Onderzoek naar de variatie in het zetmeelgehalte en de structurele eigenschappen van gerst en mout in relatie tot het mouten en brouwen

Malting of barley results in the synthesis and release of amylolytic enzymes and increases the accessibility of starch. Barley malt starch is the primary substrate from which fermentable sugars and dextrins are produced during mashing, the initial step in the brewing process. To achieve this, brewers mix milled barley malt with water and apply a carefully controlled temperature-time profile to ensure gelatinisation of starch and its maximal enzymatic conversion, while preventing excessive thermal inactivation of the amylolytic enzymes. This conversion is essential in the production of alcoholic beers, as yeast converts fermentable sugars into alcohol, carbon dioxide, higher alcohols, and aldehydes. Recently, it has been suggested that the balance between starch gelatinisation and the thermal stability of the amylolytic enzymes during mashing could be compromised by changes in intrinsic barley malt starch properties. Specifically, mashing with malts with high starch gelatinisation temperatures and high volumes of small starch granules may result in significant thermal inactivation of amylolytic enzymes (β-amylase) prior to starch gelatinisation. Consequently, high starch gelatinisation temperatures and small starch granule volumes in malt can result in a decreased conversion of starch to fermentable sugars during mashing. The starch gelatinisation characteristics and small starch granule volumes in malt are determined by genotypic factors, growing conditions, and the malting recipe. It has, for instance, been reported that high barley starch gelatinisation temperatures are associated with high growing temperatures and drought. Considering the global scale of the malting and brewing industry and their use of various barley varieties and malting recipes, large variations in starch gelatinisation characteristics and small starch granule volumes can be expected, which brewers must cope with to achieve high concentrations of fermentable sugars. Despite this, there is no clear understanding of the extent of the variability in the starch gelatinisation characteristics and the small starch granule volumes in barley and malt. Additionally, malting directly impacts barley starch properties. As changes in the gelatinisation characteristics and small starch granule volumes upon malting are often reported by directly comparing those observed in malt with those in barley, insight into the impact of each malting step on the small starch granule volumes and starch gelatinisation characteristics is lacking. Now, it has been documented that malting can result in increased starch gelatinisation temperatures. This is undesirable given the chance of already increased barley starch gelatinisation temperatures due to more extreme growing conditions. The mechanism behind this increase during malting is not clear. Against this background, the goal of the PhD thesis was to understand the variability in barley and barley malt starch properties, their evolution during the malting process, and their impact on the conversion of starch to fermentable sugars and dextrin during mashing. In the first part of the research, the objective was to study the extent of the variability of starch properties and amylolytic enzyme activities among malting barley varieties and their respective malts. Specifically, the variability in small starch granule volumes, starch gelatinisation characteristics (GTo, GTp, GTc, GTc - GTo) , and starch amylose content, as well as the variability in the amylolytic enzyme activities in α-amylase, free β-amylase, and free limit‑dextrinase, was studied in barley and malt. In barley, a broad variability in the small starch granule volumes (SSG: 13.9‑32.0 v/v%), the peak starch gelatinisation temperature (GTp: 60.1-66.5 °C), and the starch amylose content (18.2-30.7 %) was observed. The starch properties changed in a sample-dependent manner upon malting. The extent of the observed variability in barley was still present in malt, where the SSG volumes ranged from 8.3 to 26.3 v/v% and the GTp from 58.9 to 66.4 °C. Besides, a broad variability in the α-amylase (120.9‑279.7 CU/g dw), free β‑amylase (9.7-24.4 BU/g dw), and free limit‑dextrinase (0.3-0.6 PU/g dw) activity was also observed in malt. It was hypothesised that the broad variability in starch properties and amylolytic enzyme activities would impact the conversion of starch to fermentable sugars and dextrins during brewing with a single fixed mashing scheme. This hypothesis formed the basis of the final objective of the PhD dissertation. In the second part, the objective was to investigate the changes in starch properties, with a focus on the granule size distribution and starch gelatinisation temperature during the three malting steps on a lab scale and an industrial scale. On a lab scale, the SSG volumes changed in a similar trend for five different barley samples, with