Evaluation of the potential malting, brewing and antiyeast characteristics of selected Zambian sorghum cultivars

Valashiya, Rorisang Cynthia (2014-12)

Thesis (MScFoodSc)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: The potential malting, brewing and antiyeast characteristics of twelve selected sorghum cultivars from Zambia (MMSH 375, 625, 707, 1038, 1077, 1194, 1257, 1356, 1363, 1365, 1376 and 1401) was evaluated. This was done through the determination of tannin content, proximate composition, %germinative energy (%GE), micro-structure, diastatic power (DP) and free amino nitrogen (FAN); as well as the antiyeast activity of N-containing compounds for each cultivar. All cultivars were tannin-free and thus suitable for brewing potential exploration. Seven cultivars (MMSH-375, 625, 1038, 1194, 1257, 1401 and 1376) had a %GE considered appropriate for malting and brewing. A maximum of 25% of the DP level recommended for commercial malting was achieved in this study. In spite of the low DP observed in all cultivars after malting, high extract levels ranging between 18.7 and 25.5˚P were achieved. In unmalted grains a significantly FAN level (p≤0.05) was observed in three cultivars (MMSH-375, 1194 and 1077). These FAN values, however, represented less than 10% of the recommended FAN level necessary to support proper yeast functioning during wort fermentation. The 150 mg/L threshold which is necessary to support proper yeast function and is employed in modern day brewing was achieved in malts of sorghum cultivars MMSH-375 (intermediate cultivar) and 1401 (intermediate cultivar). A FAN level between 100 and 140 mg/L, which is considered enough to support optimal fermentation, was achieved in cultivars MMSH-625 (intermediate cultivar), 1077 (hard cultivar), 1194 (intermediate cultivar), 1356 (soft cultivar) and 1376 (intermediate cultivar) after malting. Mashing did not significantly increase FAN levels in all sorghum cultivars. The addition of potassium metabisulphite (KMS) in the mash did, however, lead to a further increase of FAN levels in all cultivars. Scanning electron micrographs revealed that the corneous endosperm was inaccessible to degradation by enzymes and only the soft endosperm was modified in all cultivars during malting. The N-containing compounds from selected malts of sorghum cultivars with good malting and brewing potential (MMSH-375, 625, 1401 and 1194) were isolated and their antiyeast activity was determined. It was found that the N-containing compounds were playing an essential role during fermentation by contributing positively towards yeast growth in all four cultivars. However, a 20-30% reduced growth at the lower concentrations of the putative %yeast growth promoting N-containing compounds was observed in all cultivars. Irrespective of the reduced %yeast growth observed in all four cultivars at low concentrations of the putative yeast growth N-containing compounds, it could not be concluded that the sorghum cultivars utilised in this study possess antimicrobial peptides and proteins because only small compounds (<1 kDa) were detected with UPLC-MS. The dose-response data showed that extracts of MMSH-375 and MMSH-625 resulted in generally a higher %yeast growth when compared to other cultivars. The UPLC-MS also confirmed the dose response data that MMSH-375 has a better qualitative yeast-feeding value. MMSH-375 is thus rated as the best cultivar out of the four that were screened and is recommended to be use in the commercial production of sorghum lager beer.

AFRIKAANSE OPSOMMING: Die potensiële mout, brou en teen-gis eienskappe van twaalf geselekteerde Zambiese sorghum kultivars (MMSH 375, 625, 707, 1038, 1077, 1194, 1257, 1356, 1363, 1365, 1376 en 1401) was geëvalueer. Dit is gedoen deur die bepaling van tannien inhoud, proksimale samestelling, % ontkiemings energie (%OE), mikrostruktuur, diastatiese krag (DK) en vrye amino stikstof (VAS) inhoud; sowel as die teen-gis aktiwiteit van stikstofbevattende verbindings vir elke kultivar. Alle kultivars was tannien vry en dus geskik om te ondersoek vir brou potensiaal. Sewe kultivars (MMSH 375, 625, 1038, 1194, 1257, 1401 en 1376) het ‘n %OE geskik vir mouting en brouery gehad. ‘n Maksimum van 25% van die voorgestelde DK vlak vir kommersiële brouery was gevind in hierdie studie. Hoë ekstrak vlakke van tussen 18.7 en 25.5˚P is gevind, ten spyte van die lae DK wat gevind is in alle kultivars na mouting. ‘n Betekenisvolle VAS vlak (p≤0.05) was gevind in die ongemoute graankorrels van drie kultivars (MMSH 375, 1194 en 1077). Hierdie VAS vlakke was egter minder as 10% van die voorgestelde VAS vlakke benodig om gis funksie te ondersteun tydens fermentasie. Die 150 mg/L drumpelwaarde benodig vir korrekte gis funksie en gebruik in moderne brouery is bereik in gemoute sorghum kultivars MMSH 375 (intermediêre kultivar) en 1401 (intermediêre kultivar). ‘n VAS vlak van tussen 100 en 140 mg/L, gereken as genoegsaam vir optimal fermentasie, was bereik in kultivar MMSH 625 (intermediêre kultivar), 1077 (harde kultivar), 1194 (intermediêre kultivar), 1356 (sagte kultivar) en 1376 (intermediêre kultivar) na mouting. Warm water weking het nie die VAS vlakke in die sorghum kultivars betekenisvol verhoog nie. Die byvoeging van kalium metabisulfiet (KMS) tot die weekvloeistof het egter gelei tot ‘n verdere verhoging van VAS vlakke in alle kultivars. Skandering elektronmikrograwe het aangedui dat die growwe endosperm ontoeganklik is vir ensiematiese afbraak en dat slegs die sagte endosperm van alle kultivars veranderinge ondergaan het tydens mouting. Die stikstofbevattende verbindings van die gemoute sorghum kultivars geselekteer op grond van hul goeie mout en brou potensiaal (MMSH 375, 625, 1401 en 1194) was geïsoleer en hul teen-gis aktiwiteit was bepaal. Daar was bevind dat die stikstofbevattende verbindings ‘n noodsaaklike rol speel tydens fermentasie deur ‘n positiewe bydrae tot gisgroei te maak in al vier kultivars. Verminderde groei (20-30%) was egter opgelet by laer konsentrasies van die vermeende groei-bevorderende stikstofbevattende verbindings in alle kultivars. Ten spyte van hierdie verminderde gisgroei by lae konsentrasies kon daar nie afgelei word dat die sorghum kultivars gebruik in hierdie studie wel antimikrobiese peptiede en proteïne bevat nie aangesien slegs klein verbindings (<1 kDa) waargeneem word met UPLC-MS. Die dosis-respons data het aangedui dat ekstrakte van MMSH 375 en 625 oor die algemeen gelei het tot hoër gisgroei in vergelyking met ander kultivars. UPLC-MS het ook bevestig dat MMSH 375 ‘n beter kwalitatiewe gisvoedingswaarde het. MMSH 375 word dus gereken as die beste kultivar uit die vier gekeurde kultivars en word aanbeveel vir gebruik in die kommersiële produksie van sorghum lager bier.

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