The Effect of Kilning on the Capability of Malt to Oxidise Lipids

Abstract

The influence of kilning on the ability off malt to oxidise lipids was studied. Barley was malted with a standard programme and subsequently dried with varying kilning procedures. The capability of samples taken during kilning to oxidise lipids was determined by the lipoxygenase (LOX) reaction, i.e. by measuring O2 consumption in an aqueous suspension of samples upon addition of excess amounts of exogenous linoleic acid. Various kilning programmes from isothermal low-temperature kilnings to kilnings with varying temperature profiles induced a two-to threefold increase in the rate of the LOX reaction in the samples taken during the first two to six hours of kilning. However, when lipid changes in the intact kernels were measured, oxidation of the endogenous malt lipids was very limited during the first half of kilning. Furthermore, the rate of the LOX reaction decreased in samples taken during the latter part of kilning, the rate of decrease depending on the kilning programme and especially the final curing temperatures. These results indicate that a possible risk of a marked ability to oxidise lipids remains at moderate kilning temperatures. In conclusion, kilning induces a significant capability to oxidise lipids but does not affect the fatty acid composition of whole kernels. However, in aqueous suspensions of ground malt the oxidative instability created might lead to oxidation of lipids.

Variation in quality of grains used in malting and brewing

“…Trans-2-nonenal, a common staling compound in beer, is formed from oxidized lipid components. Lipid oxidation in malt has also been reported to cause lautering (filtering) problems during the brewing process and contribute to some downstream off-flavors ( Kaukovirta-Norja et al., 1998 ; Su et al., 2022 ). In recent years, an effort to create low-lipoxygenase malt (LOX-less) has been made.…”

Green Malt for a Green Future – Feasibility and Challenges of Brewing Using Freshly Germinated (Unkilned) Malt: A Review

“…LOX enzymes are thermally inactivated, thus a reduction in enzymatic oxidation of unsaturated fatty acids is expected particularly at the onset of the wort production process. However, during malting LOX activity was shown previously 169 to increase mainly in the first 2 -6 hours of kilning, thus the risk to oxidise lipids remains at moderate kilning temperatures. Dong et al 159 , for example, showed that the trans-2-nonenal concentrations greatly increased when producing crystal malts compared to the corresponding green malt.…”

On the contribution of malt quality and the malting process to the formation of beer staling aldehydes: a review

“…This system enabled production of malt with a lower concentration of hexanal. In the case when LOX activity is high and oxygen is present in the system, enzymatic oxidation and auto‐oxidation take place resulting in a decrease during germination and following it kilning, in the levels of triglycerides and a subsequent increase in free fatty acids, intermediates of trans ‐2‐nonenal and hexanal, respectively ( 146,167,168 ) . This is most likely due to both LOX activity and lipid metabolism of the grain.…”

“…Hence the bottom layer dries faster and enzyme inactivation occurs quicker than in the top layer ( 56 ) . Therefore, the moment of inactivation mostly depends on the position of a kernel in the bed and the particular properties of the enzyme under consideration ( 56,61,145,166,168,180,181 ) . As an example, Baxter ( 61 ) stated that lipoxygenase activity in green malt is relatively stable; however, a temperature increase up to 65°C reduces its activity by 70‐90% depending on the time of grain exposure to the high temperature.…”

“…Kilning‐off at 85‐90°C reduces this activity to 2%. Generally, higher processing temperature and longer kilning times result in a significant reduction of LOX activity in malt, which is positive from the perspective of beer flavour deterioration ( 168,182 ) . Nevertheless, increased heat load also affects other malt quality parameters such as the aroma profile and, therefore could have an undesirable effect on beer flavour ( 119 ) .…”