Understanding esters

The word esters is thrown around a fair amount in popular brewing books, but is never really fully explained. Any brewer who has researched wheat ales will be able to tell you that esters are required for the style, and could likely list several variables that affect the production of esters (yeast strain, fermentation temperature, starting gravity). However, not many books go into exactly what an ester is. Broadly speaking, when alcohol reacts with a carboxylic acid in an acidic environment, an ester and a water molecule are formed.

Types of Alcohols

There are numerous alcohols, all of which have the potential to create an ester. The most common beer esters have ethanol as the reactant alcohol since it is so abundant, however the heavier fusel alcohols can react to form esters as well. Chemically speaking, all alcohols have an OH group bound to a carbon. The figure below shows the chemical structures of some of the most basic alcohols. 

Figure 1: Structure of some common alcohols

The most basic alcohol is methanol, which only has a single carbon atom. The carbon atom has bonds to an oxygen and three hydrogen that form a tetrahedral around the atom. Methanol is the alcohol created in distilling processes that can cause blindness. It has a lower boiling point than ethanol and will be a large portion of the first liquid collected from a still.

The alcohol that we are most familiar with is ethanol, which gets you drunk. It is the alcohol that is created in the highest concentration during fermentation. If fermentation temperatures are not ideal, alcohols larger than ethanol are created. These alcohols often have from 3 to 5 carbon atoms, but sometimes more. The 5 carbon species, amyl alcohol is one of the most common fusel alcohols in beer. There are several isomers where the orientation of the carbon atoms change with reference to one another. However by definition, all the amyl alcohols have an OH group bound to a carbon atom.

Carboxylic Acid

A carboxylic acid is an organic acid that contains a carboxyl group. This class of molecule is common in nature and includes the amino acids as well as acetic acid (vinegar). Esterification of amino acids does occur, however it is rare to see the concentration of any ester reacted from an amino acid to exceed 100 parts per billion, and there is no perceivable effects until at least 10 ppm. However, acetic acid plays an important role in perceivable esters in beer.

Figure 2 - Generic Carboxylic Acid

The picture above shows the generic form of carboxylic acid. It requires a carbon to be bound to a hydroxyl branch in the same was as is required for an alcohol. However, the carbon is also double bound (it shares 4 electrons versus 2) to an oxygen. The R in the figure denotes the remainder of the molecule, which is unique for each individual acid. During esterification, the two hydroxyl groups react (among other things). The end result is that a water molecule is formed, and the two molecules become bound at the remaining oxygen.

The Reaction

Two of the more common esters are ethyl acetate and isoamyl acetate. These esters are the result of acetic acid reacting with ethanol and isoamyl alcohol respectively. Acetic acid does not exist freely in solution, instead it is bound in an enzyme called Acetyl-CoA. Through hydrolysis, it can be liberated for use as a reactant in esterification.

 Ethyl Acetate can taste like green apples in low concentrations and possess a harsh solvent property at high concentrations. Isoamyl acetate is what gives hefeweizens their characteristic banana flavor. There is actually an entire industry devoted to synthesizing the chemical for use as artificial banana flavor.

It's also interesting to note that isoamyl acetate is one of the pheromones a honey bee releases when they sting.

 All esters have a similar structure: a chain of carbon atoms with a single oxygen in the middle. The reaction requires an acidic environment. This is because the free hydrogen ions act as a catalyst, promoting the reaction. The hydrogen ions momentarily become bound to the molecules priming them for reaction. However, after the reaction has occurred, they dissociated from the ester and dissolve back into solution.

Figure 3 - Acetic Acid

Figure 3 shows two models for acetic acid. The one on the left is a 3-D rendering of the molecule. The black central atoms are carbon, the red are oxygen, and the white are hydrogen. The model to the right is a simplified version common in organic chemistry. The line segment connect to carbon atoms unless otherwise noted. In Figure 4, the isoamyl acetate shows a case where a carbon chain is broken up by an oxygen molecule.

For each carbon atom, the number of hydrogen atoms attached is equal to four minus the number of lines connected to it. This is evident in the acetic acid model, where the terminal carbon atom has 3 hydrogen and one connection, and the central carbon atom has 0 hydrogen with 4 connections (one oxygen has a double bond).

