In beer brewing, protein Z is hypothesized to stabilize beer foam. However, few investigations have revealed the relationship between conformational alterations to protein Z during the brewing process and beer foam. In this report, protein Z from sweet wort was isolated during mashing and boiling processes. Circular dichroism (CD) and Fourier transform infrared spectroscopy (FTIR) were used to monitor the structural characteristics of protein Z. The results showed that the α-helix and β-sheet content decreased, whereas the content of β-turn and random coil increased. The complex environment rich in polysaccharides may facilitate conformational alterations and modifications to protein Z. Additionally, the formation of extended structural features to protein Z provides access to reactive amino acid side chains that can undergo modifications and the exposure of hydrophobic core regions of the protein. Analyzing structural transformations should provide a deeper understanding of the mechanism of protein Z on maintaining beer foam.

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Protein Z4, an abundant of serpin storage proteins of Hordeum vulgare, is one major 26 factor that benefits the stability of beer foam. In this study, protein Z4 from Canadian malt 27 Metcalfe was cloned and expressed in Pichia pastoris. The glycosylated 45kDa of recombinant 28 protein Z4 was purified by Ni-NTA affinity column. The secondary structure of recombinant 29 protein Z4 was detected via Circular Dichroism (CD) and Fourier Transform Infrared 30 Spectroscopy (FTIR), and β-sheets and β-turns were the main structure of recombinant protein 31 Z4. A few structural differences between recombinant protein Z4 and protein Z4 from malt were 32 also analyzed, that more obvious α-helical structure and more S-H contents were detected in 33 recombinant protein Z4. Meanwhile, higher structural stability of recombinant protein Z4 was 34 also observed after incubating the samples in a wide range of temperature, pH and ethanol. These 35 results confirmed the positive relationship between modifications by saccharides and high 36 structural stability of recombinant protein Z4. 37 Introduction 39Beer is the most consumed alcoholic drink worldwide, and malt is the main raw materials 40 used for brewing [1]. Beer foam, as one important trait of beer, is affected by the chemicals from 41 malt, especially the proteins [2, 3]. In previous researches, proteins were proved to be the most 42 important factors responsible for the quality of beer foam [4]. In beer, there are about 0.5mg/mL 43 of peptides and proteins which are of size 5-100kDa [5]. Most of the proteinaceous materials 44 originate from the cereal material, typically malt. During brewing process, the complex 45 proteolytic system and environment pressures contribute to hydrolyzation and degeneration of 46 soluble proteins. Among them, there are two main albumins including about 9kDa lipid transfer 47 protein (LTP) and 43kDa protein Z, which can resist the enzymatic hydrolysis and survive after 48 the complex environment pressures during brewing process [1]. Previous investigations have 49 found that both protein Z and LTP can influence beer foam. LTP mainly affect the foam 50 formation and protein Z has positive impact on foam stability. The natural LTP in malt has no 51 foaming potential, and modifications during brewing process are the key factors that affect the 52 beer foaming property of LTP [6]. Although foam stability can be enhanced by combined action 53 of LTP and other polypeptides such as protein Z, there is no significant relationship between beer 54 LTP content and the foam stability [7]. Protein Z is the major protein in beer which is one storage 55 protein in the endosperm of malt. Among beer proteins, protein Z is the first found to be 56 beneficial to beer foam for its high surface viscosity and elasticity [4, 8, 9]. Previously, a barley 57 protein Z4 gene Paz1 was identified and it is suggested that two exons are existed in a protein Z4 58 consisting 399 residues [10]. On the basis of its high structure stability during brewing process, 59 the exist...