Holmes, Calum P.
(2015)
An optimised sorghum brewing process.
PhD thesis, University of Nottingham.
Abstract
Whilst there is a tradition of indigenous opaque sorghum beer production in Africa, the manufacture of Western-style clarified lager beers from sorghum is a relatively recent development, originating in late 1980’s Nigeria. There remains substantial scope to optimise the process by tailoring the equipment and conditions utilised more specifically to the raw material. This was the context to the main objectives of this PhD project: towards an improved sorghum brewing process. Two principal approaches were employed: 1. The reduction of primary energy usage by developing novel modifications to the mashing and wort boiling unit operations and 2. Characterisation of the material properties of a substantial co-product of sorghum brewing, sorghum spent grains (SSG) as a pre-requisite to assessing its suitability for the production of value-added products and/or biofuel.
The gross chemical composition of five SSG samples sourced from commercial breweries in Africa were determined, with the sum of individual components accounting for 97.8 % material on a dry weight basis (d.b.; average value). The SSG samples contained relatively high amounts of protein (>38.0 %) as compared to values quoted for brewers’ spent grains. SSG samples were found to contain considerable amounts of residual starch (>4.55 %; d.b.), which suggested that the practice for milling and mashing with unmalted sorghum, in the breweries from which samples were sourced, could be further optimised.
By boiling at reduced temperature, required energy input is reduced as the latent heat of evaporation is not provided. In Chapter 3, the impact of reduced temperature boiling on the formation and stripping of key wort volatile compounds was evaluated at both industrial scale and pilot scales (10 hL). As compared to control boiling (3.5 % evaporation), the stripping efficiency of simmer boiling (0 % evaporation) was reduced for some volatiles, including: hexanal, linalool, and β-myrcene. One key lager flavour volatile, dimethyl sulphide (DMS), displayed similar patterns of stripping when comparing simmer and control boils.
In Chapter 4, we report the evaluation of a novel wort boiling technology in production-scale trials at a brewery in Ghana. The ‘PDX’ wort boiler utilises direct steam injection into wort and claims improved efficiency of heat transfer and volatile stripping. The present work demonstrated that steam injection technology could provide an approximate 50 % reduction in energy input during the boil, without significant deleterious effects on final product quality. Finished beverages produced using steam injection technology were determined to be within brand specifications by a trained sensory panel.
Chapters 5 and 6 report investigations aimed at reducing the energy input when mashing with unmalted sorghum and developing knowledge of how the structure and composition of different sorghum cultivars interacts with the mashing conditions employed. Designed experimentation was used to investigate the impact of mash conditions on a novel low-temperature mashing system and a high-temperature enzyme mashing system. The reduced energy, low-temperature system was comparable to the high-temperature system in terms of extract and FAN yield, when mashing with unmalted Sorghum bicolor cv. Fara Fara. Furthermore, both enzyme systems were able to produce acceptable wort using agricultural sorghum varieties, providing that the starch properties were similar to brewing cultivars in terms of their pasting characteristics and grain hardness. Poor mashing properties were associated with cultivars displaying increased physical interaction of endosperm protein with starch, resulting in reduced starch swelling during mashing.
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