Figure 4 - Reaction producing isoamyl acetate

The above figure shows the reaction forming isoamyl acetate. Acetic acid and isoamyl alcohol are the reactants on the left and isoamyl acetate and water are the products on the right. In this example, acetic acid has already been liberated from acetyl-CoA. The colors in figure 4 show the atoms destiny through the reaction. The oxygen in the acid turns into the oxygen found in the water product, while the oxygen in the alcohol forms the bond. This image was taken from the ucdavis website (Link here).

How to promote/discourage ester formation

There are several ways to promote or discourage ester formation, all of which are done during the fermentation process. The first factor is yeast strain. Different strains of yeast will produce different levels of  different esters. The temperature also plays a role in their formation; increased temperature during fermentation leads to more fusel alcohols.

Finally, pitch rate affects the formation of esters. If a beer is inoculated with fewer yeast cells at the beginning of fermentation, the beer will have more esters. Because there is low competition for the smaller yeast culture, they reproduce at a higher rate. This generates a larger concentration of acetyl-CoA, which contains a common reactant in the esterification process.


Note: Unless otherwise noted, All images were obtained from wikipedia with some MS Paint editing.

Demystifying the mash

Figure 1 - Enzymatic activities Obtained from How to Brew

The most important part of the hot side of brewing a beer is the mash. This is the stage in the brewing process where modified barley is converted to simple sugars that the yeast can metabolize. The modification process, which mostly takes place before the barley reaches the brewer, converts the bulk of the volume of the kernel to proteins and starches. There are several different classes of enzymes (which is a type of protein) that act on the proteins and starches that contribute to the quality of the finished product. The temperature as well as the pH play a role in determining which enzyme is active in the mash. Figure 1 shows several major classes that change the chemical make up and physical properties of the mash.

Step 1 - Acid Rest - 97 degrees Fahrenheit

If the pH and temperature become too high, the husks of the grain start of leech compounds that give the beer an astringent flavor. Luckily, there exists an enzyme in grain that reduces the pH of the mash that is active around 97 degrees Fahrenheit, where the first rest lies.

The modified barley is crushed up and added to a pot of water such that it has a consistency slightly more watery than grits. In antiquity, the mash would be heated to 97 degrees and let rest for up to 2 hours while the acid-producing enzymes (phytase) lower the pH. With a modern understanding of chemistry, different salts can be added to the mash to obtain the same effect. For this reason, the acid rest is rarely used in modern breweries.

Step 2 - Protein Rest - 122-133 degrees Fahrenheit

At each step of the mash a portion of the mash is pulled off, boiled, and then mixed back with the rest of the mash. This process, called a decoction mash, does two things for the to-be beer. It increases the temperature of the mash to the next rest temperature, while also breaking down the cell walls that were not broken up during the modification process. Most breweries have a steam controlled system that closely monitors the temperature in lieu of the decoction process.

So once the first decoction is complete, your mash will be between 122 and 133 degrees. At this temperature, the enzymes important in the acid rest have been denatured (become unwound) and are nonfunctioning. This rest actives three new classes of proteins: beta-glucanase, proteases, and peptidases. Now would be a good time to mention that whenever an enzyme is responsible for breaking down a different molecule, it has the suffix -ase.

Beta-glucanase and peptidases have a positive effect on the beer. Beta-glucanase breaks down a polysaccharide called beta-glucan. Beta-glucan causes the mash to have a high viscosity which can cause problems later during the brewing process. Peptidases release free amino nitrogen (FAN), an essential yeast nutrient, into the mash. The peptidases is particularly important if the malt was under-modified and has the bulk of its nitrogen bound in large proteins instead of free amino acids.

Proteases act to ruin the quality of the beer. This is responsible for breaking down the larger proteins, which are essential to a good beer. The larger proteins (along with the unconverted starch) are responsible for the perception of body. If there are to proteins in the finished beer, it will taste thin and watery. The large proteins are also responsible for the thick foamy head and its stability. A beer with low protein content will form a weak head that will dissipate quickly. It is because of this class of protein that the timing on this rest is so important: if too much time is spent on this rest the preteases will destroy all of your proteins.

Step 3 - Saccrification Rest - 145-160 degrees Fahrenheit

This is what we've been working towards. There are two enzymes responsible for breaking down the starches into sugars: Alpha- and Beta- amylase, which are together referred to as diastatic enzymes. During the protein rest, these enzymes become soluble and go into solution where they can act on dissolved starches. These two enzymes break down the starches in different ways.