an increase of 1.9 to 8.2 v/v% observed within the first 36 hours of germination, followed by a decrease of 1.7 to 7.9 v/v% later in the malting process. It was hypothesised that large starch granules (LSG) were more susceptible to hydrolysis in the early stages of germination compared to SSG, possibly due to differences in the location between LSG and SSG within the barley endosperm or due to differences in their association with the protein matrix. Consequently, the size of the LSG was reduced, and they were quantified as SSG, resulting in an apparent increase in the volume of the latter early in germination. The total change in the starch gelatinisation temperature during malting was determined by both intrinsic changes in the native starch gelatinisation temperature and the non-starch matrix of barley. Intrinsic changes in the native starch gelatinisation temperature could result in an increase in the gelatinisation characteristics, while changes in the non-starch matrix during malting could result in an increase or even a decrease in the starch gelatinisation characteristics. Kilning on a lab scale did not seem to affect the barley starch gelatinisation characteristics. The temperature-moisture conditions in a green malt bed during lab-scale kilning are, however, not comparable to those on an industrial scale due to the difference in bed size. Temperature-moisture gradients during industrial kilning impact malt characteristics and thus possibly the starch gelatinisation temperature. Lab-scale experiments were conducted in which inactivated green malt samples were subjected to temperature-moisture-time conditions relevant to industrial kilning. These temperature‑moisture-time treatments resulted in an increase of 0.5-4.3 °C in GTo and 0.5‑2.7 °C in GTp, in line with the effects of starch annealing at the given treatment conditions. Based on this, it was hypothesised that starch might anneal during industrial kilning. To test this, an industrial-scale malting process was sampled, with a focus on the kilning step. First, the evolution of the small starch granule volumes and the starch gelatinisation characteristics was studied on an industrial scale. The small starch granule volumes and starch gelatinisation temperature changed similarly during industrial‑scale steeping and germination as observed on a lab scale. This highlights the usefulness of lab-scale steeping and germination as a proxy for the industrial scale for starch‑related investigations. Second, the impact of the temperature-moisture gradient during industrial kilning was studied on the barley starch gelatinisation characteristics, starch content, and α‑amylase activity. Samples were collected in three different bed depths at two different locations per layer during the first thirteen hours of the kilning process. During this period, the upper layer retained a high moisture content (41.2 ± 0.1%) and low temperature (26.4 ± 0.1 °C). No significant changes were observed. In contrast, the bottom layer heated up to 58.0 ± 1.3 °C, and the moisture content was reduced to 25.5 ± 0.8%. This resulted in an accelerated germination process in the bottom layer, leading to a decrease from 48.6 ± 0.5 to 46.1 ± 0.8 dw% in starch content and a simultaneous increase in α‑amylase activity from 121.1 ± 4.7 to 176.0 ± 0.4 CU/g dw. Additionally, a slight increase of 0.6 °C in GTo was observed in the bottom layer. The observed increase in GTo could be due to starch annealing. α-amylase was, however, also still active during the first 13 hours of kilning, which also may have impacted the gelatinisation characteristics. In the last part of this dissertation, the objective was to investigate the impact of barley malt starch properties and amylolytic enzyme activities on the conversion of starch to fermentable sugars and dextrins during mashing. Wort maltose and dextrin concentrations ranged from 84.2 g/L to 112.8 g/L and 16.3 g/L to 38.0 g/L, respectively. The fermentable sugar and dextrin yield after mashing ranged between 73.5% to 91.3%, and 8.3% to 20.0%, respectively. Via partial-least-squares models, it was indicated that the conversion of starch during mashing was determined by a complex interplay between malt starch properties and amylolytic enzymes. High small starch granule proportions negatively influenced the fermentable sugar yield and maltose concentrations in wort. Furthermore, the models indicated that malts with high peak gelatinisation temperatures resulted in lower maltose (g/L) concentrations in wort and were more susceptible to dextrin production during mashing. High activities of α-amylase, free β‑amylase, and limit-dextrinase benefited fermentable sugar yield and maltose concentrations in wort. The results in this PhD dissertation suggest that maltsters and brewers may need to take malt starch gelatinisation characteristics and small starch granule volumes into account, as they directly influence the conversion of starch to fermentable sugars during mashing. These malt starch propertie