Structural and Thermal Characterization of Protein Isolates from Australian Lupin Varieties as Affected by Processing Conditions

Abstract

Proteins from the full and defatted flours of L. angustifolius cv Jurien and L. albus cv Murringo were prepared using alkaline extraction and iso-electric precipitation. Isolates were either freeze dried or spray dried or pasteurized at 75 ± 3 °C/5 min before freeze-drying. Various structural properties were investigated to elucidate the varietal and processing-induced effect on molecular and secondary structure. Irrespective of processing, isolated proteins had a similar molecular size, with α-conglutin (412 kDa) and β-conglutin (210 kDa) being principal fractions for the albus and angustifolius variety, respectively. Smaller peptide fragments were observed for the pasteurized and spray dried samples, indicating some degree of processing-induced changes. Furthermore, secondary structure characterization by Fourier-transform-infrared and circular dichroism spectroscopy showed β-sheet and α-helical structure being the dominant structure, respectively. Thermal characterization showed two denaturation peaks corresponding to β-conglutin (Td = 85–89 °C) and α-conglutin (Td = 102–105 °C) fractions. However, the enthalpy values for α-conglutin denaturation were significantly higher for albus species, which corroborates well with higher amounts of heat stable α-conglutin present. Amino acid profile was similar for all samples with limiting sulphur amino acid. In summary, commercial processing conditions did not have a profound effect on the various structural properties of lupin protein isolates, and properties were mainly determined by varietal differences.

Techno‐functional and rheological characterisation of protein isolates from two Australian lupin species as affected by processing conditions

“…Detailed protein extraction process and proximate analysis is presented elsewhere (Devkota et al ., 2023). The proximate composition of protein isolates obtained with processing regimes is presented in Table S1.…”

“…Based on these values, albus variety had better gelling behaviour compared to angustifolius varieties; this can be linked to higher amount of β‐conglutin (vicilin like) protein fraction present in albus lupins. As denaturation is a prerequisite to gelation, this protein fraction has lower denaturation temperature thus facilitating better gelation (Shevkani et al ., 2015; Devkota et al ., 2023). Processing showed effect on rheological behaviour, the most notable being lower gelation temperatures and lower G ′ indicating lower gel strength for pasteurised samples compared to the direct freeze‐dried.…”

“…Protein extraction, isolation and processing were done following our previous work (Devkota et al ., 2023). A brief protein extraction process is presented in the Data S1.…”

“…Notably, the two most prevalent species, Lupinus angustifolius (narrow‐leaf lupin) and Lupinus albus (white lupin), exhibit protein content ranging from 30% to 40%. Additionally, their protein compositions diverge, with L. angustifolius being rich in legumin‐like α‐conglutin and L. albus enriched in vicilin‐like β‐conglutin (Duranti et al ., 2008; Devkota et al ., 2023). Researchers have recently begun to explore lupins as a novel plant protein source due to their high protein content (dehulled lupin: 40%–50%, dehulled soy: 50%, dehulled pea: 20%–25%, and wheat: 12%–13%; Bähr et al ., 2014) and protein quality like that of soy amid low allergenicity and non‐GMO status (Fudiyansyah et al ., 2007).…”

“…In a previous analysis, we examined the impact of various processing conditions, including direct freeze‐drying, spray‐drying, and high‐temperature short‐time pasteurisation (75 ± 3 °C/5 min)—on the structural properties of lupin protein isolates derived through alkaline extraction and isoelectric precipitation from two low‐alkaloid Australian lupin species (Devkota et al ., 2023). Our findings revealed minimal changes in protein structure due to processing, with varietal differences playing a prominent role.…”