For sake of this, consider starch as a very large shrub. These two enzymes attack the shrub from two different points. The alpha-amylase is able to take off entire branches, while the beta-amylase can only sever individual twigs. Chemically speaking, the beta-amylase can only liberate glucose and maltose, while the alpha-amylase is able to liberate amylopectin, which are branched chains of glucose and maltose. Once the amylopectic "branches" have been liberated, more regions are available for the beta-amylase to free glucose and maltose (maltose is just the glucose dimer). In this way, the two enzymes work together to break apart the starch molecules.

However, they aren't the perfect team. Beta-amylase denatures before alpha-amylase. Beta-amylase is most active in between 135 and 150 degrees Fahrenheit, while alpha-amylase is most active between 150 and 160 degrees F. This disparity in optimal temperatures provides an important variable in determining the fementability of the wort. If a  low temperature is used where both enzymes are active, the starch will be largely converted to simple sugars. These will all be fermented out into alcohol and leave a thin, dry beer with a higher alcohol content. However if a higher temperature is used, the beta-amylase becomes denatured, and only the alpha-amylase acts on the starches, which leaves a portion of the starch unconverted. This means the beer will have more body, be sweeter, but have a lower alcohol content.

Apart from ingredients, the mash temperature at the saccrification rest is among the biggest factors in controlling the end product. The difference of 5 degrees can mean the difference between a malty beer that has 5% sugar by weight after fermentation and a dry beer that has barely 2% sugar by weight.

Mashing out and sparging

Once the saccrification rest is complete, the temperature of the mash is then increased to about 168 degrees F. This denatures all amylase proteins and prevents any additional conversation from taking place. At this point, the liquid containing all the sugar must be separated from the grain prior to boiling. This occurs in a process called lautering.

The mash is then strained and the wort collected. Because the grain particles are saturated with the sugary wort, hot water is rinsed through the grain to try to extract as much of the sugar as possible. The first runnings from the mash are typically between 16 and 25 percent sugar by weight, depending on the style. As the grain is rinsed with pure water this sugar concentration drops until about 3 percent. If you continue to run off after the sugar concentration drops below this point, you risk introducing harsh flavors from tannins.

Once the wort is isolated from the grain in the mash, the wort is boiled and hops are added. This boiling not only drives off different compounds from the malt that would normally lead to off flavors (mainly DMS), but also activates different flavors from the hops depending on how long they are exposed to the heat of the boil. As a hop is boiled, it first loses its aroma compounds, next its flavor compounds, then finally the alpha-acids are isomerized into compounds that are perceived as bitter.




Way more information on decoction mashes can be found here courtesy of homebrewtalk

Can your liver actually process a drink per hour? How slow do you have to drink to stay sober?

We have all heard the rule of thumb that your liver can process one beer an hour. I've never been sold on the claim, here's why.

While there is no agreement on the exact rate, most research has concluded that the liver can process about 0.25 fluid ounces of alcohol per hour (some find a higher rate, while some lower). This rate is largely independent of your weight or gender. An average drink (say a 12 ounce can of some 5% ABV american lager), will contain about 0.6 fluid ounces of alcohol. A comparable amount of alcohol will be found in 1 shot (1.5 fl oz) of 40% liquor or a 5 ounce glass of 12% wine. My calculations for this can be found below, and show you will get drunk at a pace of 1 beer per hour if you are able to metabolize 0.25 fl oz.

Calculation of Amount of alcohol in a drink

BAC is determined by the amount of alcohol per amount blood. It is measured through your breathe, which has one tenth of the amount of alcohol as your blood. This means that if you blow a 0.10 BAC, your blood is actually 1% ABW (alcohol by weight). Blood consists of roughly 7% of a person's body weight, meaning a 170 pound man will have 11.9 pounds of blood. From the definition of BAC, we can solve for the weight of alcohol in a person at the legal threshold of .08%, and then find the volume of alcohol using it's density (.786 g/mL). The calculations, which are included below, show that the volume of alcohol in a 170 pound man who just blew a .08 is about 1.8 fl oz.

A BAC of .1 means there is 0.1 gram of alcohol in 100 gram of exhaled breath OR 1 gram of alcohol in each 100 gram portion of blood. We first need to find out how many 100g units of blood there are in the body. We will then have to multiply this number by the %ABW (which is 10 times the BAC) to find the grams of alcohol in the whole blood system. This can be converted to volume from the density of alcohol. We start by converting the weight of blood from pounds into grams.

We know that there are .8 grams of alcohol in each 100g section of blood. The number of grams in the 5.397 kg of blood can be found using cross multiplication.

