Hot Side Aeration and Beer Stability

by Micah Millspaw

Introduction

This article was originally published in the American magazine Zymurgy and looks at hot side aeration and beer stability, the reactive effects of oxygen on hot wort and some methods for reducing the effects.

Oxidation and Melanoidins

It is known that oxidation plays an important part in the formation of protein haze and that compounds known as melanoidins function as anti-oxidants and prevent the oxidation of protein. Oxidation also plays an important part in the production of colloidal haze, hence the name "oxidation haze", first coined by Helm, the German brewing scientist, in early part of this century.

Moreover, the formation of chill haze is also considerably increased by oxidation. Chill haze occurs in finished beer during cold storage, the chill haze will disappear if the beer is warmed (this is only viable if the beer is to be consumed immediately, if the beer is chilled again the haze will return). Later the beer will throw an irreversible haze at ordinary temperatures. Permanent haze is the end product of chill haze. If you get chill haze permanent haze will follow in time.

The oxidation of melanoidins will result in a lower quantity of stable colloids. Unstable colloids promote chill haze and permanent haze in beer. Conversely, the presence of stable colloids inhibits chill haze. The stability of beer colloids is the result of a very complex equilibrium, and the whole problem of colloidal haze formation is very complex. A better understanding of this problem will show it possible to take some steps to minimize its effects in finished beer.

Work done by Jean De Clerk, the famous Belgian brewing scientist has shown that darker beers are inherently more stable and clear. De Clerk attributed this to the presence of large amounts of melanoidins found in darker grains.

Melanoidins were first discovered by the French chemist Maillard, the reaction between amino acids and sugars which form these substances is know as the "Maillard reaction".

By definition, melanoidins are stable complexes formed at high temperatures (i.e. mash out temps.), they are colloidal in nature and are powerful reducing agents and give an acid reaction in aqueous solution. melanoidins are reddish-brown colored substances with a characteristic aroma (malt).

Color and aroma are not the only traits that melanoidins contribute to a beer. Their colloidal nature enables them to "protect" unstable colloids present in beer and to prevent haze formation. At the same time, melanoidins are powerful reducing agents and this too can prevent beer from throwing a haze. In addition the acid character of melanoidins helps improve the quality of beer.

In 1922, Visez, a brewing scientist at Louvain, Belgium, showed that dextrins also, act as protective colloids and diminish colloidal haze. This means that beers with high dextrin levels are much less subject to colloidal haze than beers with low dextrin levels.

Reducing sugars form melanoidins more readily than non-reducing sugars. Crystal malts average 30-50% reducing sugars. Darker malts in general have higher levels of reducing sugars and dextrins than pale malts, even when the pale malts have been fully modified.

The melanoidins formed at 77°C/170°F are more stable than those formed at the lower temperatures of conventional mashing. By adding these specialty malts only in the mash out, the brewer can make his mash more efficient by optimizing saccarification, maximizing the formation of melanoidins and eliminating steeping vessels and or grain bags.

Minimising Aeration

What can you do about hot side aeration?

There are several ways to limit aeration of your hot wort. It is best to start at the beginning, with the mash.

I will describe a mashing technique that is fairly simple, efficient and not too different from what is now common practice. Infusion mashing or step infusion in a combination mash / lauter vessel is very effective at achieving adequate starch conversion when using North American grown barley malt (with its abundance of enzymes).

The grain in the mash should be underlet or infused with hot water from the bottom up. This may be accomplished by simply adding a down tube to hot water inlet or by adding an inlet below a false bottom in a mash / lauter vessel. By infusing in this manner, stirring of the grains to insure uniform mixing of the grain and hot water is not necessary. By not stirring the water into the mash, hot oxygen reactions can be reduced.

At the end of the mash it is important to do a mash out, that is an upward (temperature) infusion step and rest. This mash out is a good time to add in the specialty grains. The mash should be infused with hot water sufficient to bring up the temperature to 77°C/170°F and hold it for 15-20 minutes.

The effect on the specialty grains (dark and crystal malts) is similar to that of steeping the grains as in extract brewing. This method of utilizing dark and crystal malts will increase the quantity of melanoidins in your beer. This will lead to smoother and rounder flavors from the specialty grains, as well as more stable and clearer beers. The use of this mash out technique, with dark grains can reduce metallic flavours that often occur in dark beers, but are not actually caused by metal ions in the brewing process.

