Extraction and Fermentation of Sugars from Almond Hull to Produce Astaxanthin

Abstract

Almond hull is one of major byproducts of almond industry. It is rich in fermentable sugars which can be extracted for value-added applications. This study optimized the fermentable sugar extraction from almond hull by using response surface methodology with Box-Behnken design to optimize the operating parameters, including temperature (30, 55, and 80°C), shaking speed (200, 400, and 600 rpm), and extraction time (10, 65, and 120 min). The results showed that temperature is the most significant factor and the maximum total fermentable sugar yield of 36 ± 0.1 g/L was achieved under the conditions of 120 min, 80°C and 600 rpm for at 10% (dry weight basis) almond hull loading. A second-degree polynomial model can accurately predict the TFS extraction (R2=0.71). The extracted sugars (i.e., almond hull extract - AHE) were fermented into a high-value antioxidant, astaxanthin by using a yeast, Xanthophyllomyces dendrorhous while the effect of the type of nitrogen source on astaxanthin production was investigated, including yeast extract/peptone, ammonium chloride, ammonium sulfate, and urea. Amongst, yeast extract/peptone with the synthetic AHE and ammonium chloride with the original AHE achieved the maximum astaxanthin concentration of 3.31 ± 0.22 mg/L and 2.05 ± 0.27 mg/L, respectively. Selecting proper nitrogen sources (e.g., ammonium chloride) could mitigate the inhibition on astaxanthin fermentation of the original AHE fermentation. Astaxanthin production is a potential value-added outlet for almond hulls and a profit opportunity for the almond industry.