>>The flavor and aroma of beer is very complex. In fact, there's over 800 identified compounds
that lead to flavor in beer alone. And when we think about the styles of beer, we need
to think about what type of water, malt, hops, and yeast we're going to use to get an identifiable
style of beer. If we think about these components, water is most of what beer is, so it is an
important factor. It doesn't add a lot, or should not add a lot, to the aroma, but the
components that are in the water can have impact on many of the metabolitic cycles that
occur in the yeast and can have impact on the pH, which can have an impact on the types
of compounds that are extracted from our hops. The important factors in water are the calcium
concentration, magnesium concentration, and even some zinc. We also need to watch our
anions, such as carbonate, which can cause the pH to be too high and affect the flavors
that are coming out of hops and the yeast, and also the sulfates, which can add a little
bit too much sulfur compounds to the final product.
If we consider malt, the main component of malt, of course, is our carbohydrates. And
the flavor component we're going to get from malt is maltose, the normal sugary malt flavor
that we characterize with beer. The other factors that come out of the malt in the kilning
process will be the caramelization. Caramelization is where we take sugars and heat those up
to make the caramel color and caramel taste that are familiar to us, and also the Maillard
Reaction. This is where carbohydrates, reducing sugars, are interacting with amino acids.
Both of these start during the malting process and the kilning process. We can also see the
Maillard Reaction working with the boiling process, especially on the conction, mashing.
The next component that adds significantly to the flavor is our hops. Our hops is going
to add a lot of our flavor components. The areas that we need to watch for in hops is
resins, such as our alpha acids and beta acids, our essential oils, and then the isomerization
that can occur with the alpha acids. Before we get into the specific compounds in hops,
we need to think about what type of compounds will be found in the hops. It turns out there
are all the ones that we're going to be discussing are terpenes. Now what are terpenes? Terpenes
are biologically synthesized compounds from isoprene. Isoprene is a 5 carbon group that
has 8 hydrogens. When we think about a terpene, there are several types of terpenes. We can
have a monoterpene, where it's actually two isoprene groups together so C10 H16, or we
can have sesquiterpenes, where we have three isoprenes or C15H24. When we look at these
two types, they're going to be most of what we have. Now when we look at these structures,
what you'll see is that we can see the isoprene units within these biological structures.
So let's take a look at a monoterpene, myrcene. This is going to be a component to one of
the essential oils. If we look at that, we can actually determine that we have two isoprene
units in the myrcene. If we look at another common component in the essential oils of
hops, the humulene, this is a sesquiterpene. With a sesquiterpene, what we have is three
isoprene units. If we look at this structure, we can identify the three isoprene units,
where we have the five carbon groups. Notice the branching and notice how they are attached.
This is something that is useful to know, especially if you want to think about how
these compounds are related. One other factor that we should consider is although we usually
call this large class terpenes, often if there is any modification of an oxygen, such as
a hydroxide group or a ketone, these often are called terpenoids. So if you hear the
term terpenoid, know that's just a derivative of a terpene.
The alpha acids. If we think the hops, the main component that are often discussed is
the alpha acid and what type of alpha acids are actually in the hops. This is very important,
and this gives us most of our bitter flavors and our aromas. The three main alpha acids
are going to be humulone, cohumulone, and adhumulone. If we look at these, the only
difference in their structure is one side group, and how many carbons, and the placement
of carbons in these side groups. Besides this small group, all three of these structures
are identical. Again, the alpha acids are very important. And when you look at hops,
you'll see that it often will tell you how much alpha acid, and in modern data tables
you'll also see what the breakdown of the three alpha acids are.
Beta acids are very similar. They have a slightly different structure. We have lupulone, colupulone,
and adlupulone. And if we look at those, again, we are only differing by a small side group
on the side. The rest of the structure is same. The beta acids are less important than
the alpha acids, but still add flavors and potentially aromas. The ratio between the
alpha and beta acids is also a quantity that you could also want to know when selecting
hops for a specific style of beer.
When we look at the essential oils, most of these are just hydrocarbons, although we do
have some that have oxygens attached. The essential oils have some antiseptic properties
that again help with the shelf life of the beer, but they also add very characteristic
flavors. If you look at one of those, humulene, humulene adds a nice elegant tone to the beer.
If we look at myrcene, which we looked at earlier as our monoterpene, meaning 10 carbons,
it is seen in American hops at a higher concentration than we expect in European hops. It gives
a pungent, hoppy flavor that's less elegant than the humulene, but is still very characteristic.
