Oxidative Staling In The Brewing Process

Changes seen Why/Mechanism Strategies to avoid

1. Flavour. Changes that take place.. Sensory changes arising during beer staling (Dalgliesh, 1977) Beer staling refers to the sensory and analytical changes a beer undergoes during storage. Whilst there are relatively few studies into the actual sensory changes observed during beer storage, work done by (Dalgliesh, 1977) presents a generalised view (see fig 1). These include a constant decrease in bitterness, an increase in sweet honey like tastes and aromas, increasing levels of an aroma of wet cardboard, and an increase and subsequent decline in a flavour described as ribes (the aroma of blackcurrant leaves). In addition to the taste and aroma changes, there is a subsequent decrease in colloidal stability as well as an increase in colour during storage.   It should be noted that most studies into the sensory and analytical changes that take place in beer during the aging process have looked at beers with a rather different sensory profile and colloidal stability to the hop forward, and often hazy, beer produced by many craft brewers today. My own experience suggests that sensible additions to Dalgliesh’s chart would include a decline in dry hop aroma as well as a sometimes dramatic increase in colour in hazy, hoppy beers.

1. Stability. Changes that take place.. Protein + Polyphenol Haze free beer Precipitates at low temperatures Reversible Chill Haze H-bond formation colloidal particles of < 1 m Protein-Polyphenol Complex Irreversible, does not dissolve when warmed Polymerization Permanent haze colloidal particles of > 1 m Protein-Polyphenol Complex Colloidal haze formation in beer results of the interaction of proteins and polyphenols. Initially these complexes are quite small and the reaction is reversible. This haze will disappear upon warming – this is known as ‘chill haze’. As the complexes become larger and more complex, mainly through the increased polymerisation of the polyphenolic component, the haze becomes ‘permanent’ and will not disappear on warming. The formation of this permanent haze is promoted by the presence of oxygen and certain metal irons (Fe and Cu in particular).

2. Factors which can affect beer staling Colloidal haze formation in beer results of the interaction of proteins and polyphenols. Initially these complexes are quite small and the reaction is reversible. This haze will disappear upon warming – this is known as ‘chill haze’. As the complexes become larger and more complex, mainly through the increased polymerisation of the polyphenolic component, the haze becomes ‘permanent’ and will not disappear on warming. The formation of this permanent haze is promoted by the presence of oxygen and certain metal irons (Fe and Cu in particular).

2. Reactive Oxygen Species ROS Superoxide Hydrogen peroxide Hydroxyl Radical Cu2+ or Fe2+ Molecular oxygen (O2) itself is inactive. It is the ‘reactive’ species of oxygen that cause the oxidation of beer components. These ‘Reactive Oxygen Species’ are generated from molecular oxygen in the presence of even trace amounts of transition metals (Fe/Cu). The hydroxyl free radical is extremely reactive and rapidly oxidises other components of the beer to generate other free radicals which then react further to promote chain reactions which will eventually result in the formation of carbonyl end products such as aldehydes and ketones. The oxidation and subsequent polymerisation of polyphenols is a major factor in colloidal instability in beer. We will briefly consider the mechanism of colloidal haze instability in the next slide.

2. Effect of ROS Aldehydes Ketones Alcohols Organic Molecules ROS +

2. Effect of ROS Paper/Cardboard Threshhold 0.5ppb Colloidal haze formation in beer results of the interaction of proteins and polyphenols. Initially these complexes are quite small and the reaction is reversible. This haze will disappear upon warming – this is known as ‘chill haze’. As the complexes become larger and more complex, mainly through the increased polymerisation of the polyphenolic component, the haze becomes ‘permanent’ and will not disappear on warming. The formation of this permanent haze is promoted by the presence of oxygen and certain metal irons (Fe and Cu in particular).

2. Effect of ROS ROS Lox-1 Enzyme + O2 Malt Lipids E-2- Nonenal

2. Influence of ROS on colloidal stability Protein + Polyphenol Haze free beer Precipitates at low temperatures Reversible Chill Haze H-bond formation colloidal particles of < 1 m Protein-Polyphenol Complex Polymerization (O2 and metal ions, heat, agitation) Irreversible, does not dissolve when warmed Permanent haze colloidal particles of > 1 m Protein-Polyphenol Complex Colloidal haze formation in beer results of the interaction of proteins and polyphenols. Initially these complexes are quite small and the reaction is reversible. This haze will disappear upon warming – this is known as ‘chill haze’. As the complexes become larger and more complex, mainly through the increased polymerisation of the polyphenolic component, the haze becomes ‘permanent’ and will not disappear on warming. The formation of this permanent haze is promoted by the presence of oxygen and certain metal irons (Fe and Cu in particular).

