Saturday, June 27, 2009

Lipids

Lipid is the chemical name for the many types of fat in animal and plant matter. They are the things of love handles, saddle bags and muffin tops. We're drawn to them and can't help the attraction because they are packed full of energy, and our modern lifestyles are still dictated by the needs we had on the plains of Africa 20 thousands years ago when fat was in short supply and the humans that survived tended to be those that deposited fat for later use.

Lipids take the generic form shown below:

The simplest form is glycerol and lipids of this structure are called triglycerides. The R group on the left hand molecule represents long chain fatty acids of various lengths. Triglyceride is the storage form of fatty acids and these long chains can be cleaved off the molecule and metabolised when energy is required. The long chains have further variation in the extent to which they are saturated. Saturation refers to the type and number of bonds between the carbon atoms in the chain. Fatty acids are grouped into saturated and polyunsaturated. The classic animal fat that we eat is stearate and looks like this:
It contains 18 carbon atoms with single bonds between each of the atoms and forms a solid at room temperature due to the tendency of the long chains to tangle and clump together. Lard is the classic saturated fatty acid presentation.

Unsaturated fatty acids contain double bonds between one or more of the carbons in the chain of the molecule. This prevents the chains from tangling and produces the liquid forms of fat we usually associate with vegetable oils. Linolenic acid is a polyunsaturated fatty acid (PUFA) with 3 double bonds that kink the molecule and prevent packing together of the chains:



Lipids are pesky things in the brewing process. Malt contains about 3.5% lipid material but less than 5% of this material usually makes its way into wort. Mash filters, used by the big time brewers are the worst offenders for this. During the mash filter process the mash is squeezed by an air compressed membrane, removing every last drop of extract from the process but squeezing out a lot of undesirable elements too. This represents the higher end of lipid extraction, while traditional mash tuns can cut this back to around 0.3%.

The presence of lipids is quite obvious; turbid wort contains greater lipid content. This can be clearly seen during the recirculation of wort in batch sparging at home. In general it is beneficial to reduce lipid content in wort, and brewers have differing requirements for the turbidity of the wort they produce. Yeast benefit from lipids in wort and a more vigorous fermentation is often noted, but this must be balanced against the potential problems later in the production line.

Lipid extraction can be increased through a number of ways:
  • The use of over modified malt

  • A finer malt grind - particularly hammer milling used in mash filters

  • Higher mash and sparge temperatures

  • Fast wort run off

  • Lack of adjuncts

  • Squeezing the mash to recover residual extract
Lipids can be removed later in the process during boiling and clarification steps, particularly through the removal of break material, but generally brewers like to prevent excess lipids entering the wort in the first place. The most common supply of excess lipid in wort comes from the last runnings. In the home brew setting this is of little concern because we rarely look to extract every last drop from the wort, but for commercial brewers working on tight margins the few degrees of extract left in the mash tun at the end can really start to add up. It is common for brewers to remove the lipid laden last runnings and add them to the strike water in the next mash.


Lipids can cause a number of problems in the brewing process. The most common of these is the undesirable oxidation steps they undergo at the hands of lipoxygenase enzymes, the result of which is potent, unpalatable off flavours that make the beer stale. The classic off flavour stems from trans-2-nonenal, the molecule that provides the cardboard flavour in oxidised beer. The main concern about oxidation of lipids in the mash is the formation of compounds that are more polar and therefore more water soluble. Water soluble compounds are far more likely to make it all the way into the finished beer and cause problems with shelf life.

Further to this most obvious and common lipid problem is the more obscure interaction of lipids with other malt constituents. For example, amylose - the form of starch found in malt, complexes with lipids forming molecules that are inaccessible to the malt amylase. The unsuccessful break down of malt starch during mashing causes all sorts of problems with extract recovery and, later on, haze in the finished beer. Alterations to beer flavour also stem from lipid interactions through the formation of complexes with esters during fermentation. Esters provide most of the fruity and aromatic flavours in beer that do no stem from hops. These flavours can be diminished by lipid interaction and alter the flavour of the beer.

13 comments:

Oblivious said...

Very nice post, also those unsaturated fatty acids are quite capable of been oxides with out the presence of LOX (lipoxygenase enzymes). In fact there is a whole wing of lipid metabolism devoted to non enzymatic lipid products from free radical damage. Ah this brings me back to my my PhD days!

Ed said...

The majority of barley starch is amylopectin.

The thing I find fascinating about lipids in beer is that wort oxygenation is needed so that the yeast can convert saturated fats to unsaturated ones required for cell growth.
The New Belgium brewery developed a method of adding olive oil to their yeast so they don't need to oxygenate the wort.

Oblivious said...

oxygen is need for sterol production, but I have heard the new Belgium may have not stop this method

Here is a short thesis on the method
from New Belgium

http://www.brewcrazy.com/hull-olive-oil-thesis.pdf

Thom said...

Looking back at my notes I can see that that the amylose suffers most from interaction with lipids, but it is not the main constituent of malt starch. I must have confused the two.

I too have always thought it a little strange that brewers go to such lengths to prevent oxidation of wort and then blast it with pure oxygen when the yeast is pitched. It could be a case of having no choice because the yeast must have the oxygen for sterol production and the low oxygen alternative is even worse for the beer.

I have heard of the auto oxidation mechanism for some PUFAs. George Fix mentions a major European lager brewer who had trans-2-nonenal issues despite running an entirely oxygen free set from beginning to end, except of course for the yeast requirements, but the wort is cool at that point, and this might help explain why brewers get away with oxygenating.

Briggs suggests that staling aldehydes stem from very water soluble unsaturated trihydroxyacids that readily come out with the wort and that the run of the mill lipids don't cause the problems.

I heard about the olive oil supplementation in Brew Your Own initially, and found myself astounded at how little oil they suggested for use. It was akin to dipping the the head of needle in oil and then adding it to the wort.

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Oblivious said...

Yea the olive oil it think work out to be 6 micro liters per 20 liters. Time to borrow some stuff from the lab!

Ed said...

The quantity looks a bit dubious to me. I'm suspect the thing about using homeopathic quantities of olive oil is a typing or calculation error.

Oblivious said...

Ed 6ul is not homeopathic, which is past Avogados number an there non existent

Its there to prove trace amount of unsaturated fatty acid for sterol membrane production

Ed said...

I know it's not literally homeopathic, I was trying to humourously refer to the very low amount of oil that is added. Thought come to think of it oil coated pins is probably more voodoo than homeopathy ;-)

I'm still suspicous that 6 micro grams of oil in 20 litres can replace 8 mili grams per litre of oxygen. Doesn't seem to add up to me.

viagra said...

the main componentof the fat, and for disgrace in a excesive amount the cause of many health problems, related with overweight.

Bradley Popkes said...

Curiously as a home brewer and having an elementary knowledge of chemistry, I question the seemingly pervasive theory that fats in beer are expressly bad. Perhaps I am having a misunderstanding somewhere down the cascade, but are hop "acids" or "oils" not just hop fats, which impart a much desired flavor and character to the beer especially when brewing hop forward beers?

I'm not confusing that fats in general tend to create off flavors, but genuinely interested in the significant differences between hop fats and grain fats.

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