueous isopropanol at three different concentrations (20%, 50%, and 80%) to remove the unpleasant beany, earthy and astringent flavours. Headspace solid phase microextraction followed by GC-MS was used to identify the flavour compounds in untreated and treated PPEF. Besides the flavour profile, changes to their proximate composition, colour, functionality and protein quality were compared among untreated and treated samples. There were 28 volatile compounds extracted from untreated PPEF. Total peak area of the compounds extracted from untreated PPEF was 261×106 with 64% contributed by aldehydes and 30% from alcohols. Most alcohol treatments reduced the total volatile compounds extracted of PPEF expect for the 20% isopropanol treatment. The level of flavour compound reduction rose with an increase in solvent polarity. Higher concentrations of ethanol and isopropanol (50% and 80%) showed greater effectiveness in removing flavour compounds by reducing the total peak area by 82%–94%, whereas solvents at the lowest concentration (20%) were less efficient. Treatment with 80% isopropanol resulted in the removal of 20 compounds from PPEF to below the detection level, followed by 50% isopropanol, 80% ethanol and 50% ethanol. However, there was no significant difference among the 50% and 80% treatments in the total peak area extracted. The protein contents of all treated samples (58.2%–64.3% d.b.) increased compared to untreated PPEF (55.5% d.b.) due to the decrease in ash, lipid and carbohydrate content. All treated samples were significantly darker in colour compared to the untreated sample. The surface charge of the protein in the PPEF decreased after alcohol treatment. Also, the protein solubility was negatively affected by solvent treatment, i.e. reduced from 85% to 21%–52%, with isopropanol treatments and lower alcohol concentration treatments having greater negative impact on the solubility. Water hydration capacity was positively affected by alcohol washing, while oil holding capacity was negatively affected. Emulsion stability of PPEF was maintained in all isopropanol- treated samples but reduced in 50% and 80% aqueous-ethanol-treated samples. Although in vitro protein digestibility was improved with the solvent treatments, from 79% to 82%–85%, the amino acid scores became lower with the treatments. Amino acid scored dropped from 0.9 in untreated PPEF to 0.66–0.84, due to the decrease in methionine and cystine in the treated PPEF. The more significant drop in amino acid score compared to the rise in protein digestibility resulted in lower in vitro protein digestibility corrected amino acid scores (PDCAAS). Both aqueous ethanol and aqueous isopropanol at 50% and 80% concentration were proved to be effective in removing flavour compounds with PPEF with remarkable modification of the chemical composition, protein functionality and protein quality in both positive and negative way. There is potential to use aqueous alcohol to treat pea protein enriched flour to obtain a functional high protein pulse ingredient with milder volatile flavour profile.