Lipids

Lipids are very important elements for proper beer stability. Lipids are unsaturated fatty acids, this means that they are available to form new bonds with other elements of the wort. Although only a few PPM of lipids are present in finished wort, they can have far reaching effects on factors such as yeast viability, ester formation, gushing, and flavor staling.

Small variations in brewhouse procedure can produce large variations in wort lipids. Lipids tend to adhere to trub particles (trub contains up to 50% lipids) and to filter materials. Because of this, spent grains hold high amounts of lipids. A turbid top runoff from the lauter tun can contain 5 times, and even 40 times as many lipids as the clear wort runoff from the same mash. Fermentation can be effected as yeast will autolyze if it does not receive adequate amounts of ergosterol, or unsaturated lipids.

North American grown barley malt contains very small amounts of free fatty acids (3.2-3.5 mg/l) opposed to European malts (18-26 mg/l). Insufficient fatty acid levels can result in high esters in the finished product and can also be responsible for gushing problems in the finished beer. The addition of unsaturated fatty acids can cure gushing. While the addition of saturated fatty acids tends to increase gushing. The content of unsaturated fatty acids has a strong influence on the formation of fermentation volatiles, notably the acetate esters. It is possible for a wort that has been stripped of lipids could produce a beer too high in esters.

I believe that a shortage of lipids may be a problem that homebrewers encounter because of their obsession with mash extraction yields. This need to eke out every trace of sugar from a mash, leads home brewers to practice wort recycling, vorlauf, and / or flaufing. These can be risky sparging techniques with regard to hot side aeration as well as stripping lipids from the wort.

Recycling is the collecting of the wort as it runs out of the lauter tun and pouring it back over the grain bed. Many brewers claim that recycling should be done to settle the grain bed.

Flaufing is the collecting of the wort as it runs out of the lauter tun, boiling it and then returning it to the top of the grain bed.

These practices not only give opportunity for hot oxygen and wort reactions, but also strip out the fatty acids (which North American grown malts are low in) that are essential for proper yeast nutrition.

Particulates

I have long felt that mash recycling was a bad thing, in that it tends to remove a lot of large particulate matter that would otherwise be present in the boil. I feel that the particulates (husks and grits mostly) provide a place for proteins to clump onto during the boil and then settle out more effectively in cooling.

I have observed much clearer finished wort (cooled) from my boils, when the mashes were conducted with no recycling of wort than from those of other brewers whose worts were made by recycling the mash. Some care should be taken in the transfer of hot wort from the mash / lauter vessel to the kettle. Splashing of the hot wort should be avoided. A grant should not be used. If the sweet wort is to be pumped or gravity fed into the kettle, a simple tube going to the bottom if the kettle will do the job. The down tube removes the splashing free fall of the hot wort when it is most likely to react with oxygen in the atmosphere. If the wort must be transferred by means of a bucket there is not much that can be done except to pour slowly and carefully so as not to splash the wort.

It should be of interest to most brewers to have a viable method of comparing beers made using their usual brewing procedure with beers made by implementing the above procedures.

Evaluation

Jean De Clerk developed a method for the critical evaluation of finished beer and for predicting the colloidal stability of a beer. He found that the best method of predicting shelf-life in practice, was to store the beer at a high temperature (60°C) and to immerse the bottles in melting ice for three hours each day, and to note the number of days until a haze appears. Beers subjected to this treatment throw a haze eight to ten times more quickly than when stored at 25°C. With this test it is possible to predict the number of days of shelf life of a beer.

Oxygen

Although it is very important to limit the exposure of the hot wort to oxygen, it is equally important that enough oxygen be present in the post-boil wort to insure a proper fermentation. Oxygen should be dissolved in the cooled wort prior to pitching the yeast. Brewing yeast require oxygen and other trace gases for their reproductive phase, this is essential for a normal fermentation. When the oxygen is introduced to the wort at low temperatures (21°C/70°F and lower) the problematic reactions which can damage the beer do not occur.

It is my hope that this information will provide brewers with a better understanding of beer stability, and some ways to reduce the factors which cause instability and haze in beer and at the same time enhance the effect of melanoidins to bring out malt aromas.

Micah Millspaw 4/1/92