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.