Malt COA Series #2 – Proteolysis
By: Malteurop Malting Company

Those of you who read Part 1 of this series on Cytolysis know just how important the Endosperm’s cell walls are. Protein breakdown or Proteolysis, is just as important, and it occurs in multiple structures of the barley kernel*. Let’s begin with why protein breakdown is important in the field, why it’s so important that Maltsters get this done well before the malt heads to the Brewery or Distillery, and how we measure how well we maltsters did our job.

Plant and animal tissues are both created from the reassembly of amino acids into proteins. For plants, proteins help utilize light and energy to cause growth of structures like the roots and the stalk that are so important in the field. Without the ability to form roots under the soil the germinating barley kernel would not be able to find the moisture and nutrients required to create a new plant. The amount of protein contained in a barley kernel varies by variety, kernel size, fertilization, soil quality, and weather during the growing season. Add into the equation the fact that higher protein in barley usually means less starch was formed out in the field. That’s a lot of variability for the maltster to smooth out!

According to Dennis E. Briggs in Malts and Malting, Chapman & Hall, 1998, about 40% of the kernel’s protein can be found in the Endosperm, 35% in the Embryo, and 20% in the Aleurone Layer. Two main groups of proteins exist in barley at the time of harvest, and they have different functions: Hordeins and Glutelins. Glutelins are primarily structural, and together with Beta-glucan and Pentosan form the rigid endosperm cell wall. Two very important plant foods are protected inside the cell walls: protein (predominantly Hordein) and starch. Well-modified malts contain only half of the Hordeins originally in the barley (Briggs 1998). During germination, some of the proteins broken down into small peptides and amino acids are brought to the Embryo for reassembly into growing roots and the plant stalk. Others remain in the Endosperm, waiting patiently to be solubilized in the Brewer’s or Distiller’s mash (Image 1).  

So why is it so important that the Maltster gets the breakdown into small peptides and amino acids done in Germination?  The answer, as usual, is enzymes. Enzymes are very finicky keys that unlock (break down) things like Beta-glucans, proteins, and starches. Heat distorts their shape, rendering them inactive (denatured). As the green malt is heated in the kiln, the proteinases and peptidases are among the first enzymes to be denatured, meaning there is little activity that is retained for Proteolysis in Mashing. Some of the amino acids will also be used to form good things like Color, Melanoidins, and Reductones during kilning. That’s why it’s our job to break down the proteins, since kiln and mash temperatures are just too high for enzyme survival. We need to provide Free Amino Nitrogen (small peptides and amino acids) to promote healthy yeast behavior. We also need to leave some proteins as large molecules for head formation and retention, as well as body and mouth feel (Image 2).

Malt Certificates of Analysis (COAs) have a number of parameters that can be used together to check how well we did our job. One that we can only manage by barley selection is Total or Malt Protein. Mother Nature controls most of the variables we talked about earlier. We maltsters then use the malting recipe chosen to influence the extent of Proteolysis. In the laboratory we mimic a low-gravity brewers’ mash, and then measure how much of the Protein is now soluble. Dividing the Soluble Protein by the Total Protein gives the Kolbach Index or S/T Ratio. My personal favorite of the protein assays is Free Amino Nitrogen (FAN), also called Assimilable Amino Nitrogen (AAN) in some parts of the world. I think this assay gives the most information to the Brewer or Distiller about the amount of food available to keep yeast happy and healthy. Unhappy yeast will not work the magic required to make these luscious, malty libations!

Image 1

C.Lekkas, A. E. Hill & G. G. Stewart (2014) Extraction of FAN from Malting Barley during Malting and Mashing, Journal of the American Society of Brewing Chemists, 72:1, 6-11, DOI: 10.1094/ASBCJ-2014-0113-01

Image 2

Y. Pomeranz & I.B. Sachs (1972) From Barley to Beer – As Seen Under the Scanning Electron Microscope, Proceedings. Annual Meeting – American Society of Brewing Chemists, 30:1, 24-29, DOI: 10.180/00960845.1972.12005953

1a) Transverse section through pericarp, seed coat, and endosperm of barley (1,000x) PE = multilayered, cord-like pericarp; AW = aleurone cell wall; AG = aleurone grain; PM = protein matrix of subaleurone layer; S = starch granule; EW = cell wall of subaleurone layer.

1b) Central endosperm of malted and kilned barley (1,250x); AP = adhering proteinaceous material; DS = highly damaged starch granules.