The Potential of Sweet Sorghum [Sorghum Bicolor (L.) Moench] As A Bio- Resource for Syrup and Ethanol Production in Kenya

Abstract

Sweet sorghums (SS) (Sorghum bicolor L. Moench) are sorghum varieties that accumulate sugar (>8°Brix) in the stalks and produces grain. The SS crop is a multifunctional crop that can be cultivated for simultaneous production of grain for food or feed and utilization of juice from stalk in production of value-added products like syrup and ethanol. Demographic growth, diminishing arable lands, food insecurity, and climate change resulting from fossil fuels, are issues that demands attention. Sweet sorghum has shown potential to provide, food, fuel (ethanol), feed and fiber hence the need to explore its adaptability in Kenya. The objective of this study was to screen high sugar yielding SS varieties, determine their optimum harvesting time, extract the stalk juice and characterize the syrup and ethanol produced from the SS juice. Twenty five SS varieties and Mtama 1 were cultivated and evaluated in two locations, JKUAT and Awendo, Rongo District. SS stalks were harvested at three different maturity stages with an aim of obtaining the optimum harvesting time, then stripped, weighed and crushed to extract SS juice, which later was used for analysis and production of syrup (>70 °Brix) and ethanol. Standard methods were used for analysis. Total soluble solids were analyzed by refractometry, total sugar by phenol-sulfuric acid method while sucrose, glucose and fructose content by HPLC method. The total titratable acidity was determined by titration against 0.1N NaOH to a phenolphthalein endpoint, the pH using a pH meter, while moisture and ash content by AOAC methods. Minerals composition was evaluated by spectrophotometry. xv The radical scavenging activities of the SS syrup against 2, 2-Diphenyl-1-picryl hydrazyl (DPPH) and its color were evaluated by spectrophotometry. Total phenolic acid and tannin contents were evaluated by Folin –Ciocalteu method while flavonoid content was determined by aluminum chloride method. SS syrup viscosity was evaluated using a rotational viscometer, while density and alcohol content were determined by pycnometry. GC methods with external standards were used to qualitatively and quantitatively determine acetaldehyde, propanol and ethanol concentrations in bio-ethanol. The SS varieties that had high sugar content included IESV 91018LT, IESV 92008DL, IESV 92038/2SH, SPV 1411, IESV 930042 and Madhura which ranged between 14-23 °Brix. Optimum harvest time varied between soft and hard dough stages depending on variety. SS juice extractability varied significantly (p=0.05) with variety, harvest stages and location. Total soluble solids which averaged 18 °Brix varied significantly (p=0.05) with variety, maturity stage and location. Location significantly (p=0.05) influenced total sugar, sucrose and glucose, ash and moisture content while fructose, pH and total titratable acidity had insignificant effect. The fructose, glucose and sucrose concentration averaged 22.65, 28.13 and 26.4 mg/ml respectively. Moisture content averaged 81.57 %, pH 5.51, total titratable acidity 0.99 % and ash content 2.21%. SS juice minerals namely, potassium, calcium, and magnesium content averaged 0.25%, 0.16%, and 0.11% respectively, while sodium, manganese, zinc, copper, iron and phosphorous had values less than 0.1%. The syrup contained high sugar, appreciable macro and micro-minerals, bio-active compounds; total phenols, flavonoids and tannins xvi with the highest yield being 256.9g/kg. The syrup radical scavenging activity against DPPH radical revealed more than 50% inhibition, indication of potent anti-oxidants. Time significantly (p=0.05) influenced total residual sugar, pH and ethanol levels (13%) with variety and harvest stage being insignificant. The one-factor-at-a-time method showed temperature of 30 °C, sugar content of 20 °Brix, pH of 5 and 2 % yeast concentration for 72 hours, optimized ethanol content. In conclusion, location, variety and stage of maturity affect SS juice composition hence influencing yields and characteristics of syrup and ethanol.