Brewing Chemistry: Part 4- Iodide and starch

During the mashing of the grains, the amylase found in the barley is used to break down unfermentable starch into fermentable glucose. The goal is to break down as much of the starch as possible.

Starch comes in two different forms, amylose and amylopectin. They are both polymers of glucose. Amylose is a linear polymer, and amylopectin is a branched polymer. There is usually more amylopectin in a given sample starch, but it is the amylose that can be used to identify the mashing progress.
A common and simple test for the presence of starch is the iodine test. A solution of iodine (I₂) and iodide (I^-1 from something like potassium iodide, KI) is added to the sample. The iodine/iodide solution is reddish, but in the presence of amylose it turns dark blue (almost black).

First of all, iodine (I₂) is not terribly soluble in water. The addition of iodide (I^-1) makes it soluble by forming an I₃^-1 complex ion.

Amylose adopts a coiled or helix shape. When I₃^-1 is added it gets trapped in the coils of amylose. The iodine atoms trapped in the coils can be in the form of I₃^-1 or I₅^-1 or more iodine atoms (I'm not sure and I don't know if anyone is). The resulting interaction between the amylose and the iodide complex results in a shift of the light being absorbed by the iodide complex. This happens because there is transfer of charge to the starch molecule. This affects the energy spacing of the electrons in the iodine.

The result is a complex that does not absorb light in the blue region of the visible spectrum.

Brewing chemistry: Part 3- Mashing: Amylase

Once the barley has been malted and roasted, the grain is full of starch and enzymes. The grains are then dried and roasted. At his point, it is critical that the amylase enzymes not be denatured. Denaturation is any process that renders enzymes inactive. Heating (i.e. cooking) will denature proteins (enzymes are proteins). Roasting the barley too hot could destroy the critical enzymes. But, fear not, the folks who do the roasting know exactly what they are doing.

When the barley is roasted it is ready to be mashed. For extract brewers, this is something that isn't done. For all-grain brewers, mashing is a critical process. A screw up here and the whole batch could be ruined. Mashing essentially breaks up the starch into fermentable sugars. No sugar, no beer.

As I have mentioned before, I am now doing partial mashes (I'll post later about my method). In fact, all four batches featured recently were partial mash batches.

There are a lot of things that happen during a mash. I will focus on the action of amylase in the context of doing a partial mash.

During mashing, malted barley is heated in water. For partial mashing about 1 liter of water is used per pound of grain. The temperature control is critical since the amylase activity is dependent on temperature.

There are two main amylase enzymes at work. The enzymes are formed during the malting process thanks to the action of gibberillic acid. The two enzymes are alpha-amylase and beta-amylase.

They both break glycosidic bonds between the glucose molecules in starch. However, alpha-amylase does so randomly and beta-amylase starts at the end of the starch chain (the non-reducing end) and chops off two glucoses (maltose) at a time. The optimum temperature for alpha-amylase activity is around 158° F and that for beta-amylase is around 145° F. In all grain brewing temperature control is absolutely critical since the amylase activities must be balanced out. In partial mashing a temperature compromise is reached. At 152-154° F the activities of both enzymes are good enough to result in decent conversion.

The goal is to not convert all of the starch into glucose or maltose. Unless you are trying to brew a very dry beer with very low residual carbohydrate levels (i.e. T.A.L.L.). If beta-amylase activity is allowed to dominate, the result is a highly fermentable wort and a dry beer.

A great description of partial mashing is available on the BYO website

Brewing chemistry: Part 2- Kilning

Once the barley has been malted, and the barley has just started to sprout, the next step is to kiln dry the grain. In some cases, the grain is roasted.

The purpose of the kilning (if that is a word) is to dry the grain, but also, in some cases, to roast it.

During the heating, some enzymes become denatured. However, most of the amylase enzymes survive. These will be utilized during mashing to liberate fermentable sugars from starch. Fortunately, during the drying phase, most lipase and lipoxygenase enzymes are destroyed. These enzymes are implicated in the formation of off flavors in beer as it ages.

A second goal of kilning, in some cases, is to roast the grain. Pale malt is typically not roasted, whereas, roasted barley is (umm, that's why they call it 'roasted' barley). During the roasting process a glorious reaction called the Maillard reaction occurs.

The Maillard reaction is a general reaction between an amino acid and a reducing sugar. Considering there are a lot of types of sugars and amino acids available, the Maillard reaction can form a variety of products. The Maillard reaction is a very important reaction in food chemistry. The products contribute to the color and flavor of browned bread, chocolate, seared meat, caramel and deep-fried death. Now, I'm not a food chemist. I'm just a chemist who likes food. My understanding is the Maillard reaction is central to what food chemist study. The following is my understanding of the Maillard reaction.

