Effect of germination on the protein fraction composition of different lupin seeds

Abstract

Sweet lupin seeds (Lupinus luteus cv. 4486 and cv. 4492 and Lupinus angustifolius cv. troll and cv. zapaton) were germinated and investigated according to protein composition, nitrogen and amino acid content of Osborne fractions. In raw lupins, globulins (G) comprised the main fraction of lupins, followed by albumins (A) and glutelins+prolamines (Gt + P). Differences in the protein profile of the Osborne fractions were found among species whilst cultivars did not show electrophoretic differences. Amino acid content of protein fractions was also studied and differences among cultivars were found. In general, Glu, Gly, Arg and Ala (as non-essential amino acids, NEAA) and Lys (as essential amino acid, EAA) were predominant in the A fraction, Glu and Arg (NEAA) and Leu and Thr (EAA) were the main ones in the G fraction; while Asp, Glu, Gly and Arg (NEAA) and Leu and Lys (EAA) were the major components of the Gt + P fraction. Germination increased the protein content of L. luteus cv. 4486, L. angustifolius cv. troll and cv. zapaton and caused sharp changes in the protein profile of the Osborne fractions. After germination, the A fraction almost disappeared in the protein profile while G and Gt + P fractions were modified, depending on the lupin species and cultivar.

Introduction

Lupin is an economically and agriculturally valuable plant which is able to grow in different soils and climates. Interest in lupin production is increasing, not only because of its strong capacity to fix nitrogen, making accessible macro and micro elements elute to the soil sublayer (Gulewicz, Peretiatkowicz, Bratek-Wiewiórowska, & Wiewiórowski, 1993) but also because of the high protein content of lupin seeds (Duranti and Gius, 1997, Hudson, 1979, Petterson, 1998, Sujak et al., 2006). The utilization of lupin seeds can be extended to the production of protein concentrates which, added to other food products, can enrich their nutritional values and improve their technological properties, thus giving higher quality foods (Dijkstra et al., 2003, Gladstones, 1998, Torres et al., 2007b). A recent in vivo study has shown that lupin protein is a good quality ingredient, as demonstrated by the biological indices assayed (higher nutritive utilization of protein, improvement in weight gain and the food transformation index), which show that lupin is an excellent protein source for human and animal nutrition (Martínez-Villaluenga, Urbano, Porres, Frías, & Vidal-Valverde, 2007) that could replace soy concentrates in countries where soybean must be imported (Ruiz & Hove, 1976).

Lupin seed protein is considered to be a good source of lysine (Lys) and, generally, poor in the sulfur-containing amino acids (Met and Cys) (Hudson, 1979, Petterson, 1998). According to Osborne fractionation (Osborne & Campbell, 1898), lupin proteins can be divided into water-soluble albumins, salt-soluble globulins, alcohol-soluble prolamines and acid/alkali-soluble glutelins (Mandal & Mandal, 2000). The main storage proteins of lupin seeds are globulins while prolamines and glutelins are detected in small amounts (Blagrove and Gillespie, 1975, Duranti et al., 1990, Mandal and Mandal, 2000, Peretiatkowicz et al., 1988b).

The presence of antinutritional compounds, such as alkaloids and α-galactosides, in lupin seeds has played an important role in the utilization of lupin seeds as a protein source for purposes of human and animal nutrition. With the appearance of sweet (low alkaloid content) lupin varieties, the single most important constraint for the consumption of this nutritious food is the high level of α-galactosides (Frias et al., 1995, Martínez-Villaluenga et al., 2006, Price et al., 1988) which have been reported to cause several negative nutritional effects, including flatulence, osmotic effects and a reduction in protein utilization and net dietary energy (Martínez-Villaluenga, Frias, & Vidal-Valverde, in press-a).

Germination, among other technological processes, has been widely used for its ability to decrease levels of antinutritional factors present in legume seeds, at the same time improving the concentration and bioavailability of their nutrients (Ghorphade and Kadam, 1989, Urbano et al., 2005a, Vidal-Valverde et al., 2002). The extensive breakdown of seed-storage proteins that takes place during this process improves protein digestibility and the essential amino acid content, thus enhancing the nutritional value of legumes (Duranti et al., 1984, Kuo et al., 2004, Rozan et al., 2001). Less information, however, is available about the effect of germination on the profile of albumins, globulins, glutelins and prolamines, or their amino acid composition in legume seeds, which might show that lupin sprouts are an unexploited potential source of dietary protein.

The aim of this work was to study the nitrogen content, protein composition and amino acid content of Osborne fractions in different cultivars of Lupinus luteus (cv. 4486 and cv. 4492) and Lupinus angustifolius (cv. troll and cv. zapaton) in order to establish the effect of the genotype. Afterwards, the effect of germination on nitrogen content, protein composition and amino acid content of the Osborne fractions of these lupins was also studied.