Alkaline extraction of starch from Australian lentil cultivars Matilda and Digger optimised for starch yield and starch and protein quality

The Australian lentils, Lens culinaris: Matilda (Green Lentil) and Digger (Red Lentil) contain 44–45% starch (HPLC method) and 30–33% protein (LECO Analyser). This paper describes, first, the identification of optimum starch extraction conditions where yield is high and starch and protein damage is acceptable and, second, the properties of the starch and protein recovered. Starch was extracted from the flour of each variety with water at four temperatures (ambient 22, 30, 35 and 40 °C) and at five pH conditions (distilled water and pH adjusted with NaOH to 8, 8.5, 9.0 and 9.5). Extraction at pH 9.5, for all temperatures, achieved the maximum starch yield (85–95%) for both varieties. Digger gave a lower starch yield than Matilda when extracted in distilled water at all four temperatures. The yields of Digger starch were not significantly different at the four alkaline pHs. Matilda flour showed gradual increase in % starch yield with increase in extraction temperatures and pH (significant to 95% confidence level). Protein yield achieved, from both Digger and Matilda flour, was relatively low: 43–60% of the analysed protein content for Digger and 48–63% for Matilda. No significant difference was observed for extracted Digger protein at the various extraction pHs and temperatures. Matilda protein yields were significantly different between the various extraction conditions. The % starch damage was high for both varieties when extracted at higher temperature and pH. The DSC ΔH value increased with increasing pH and temperature. Extraction at higher pH resulted in a smoother and more symmetrical peak, denoting the absence of protein adhering to the starch surface. Even though low pH and low temperature caused less starch damage, these conditions were undesirable because they resulted in lower starch and protein yields. Taking all factors into account, pH 9.0 at 30 °C was chosen as an optimum extraction condition for Matilda while pH 8.5 at 35 °C was chosen for Digger.