2. Polyphenols and reactivity Polyphenol / Catechin Flavinoid Increasing reactivity Colloidal haze formation in beer results of the interaction of proteins and polyphenols. Initially these complexes are quite small and the reaction is reversible. This haze will disappear upon warming – this is known as ‘chill haze’. As the complexes become larger and more complex, mainly through the increased polymerisation of the polyphenolic component, the haze becomes ‘permanent’ and will not disappear on warming. The formation of this permanent haze is promoted by the presence of oxygen and certain metal irons (Fe and Cu in particular).

2. Higher molecular weight polymers more reactive to protein Oxidised flavanoids (anthocyanidins) contribute to colour Higher molecular weight polymers give increased astringent character to beer The removal of polyphenols is desirable to achieve colloidal stability however, in practice, it is not usual to remove 100% of polyphenols from beer. Removal of haze forming proteins using silica gels is also an important part of a typical strategy to achieve colloidal stability. It is important that the oxidation of polyphenols (flavanoids) is minimised in package in order to inhibit the formation of colloidal hazes. Polyphenols are themselves reductants (so they will react with oxidising materials). It is important to minimise the input of oxygen during the hot side of the brewing process (mashing, lautering, boiling) in order to prevent the oxidation of these (and other) beer components which would otherwise act as protectants against oxidation in the packaged beer.

3. Improving colloidal stability Protein Reduction use high adjunct level low nitrogen malt low malt modification vigorous wort boil temperature control Gels Polyphenol Reduction low proanthocyanadin malt pH control Wort run-off control ( no weak worts) PVPP

3. Improving flavour and colloidal stability Low metals Low oxygen Brewing Operations Packaging Operations – (biggest area for O2 pickup) Low storage temperature Careful transport The brewer can influence other factors that will contribute to colloidal instability, such as specifying low Fe levels in filter powders. Oxygen pick up during brewing can achieved in several ways – by using de-aerated brewing water, filling vessels from the bottom to avoid turbulence, ensuring CO2 blankets are maintained above the beer in storage vessels. BUT the most important area is probably in the packaging lines where oxygen control is one of the most important factors. Filling machines should be designed to flush the bottle with inert gas, typically CO2, before filling and should minimise the headspace volume. Oxygen levels in beer post fermentation will typically be around 50ppb but the packaging process can result in ingress of air to give typical packaged oxygen values of 150-300ppb. One major US brand achieves O2 levels in the final bottle of just 50ppb through its very close attention to the packaging process (and significant investment in its packaging operation)! Control of the supply chain, in particular temperature control and movement of the beer, can be more difficult in countries where ambient temperature is high and transport is by roads that will shake the beer a lot (such as Vietnam!).

3. Antioxidants + ROS The brewer can influence other factors that will contribute to colloidal instability, such as specifying low Fe levels in filter powders. Oxygen pick up during brewing can achieved in several ways – by using de-aerated brewing water, filling vessels from the bottom to avoid turbulence, ensuring CO2 blankets are maintained above the beer in storage vessels. BUT the most important area is probably in the packaging lines where oxygen control is one of the most important factors. Filling machines should be designed to flush the bottle with inert gas, typically CO2, before filling and should minimise the headspace volume. Oxygen levels in beer post fermentation will typically be around 50ppb but the packaging process can result in ingress of air to give typical packaged oxygen values of 150-300ppb. One major US brand achieves O2 levels in the final bottle of just 50ppb through its very close attention to the packaging process (and significant investment in its packaging operation)! Control of the supply chain, in particular temperature control and movement of the beer, can be more difficult in countries where ambient temperature is high and transport is by roads that will shake the beer a lot (such as Vietnam!).

3. Antioxidants Carbonyl + Antioxidant Flavour neutral reaction product

3. Vicant SB

3. Antioxidants

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