Now that we know the weight of the total amount of alcohol in the body, we can use its density to find the volume and see how many beers it took to get there. Because we are working in metric, will we also have to convert the mL into fl oz in order to compare to the amount of alcohol in a single beer.

These calculations show that a 170 pound individual will have to have 1.84 fl oz of alcohol in his system to blow the .08. This is roughly equivalent to 3 beers, though the liver will be able to process some as he is drinking.


But back to the original question: What rate of beer consumption can the liver handle without becoming intoxicated?

Because each beer contributes 0.6 fluid ounces, and if liver is able to process 0.25 fluid ounces per hour, you would have to slow your consumption to a beer every 2.5 hours.



NOTE: The BAC analysis only accounts for the liver's contribution to clearing the alcohol from the body. Alcohol also leaves through your breathe, urine, and sweat. However, ,the majority of it (>85%) is processed by the liver. Another simplification I used was the method to calculate the amount of blood given someone's weight. This method of 7% of body weight was taken from the following book



Cameron, John R.; James G. Skofronick & Roderick M. Grant. Physics of the Body. Second Edition. Madison, WI: Medical Physics Publishing, 1999: 182.

Hops: Oils/Acids, Varieties, and the Struggle

Hops are one of the four major ingredients in beer. They provide a complexity through bitterness, aroma, and taste. During the brewing process, they are added during the boil for a length of time dependent on their role in the beer. As the time the hop is exposed to the heat of the boil increases, the flavor components will go from aroma to flavor to bitterness. This is due to the fact that different chemicals are created or driven off depending on the boil time.

Oils and Acids

They glory of hops lies in their oils and acid and each variety contains a unique chemical package. The acids are typically described with two different quantities: %alpha- and %beta- acids. Alpha acids are largely responsible for the bitter flavor in beer, while beta acids contribute to the bitterness to a lesser degree. It is interesting to note that hops high in alpha acids posses strong anti-bacerial properties that promote exclusively brewer's yeast to drive the fermentation (as opposed to countless bacteria).

Alpha acids refer to a class of chemicals that posses a carbon hexagon with branch groups containing ketones and alcohols. Under enough heat, these chemicals can be isomerized. That is, the atoms contained within the molecule will change in relation to one another. Isomerization does not refer to a chemical reaction where atoms are switched between molecules, only a restructuring of the molecule. Heat is required to isomerize the compounds, which is obtained in during the boil. As the hops boil a longer period of time, a greater proportion of the alpha acids are converted to iso-alpha acids, resulting in a more bitter beer. Bittering hops are boiled for at least 45 minutes for most beers. Hops varieties used for bittering will have high levels of alpha acids, up to 20% alpha acid by weight, while those for flavor and aroma can be less than 5% by weight.

Beta-acids differ from alpha-acids in that they cannot be isomerized. However, a chemical reaction with oxygen will yield a compound that is perceived as bitter. Iso-alpha acids will degrade with time, making the oxidation of beta-acids play an important role in lagers and aged beers. The ratio of alpha acids:beta acids changes depending on the variety, but there is almost always more alpha acids than beta acids. The alpha:beta ratio ranges between 1:1 to 2:1.

The majority of the weight in flavor/aroma hops comes from essential oils as opposed to the acids. These oils, which vary in concentration among different varieties, are highly volatile and are driven off during a long boil. Brewers will add these hops either: 1)in the mash which doesn't get above 170degF,  2) in the last few minutes of the boil ( <5 minutes), or 3) into a fermentor in a process called 'dry hopping'. By minimizing the amount of time the hop tissue is exposed to high heat, the volatile compounds are maximized.

Varieties

Noble Hops - Europe. Much like wine, the specific climate and soil conditions have a huge effect on the chemical profile of hops. There are four types of noble hops: tettnang, saaz, haullertau, spalt. These hop varieties are named after the region in which they are grown. Each variety is named after the town or region in which it is cultivated, in the same way burgundy wine is named after the region in France. Geographically speaking, saaz is from the Czech Republic, while the others are from Germany. Noble hops have low levels of alpha- and beta- acids (<5%) and an alpha:beta ratio ≤1. They are renowned for their consistent bitterness with age, and are the most common hop variety in most German/Czech lager and pilsners.

