Effect of cultivar and processing on anti-nutritional factors and bioaccessibility of minerals of Australian sweet lupin (Lupinus angustifolius L.)

Abstract

Australian sweet lupin (ASL) (Lupinus angustifolius L.) is an underutilised grain legume with a unique chemical composition. It contains high protein and dietary fibre and is a good source of vitamins, minerals and bioactive compounds. Due to various health benefits, there is an increasing interest in developing lupin incorporated functional foods. However, the presence of anti-nutritional factors is one of the reasons that limit commercial production of lupin based foods. Major anti-nutritional factors in lupin include raffinose family oligosaccharides (RFOs), phytate and polyphenols. Phytate and polyphenols have negative effects on the minerals bioavailability. RFOs also cause flatulence and abdominal discomfort. Published information on the anti-nutritional factors and mineral bioavailability of ASL is limited. The present study aimed at investigating the effect of cultivar, cultivation year and dehulling on mineral (calcium, iron, magnesium, potassium and zinc), anti-nutritional factors (RFOs, phytate, total phenolics, total flavonoids and condensed tannins) and mineral bioaccessibility (calcium, iron and zinc) of ASL. The relationships between minerals or anti-nutritional factors, and mineral bioaccessibility were also determined. Ten cultivars of ASL (Belara, Corumup, Gungurru, Jenabillup, Mandelup, PBA Barlock, PBA Gunyidi, Quilinock, Tanjil, and Walan 2385) cultivated at Wongan Hills Research Station in 2011, 2012 and 2013 were obtained from the Department of Agriculture and Food, Western Australia. Lupin samples were analysed for RFOs, phytate, total phenolics, total flavonoids, condensed tannins, calcium, iron, magnesium, potassium and zinc contents. Bioaccessibility of calcium, iron and zinc in heat treated lupin samples were determined using a dialysability method. The results showed that most of ASL cultivars are good sources of RFOs. Average total RFOs content in ASL (dehulled seeds) was 10.5 g/100 g DM which is higher than most of the other pulses such as black gram and mung bean. Phytate content in lupin is similar to some other pulses such as chickpea and mung bean but lower than kidney bean and soybean. ASL cultivars had low levels of total phenolics (< 100 mg GAE/100 g DM), total flavonoids (< 20 mg CE/100 g DM) and condensed tannins (< 80 mg CE/100 g DM). Dehulled seed of ASL contained relatively high amount of calcium (95 mg/100 g DM) and potassium (1120 mg/100 g DM). Iron (3 mg/100 g DM) and zinc (4 mg/100 g DM) contents in ASL (dehulled seed) were similar to some other grain legumes such as soybean and lentil. The results showed that cultivar has a significant influence on RFOs, phytate, total phenolics, total flavonoids, condensed tannins, calcium, iron, magnesium, potassium and zinc contents in lupin. Belara and Mandelup contain high levels of total RFOs and recommended for prebiotic rich functional food product development. Gungurru and PBA Barlock had low RFOs and suitable for lupin-enriched foods with low flatulence effect. PBA Barlock contained higher level of polyphenols and flavonoids than most of the other lupin cultivars. Walan 2385 has the highest condensed tannin content and high in flavonoids contents. The highest iron (3.2 mg/100 g DM) and zinc (3.8 mg/100 g DM) contents were found in Belara and Quilinock, respectively. Belara and Quilinock also contain high calcium contents. These findings on the effect of cultivar on anti-nutritional factors and minerals are helpful in selecting suitable cultivars for particular food applications. Lupin flour for food applications is commercially produced by the dry dehulling technique. Lupin flour can be incorporated into various foods. The effect of dry dehulling on iron, magnesium, zinc, RFOs, phytate, total phenolic and total flavonoid contents depends on the lupin cultivar. Dehulling increases condensed tannin and potassium contents in most lupin cultivars. Calcium contents in all lupin cultivars were reduced as a result of dehulling. Average calcium, iron and zinc bioaccessibility values of ASL (dehulled seeds) were 11, 21 and 12%. Calcium and zinc bioaccessibility values of lupin were poor and lower than some other grain legumes such as cowpea and mung bean. The low calcium and zinc bioaccessibility values indicate that these minerals may form large complexes which are difficult to be digested by the human digestive system. In contrast, iron bioaccessibility of ASL is higher than values reported for other pulses including red grams and black grams. Cultivar had a significant effect on calcium and iron bioaccessibility of lupin but had no effect on zinc bioaccessibility. PBA Gunyidi had higher calcium bioaccessibility than some of the other cultivars. High iron bioaccessibility values were found in Gungurru and Mandelup. Calcium bioaccessibility of most of the lupin cultivars were increased after dehulling. Phytate to calcium molar ratios of dehulled lupin samples present poor calcium bioavailability. All tested lupin cultivars had high phytate to iron molar ratios indicating poor iron bioavailability. High phytate to zinc molar ratios were recorded in almost all of the lupin samples implying poor zinc bioavailability. Although the phytate to mineral molar ratios are widely used as predictors of mineral bioavailability, the result of the study shows that the phytate to mineral molar ratio is not correlated to the mineral bioaccessibility of lupin. Therefore, the phytate to mineral molar ratio may be not a suitable predictor of the mineral bioaccessibility of lupin. Bioaccessibility was not directly related to calcium, iron or zinc contents of lupin which shows that high mineral content is not always related to high mineral bioaccessibility. Calcium content was negatively correlated to bioaccessibility of calcium of lupin. Results also showed negative trend between iron content and iron bioaccessibility as well as zinc content and zinc bioaccessibility. Poor correlations between minerals (calcium, iron and zinc) bioaccessibility values and anti-nutritional factors (RFOs, phytate, total phenolics, total flavonoids and condensed tannins) were found. This finding indicates that the anti-nutritional factors studied are not likely to significantly affect the minerals bioaccessibility of lupin. A stepwise multiple regressions were performed to develop predictive equations to predict mineral bioaccessibility using the minerals and anti-nutritional factors contents. A predictive equation using calcium, phytate and RFOs contents can predict 69% of the calcium bioaccessibility of lupin. Iron, calcium, RFOs and polyphenols contents can be used to predict 70% of the iron bioaccessibility of lupin. A regression equation using zinc, calcium and condensed tannin contents can estimate 59% of zinc bioaccessibility of lupin. These predictive equations indicate that there are other factors affecting the minerals bioaccessibility of lupin in addition to the factors studied