In the first step, an amino acid reacts with a reducing sugar (glucose is shown) to make an amino glucose. This part of the reaction is known as the Amadori reaction.



The Amadori complex can react with dicarbonyl compounds in a reaction known as the Strecker degradation. This results in a number of aldehydes such as, isobutyraldehyde and furfural and others. Some examples include:



Another important product of this breakdown are the melanoidins. This class of poorly characterized heterocycles contribute a dark color and a toasty aroma.

In reality, the Maillard reaction is more important during the boiling of the wort (a future post), but it does play a role in the roasting of grain.

UPDATE:
Try #2 for the images. I don't know why, but my images stopped appearing. I reloaded them above as .gif files and below as a .bmp file. Can you see it?

Brewing chemistry: Part 1- Gibberellic acid

Gibberellic acid is a plant hormone that induces the formation of a number of key enzymes, and in a sense, it gets things started in the brewing process. So, I thought I would start with gibberellic acid.

Beer is made from the sugars in malted barley (along with a few other key ingredients). In brief, the malting process involves soaking the barley in water to induce germination. During germination cell walls are broken down and starch is released within the grain [there is a bunch of plant anatomy that I could get into, but I'm not really interested in that]. Enzymes are formed that will eventually be used by the brewer to break down the starch into maltose. At a certain point germination is stopped and the grain is kiln dried and/or roasted. This is now malted barley, and it is ready to be mashed. I will deal with mashing in another post.

I want to zero in on what causes the starch-hydrolyzing enzymes to form. As the barley is germinating, any free carbohydrates are consumed during respiration (i.e converted to pyruvate and then CO2). Once the carbohydrates are depleted, the starving barley grain turns to its starch reserves. Since starch doesn't just fall apart into glucose, enzymes are needed. When the free glucose gets low, a signal is sent to start forming enzymes (amylase) to break up starch. The "signal" that triggers the formation of these enzymes is the plant hormone, gibberellic acid (GA3).

The structure of GA₃ is:



This is a very interesting molecule. It was first synthesized by a fellow by the name of Elias James Corey in 1978. I'd love to write a bit about his synthesis, but thanks to very limited (and embarrassingly so) library access at my institution, I can't easily get the papers. Thanks to Peter J. Stang, I can at least see the first page of the 1978 communications [JACS, 1978, v.100, p.8031 and p. 8034]

GA₃ gets the germinating plant to form mRNA that codes for the formation of things like amylase and other starch hydrolyzing enzymes. How any of the gibberellins work is not well understood. What is known is that the cells in the aleurone layer of the barley seed contain a membrane-bound receptor for GA₃. When GA₃ binds, a Myb transcription regulator is produced. This Myb protein induces transcription of the amylase gene. The amylase is sent on a tour of duty to destroy starch, but a well trained maltster will stop the malting before this takes place. The amylase is going to be used during the mashing process by the brewer.

To be continued...

Brewing chemistry: Intro

I am an extract brewer. Not by choice, but by necessity. My 5 year goal is to take the plunge and move to all-grain brewing. My limitations right now are time, space and acclimating my patient and understanding wife to the realities of my hobby.

Extract versus all-grain: The difference between the two is the source of fermentable sugars. In all-grain brewing the sugars are extracted directly from malted barley in a process known as "mashing". In extract brewing, the sugars come in the form of an extract, either dry or liquid. The sugars are extracted from the grains and dried to a powder under vacuum (or thick syrup) in a glorious factory somewhere. A typical extract recipe calls for 5-6 pounds of dry malt extract. In all-grain brewing recipes typically require 5-15 pounds of barley.

All-grain brewing requires more specialized equipment and more time. Is it superior to extract brewing? Well, that depends on who you ask. The result for either is superb beer, but with all-grain brewing there is more control over the final character of the beer. I hope to get into all-grain brewing someday.

I started writing this post and realized there were so many tangents I wanted to go on, that the post would be too long, too cumbersome and probably confusing. So, I am going to write a serial. [One of my favorite books of all time is "The Count of Monte Cristo." It was written as a serial. According to a legend I may have made up in my mind, Dumas started writing it and due to its popularity, he kept adding chapter after chapter without knowing where everything was headed.]

For every entry I am going to pick one aspect of the brewing process and write about the Chemistry involved. Hopefully, it will become a list of interesting and useful tidbits of brewing chemistry.

I must warn you: I am NOT a brewing chemist. I am a chemist who brews. Brewing is a hobby and not my career. Therefore, don't expect me to know everything.