Sorachi Ace - Japan. Sapporo brewing company in Japan developed this hop from Saaz and Brewer's Gold. Their intention was to create a hop that would be similar in flavor to the Czech Saaz, but would possess the intense bittering power of the high alpha acid Brewer's Gold. While it does possess a high alpha acid content, the hop is still viable as a late addition hop due to it's unique lemon/spice flavor and aroma. It was first cultivated in the 1970s in Japan, but the hop wasn't available in the US until 2006. During the hop shortage in 2008, many breweries and home brewers began to experiment with it, with some having commercial success (Brooklyn Sorachi Ace saison).

There are dozens of other hop varieties grown though the US, Canada, and the UK. Each region has its own unique varieties breed for bittering, aroma, or flavor.

The Struggle

Roy Farms Hops

Hops are a pain in the ass to grow and harvest. The picture above shows a typical hop farm. The hops grow on bines which require a trellis for support. As the hops grow up the trellis, it increases the area of sunlight they are exposed to, which in turn increases the total yield of the plant. The trellises are constructed by hand, and are created whenever a new crop is planted. Each spring the vine starts from scratch, and will grow to the top of the trellis by the end of June. Each plant has to be trained by hand to grow up the trellis and not across the ground.

The recent craft explosion has put a strain on the hop growers around the world, particularly the United States. Hops can only be grown in certain regions, and the only areas suitable in the united states are in the northwest (Washington, but some in Idaho and Oregon). While the recent craft beer explosion provides a new market for hop growers, the constant thirst for something new creates demand for new hop varieties. Maintaining a hop crop is hard enough, but to switch between varieties can lead to financial ruin. Hop plants do not produce a crop their first year, so farmers will typically sign contracts with hop distributors guaranteeing that their crop will be purchased for several years regardless of demand. It is for this reason that hops are so damn expensive.

Preparing the Malt

Before the brewing process begins, the barley has to be partially germinated in a process called modification. The modification metabolizes the embryonic nutrients to produce a nugget of starch, which is in turn converted into simple sugars in the mash. After the barley is modified, it may be roasted to add complexity to the malt profile. While kilning produces a more complex flavor, it destroys the enzymes responsible for converting the starches into sugars. A beer will often get the enzymatic power from lowly/un-roasted barley, and will use kilned malts to add flavor complexity.

Anatomy of barley kernel  (Palmer, How to Brew)

The modification process was likely discovered by accident. Barley kernels which are soaked in water and then kept moist for several days will begin to germinate. The figure to the right shows the inside of a barley kernel a few days into the modification process. Endosperm contains globules of starch contained within tough cell walls. As the kernel germinates, the acrospire metabolizes the cell walls of the endosperm as it beings to engulf the volume of the kernel. If the acrospire takes up less than 75% of the volume, it is considered under-modified; if it beings to sprout, it is over-modified.

As the acrospire metabolizes the endosperm, it liberates the starches which will eventually become the sugars that the yeast turn into alcohol. The modification process changes the physical as well as the chemical properties of the kernel. Prior to modification, the kernel is hard as a rock due to the cell wall infrastructure. As the kernel is modified and the cell walls metabolized, the kernel becomes more friable and won't break your tooth during a sensory test.

The modification process requires the kernel to be saturated; up to 50% water by weight. This water content must be reduced to below 5% by slowly increasing the temperature to 100degF. After the malted barley has a low enough water content, it is then kilned to halt enzymatic activity as well as impart a complexity of sweetness. Pale and pilsner malt are kilned and low temperatures for a brief duration, while chocolate and Black Patent malt are essentially broiled for hours. There is a spectrum of malts between the two classes that is kilned at intermediate temperatures and duration. The kilning process was mechanized during the Industrial Revolution by Daniel Wheeler's invention of the drum roaster.

The kilning process provides heats that destroys some of the proteins required to convert starches into sugars. For this reason, you cannot make a beer strictly out of darkly roasted malts; you must have some pale/pilsner malt to provide the catalysts involved in saccrification. The proteins in highly roasted malts are broken down and often react with sugars in a Maillard Reaction

The Maillard Reaction is what gives steak and bread their color and flavor. It occurs between 140 and 160 degrees Fahrenheit, when sugars form compounds with amino acids. This reaction "stores" the sugar as yeast are unable to metabolize it. This reaction is responsible for the sweetness of Samuel Adam's Boston Lager. 

The Reinheitsgebot forced brewers to be creative with the four ingredients allowed. This lead to the spectrum of malts as well as the variety of different styles that exist today. The modification and kilning process have become so intricate, that they are typically preformed by maltsters and not your average brewery. While the brewer does not have to prepare their own malt, they are still responsible for converting the starches stored in the malted barley into fermentable sugars in a process known as mashing.