Dry heat treatments of sorghum grains: influence on the flour storage stability and porridge sensory characteristics

Abstract

Sorghum, a climate-smart crop, is an important cereal food crop in Africa with a vital role in nutrition and food security. However, utilization of its flour is limited by its susceptibility to deterioration by lipid hydrolysis and oxidation during flour storage. These chemical changes are associated with the development of rancid off-flavours. Application of dry heat treatments of the grain could be a practical approach to stabilize the flours. This study evaluated the influence of dry heat treatments (microwaving and roasting) of sorghum kernels on the storage stability of the flour as evaluated based on the sensory characteristics of porridge prepared from the flour. Firstly, the effects of microwaving (36 kJ/100 g-90 kJ/100 g) and roasting (150 ºC, 10 min-25 min) non-tannin and tannin sorghum grains on the physicochemical and functional properties of their flours (wholegrain and decorticated) were investigated. The results showed decreases in moisture content, lightness, fat acidity and pasting viscosity of the flour due to treatments at the extreme levels. With decortication, flour fat acidity was reduced further. The total phenolic content (TPC) and antioxidant activity (AOA) of flours decreased with increasing microwave energy inputs. Whereas the phenolic content of grain flours decreased significantly (p < 0.05) due to roasting of the kernels, the AOA increased. Secondly, flours of non-tannin sorghum after heat treatments (microwaving with 30 kJ/100 g and 90 kJ/100 g inputs, roasting at 150 ºC, 10 min and 25 min) were tested under accelerated storage conditions (50 ºC with UV light-16 W/cm2) for 6 weeks. The effects of heat treatments of sorghum grains on rancid-indicating parameters (fat acidity and anisidine value) of the stored flour and the texture, colour and descriptive sensory characteristics of porridge prepared from flour were investigated. Both microwaving and roasting resulted in a substantial reduction in the flour fat acidity and anisidine value throughout storage; the higher levels of both heating types being more effective. Microwaving of sorghum kernels made the porridges slightly more darker brown, with more specks, less cooked maize porridge but more intense sorghum aroma. With flour storage, porridge of control was characterized with more intense oily, fermented, rancid and painty aromas than those of the microwave-treated samples. Fat acidity of flour samples decreased due to heat treatments from 50.1 to 21.1 mgKOH/100 g at baseline (week 0). Sensory indications of porridge rancidity were identified less intensely and much later during flour storage for heat-treated samples. With roasting, flour fat acidity of roasted kernels for shorter (10 min = 48.9 mgKOH/100 g) and longer (25 min = 34.4 mgKOH/100 g) duration was substantially lower than those unroasted (50.1mg KOH/100 g). The lower fat acidity indicates greater stabilization of the flour against lipolysis and oxidative rancidity. With flour storage, porridges from unroasted non-tannin wholegrain flours had dominant painty, oily, rancid and fermented aromas while those prepared from decorticated flours displayed rancid and fermented aromas, brown colour, visible specks and residual aftertaste. Roasting sorghum kernels for 25 min and decorticating was most effective at reducing fat acidity (lipolysis) from 50.1 to 3.3mgKOH/100 g and pAV (oxidation) from 80.2 to 38.2 units. The treatment therefore has potential to improve the flour stability but had an impact on sensory quality of products made from the flour. The treatments partially inactivated the flour lipases and consequently retarded free fatty acid oxidation. The treatments had no substantial adverse effects on other flour and porridge attributes. Microwave pretreatment of sorghum grain slows the development of rancidity and improves flour shelf stability, more so at 90 kJ/100 g than 36 kJ/100 g. Microwave treatment of sorghum could be recommended over roasting and thus be an effective technology to stabilize sorghum flour and thereby enhance its food product quality. Also, roasting operation showed to be rather promising.