Impacts of starch breakdown during germination and stewing on aroma formation in crystal malts.

Abstract

Why was the work done: Crystal malts are used in brewing to add colour, flavour and mouthfeel to beers. In their manufacture, aroma compounds are formed through thermal flavour generation during roasting of stewed green malt. As part of a wider project to generate novel flavour extracts from roasted products, strategies to maximise aroma compounds in crystal malt were investigated.

How was the work done: Crystal malts were prepared at laboratory/micro-malting scale using a Box-Behnken experimental design where three parameters were varied: (i) germination time (4-6 days), (ii) stewing time at 65^oC (1-24 hours) and (iii) roasting temperature (120-160^oC). The major aroma compounds in the malts were quantified using Solid Phase Micro Extraction Gas Chromatography with Flame Ionisation detection.

What are the main findings: Response surface models are reported which predict the concentration of the quantitatively dominant compounds: acetic acid, furfural, 2-furanmethanol, maltol, HMF, 2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one (DDMP), 5-methylfurfural and phenylacetaldehyde as a function of green malt germination time (2-6 days), stewing time (1-24 h) and roasting temperature (120-160^oC). By optimising these factors, the concentration of bulk aroma compounds (sugar degradation products) increased up to sixfold relative to ‘standard’ conditions (four days germination, 1 hour stew at 65^oC and roasting at 140^oC). Optimal conditions for the formation of each compound are discussed in mechanistic terms. An image analysis method using cross polarised light microscopy was developed to quantify the degree of crystallinity of starch granules during stewing. Data indicate that under conventional manufacturing conditions there is a pool of ungelatinised starch in crystal malt kernels which represents an untapped source of aroma compound precursors.

Why is the work important: The results show that varying the production conditions changed the amounts and relative proportions of the main aroma compounds and hence the flavour of crystal malt. This may help maltsters understand differences in the flavour of products from different production facilities. Application of cross polarised light microscopy with image analysis revealed that a proportion of starch remains ungelatinised following standard industry stewing conditions. This demonstrates the potential to generate novel crystal malts with more flavour.