Degree of maturity and dry-heat processing affect visual quality and volatile profile of roasted immature rice grains

Abstract

Foods from unripe cereals, such as pepeta, i.e., a flakes prepared in Tanzania from immature rice, are popular. However, factors determining sensory quality of these immature rice flakes are inadequately characterised. In this study, grain size, colour, and volatile compounds of immature rice destined for pepeta production were investigated at different maturity stages (i.e., dough-grain, mature-grain, fully-ripe, and over-ripe) and processing conditions (i.e., roasting at 80, 100 and 120 °C, with and without prior to water-soaking for 12 h at room temperature). The highest brightness, lowest redness and yellowness values in pepeta resulted from the pounding process. About 53 volatile compounds were identified in Lawama and TXD306 variety; their concentrations decreased as grain matured except for Lawama at the mature-grain stage. Odour-active compounds such as 2-isopropyl-5-methyl-1-heptanol, butyl acetate, and butyl propionate were formed upon processing into pepeta, whereas abundant 2-pentyl-furan and 2-methoxy-4-vinyl phenol significantly increased with roasting temperature. No characteristic volatile pattern was observed to differentiate pepeta from other processed rice products. Soaking before roasting had no significant (p < 0.05) effect on grain size, colour, or volatile compounds. Degree of maturity and roasting temperature both had impact on colour and volatile compounds, and subsequent sensory quality of the pepeta product.