The impact of malt derived proteins on beer foam quality. Part I. The effect of germination and kilning on the level of protein Z4, protein Z7 and LTP1

The effect of barley germination and kilning on three putative beer foam-po sitive proteins was investigated by immunoblotting and ELISA procedures. Th ese procedures involved the use of specific antibodies raised to purified l ipid transfer protein 1 (LTP1) and the two protein Z forms, Z4 (BSZ4) and Z 7 (BSZ7). The free fraction of BSZ4 and BSZ7, and all LTP1 were extracted b y aqueous salt-solution from barley and malt. The addition of reducing agen t allowed the extraction of bound BSZ4 and BSZ7. A previously undescribed f raction of BSZ4 and BSZ7, refered to as latent, was extracted with SDS and reducing agent. The barley combined fraction (free + bound fractions) was s urveyed in 93 barley varieties to show that BSZ4 was the dominant isoform, on average constituting similar to 80 % of all protein Z. Considerable vari ation was observed between varieties in the level of LTP1 (502-1144 mu g/g) and the combined fractions of BSZ4 (18-2136 mu g/g) and BSZ7 (38-771 mu g/ g). The free fraction is expected to be more available for extraction into wort during mashing than the bound or latent fractions. The level of LTP1 d id not change substantially during germination, but a significant proportio n of the latent and/or bound protein Z fractions was converted into the fre e fraction. In the seven varieties studied the free fraction of BSZ4 and BS Z7 increased 149-300% and 49-141%, respectively. Proteolytic cleavage in th e reactive site loop converts protein Z to heat- and protease-stable forms that survive the brewing process. During germination most of the free BSZ4 and 30-70% BSZ7 was converted to the cleaved form. Kilning was found to red uce the amount of protein Z and LTP1 that could be extracted by 10-30% and 7-37%, respectively, which is likely to be counter productive for foam qual ity. These results suggest that barley variety selection and optimisation o f germination and kilning protocols during malting may be opportunities for improvement of beer foam quality.