And most American pale ales will have this in a higher concentration than you would expect
in European pale ales. The other two, caryophyllene and farnesene, are also essential oils that
are known and are characterized in most hop varieties. The last part of what the hops
does is the isomerization. Alpha acids are not overly soluble, but they can be converted
to iso-alpha acids with heat. This is a thermal isomerization that we'll be looking at in
more detail in the course. What happens with the iso-alpha acids is they are more soluble,
which means when we get this conversion to occur, we get more of the alpha acid moiety
into the solution. The isomerization is characterized with the boil. So if we put the hops into
a vigorous boil, we'll get more of the isomerization. If we do a late hopping or a dry hopping,
we're going to see less of this isomerization. Again, just like with many of the components
in hops, since they are so specific to different types of beers or styles of beers, they have
information on the percentage of isomerization and even how much time it will take at a full
boil to get the isomerization to occur.
Yeast adds a lot of flavor components, in fact over 600. Often we think of yeast as
just forming the ethanol, taking glucose to pyruvate, and pyruvate into ethanol. And of
course, the ethanol is going to have very specific flavors. Except for a few select
styles of beer, ethanol should be an after note and not the main flavor that we taste.
But in some high alcohol styles of beer, you will have that liquor flavor from the ethanol.
So if yeast is mainly doing this, how do we get all those other components? Well, yeast
is also having secondary and tertiary metabolitic cycles. These other metabolitic cycles are
turning other components that the yeast may need. So if we look at those, the yeast can
produce larger alcohols, esters, and ketones. The yeast may also have to produce some amino
acids or some fatty acids if our wort does not contain the proper amount of amino acids
or fatty acids from the lipids from the malt. In high adjunct beers we have to be careful
that we have the right amount of amino acids and lipids, so that the yeast can produce
and grow well, so it does not have to take its time to create these. Again, these are
all based on the style. So different yeast strains will produce different amounts of
alcohols, esters, and ketones. If we think about our alcohols, ethanol is just two carbons
and a hydroxide group. If we think about our larger ones, like propane, we have three carbons,
or butane, four carbons, these are just larger alcohols and will be contained depending on
the amount of oxygen that is in the solution and the strain of yeast.
Propanol and butanol would not be a favored flavor component because it's going to give
you a very alcoholy or solvent-like tone. Other alcohols like isoamyl alcohol by itself
may not be as solvent-like and can even give you kind of a banana flavor. As we'll see
in a second, creating this alcohol is important to creating the note of bananas that we'll
see in some beer styles. The other alcohols that we'll see are actually from amino acids.
Converting the amino acids into alcohol, an example of this is like tyrosol, again not
a alcohol that we want to produce in high quantities because it will give an off flavor.
Esters are created by taking alcohols that are produced and combining those with carboxylic
acids, most often as acetyl coenzyme A. These combined together to give us esters, so therefore
if we have a type of alcohol and either an acetate or a fatty acid, we can create these
esters. Esters give us our fruity tones and are characteristic of ales. Esters can overwhelm
a beer if we increase the fermentation temperature too high. So it's important to understand
the proper techniques to make sure that we keep esters as subtle tones and not overpowering
the beer. If we look at these, ethyl acetate is very common. In fact, it's used in non-acetone
fingernail polish, but in very small quantities it actually has nice tones that are very subtle.
Isoamyl acetate is the component that gives us the characteristic flavor of bananas. Again,
if we have this in small amounts it will give us a little hint of a banana. In a Hefeweizen,
this may actually be a little bit high. You might be able to tell that it's bananas. But
at very high concentrations, it would taste like Laffy Taffy, and we don't want a beer
that tastes like Laffy Taffy.
The last one, ethyl hexanoate. Again, here we're taking a fatty acid and combining it,
and again, in small quantities this ester can add a lot of nice flavor components. When
we look at our ketones, we often are focused on diacetyl. Diacetyl is a four-carbon group
with two ketones in the middle. It has a buttery flavor. It usually gives us an indication
that something went wrong in the brewing process: either we did not have the right oxygenation
of our wort, we did not leave our wort in contact with the yeast long enough, or there
was something else going wrong, such as a foreign bacteria that made it throughout the
process. So diacetyl is one of those compounds we really have to watch, especially if you're
trying to do this at home. Industrial brewers spend a lot of time to try and control diacetyl.
Now one of our other ketones that again only shows up in certain varieties of beer is the
2,3-Pentanedione. 2,3-Pentanedione gives us a honey tone and again is only specific for
very few styles of beer.
Lastly, we need to talk about off flavors. Most of the things we've been talking about
so far have been things that we want to have in our beer, such our characteristic hop and
malt flavors. But there are other things we need to avoid, especially if we're home brewing.
Again, industrial brewers spend a lot of time avoiding these. First one, acetaldehyde. Acetaldehyde
is the flavor of green apples, not as Granny Smith apples but as unripe apples. This is
an indication that the beer was taken off of the live yeast too fast. Just by leaving
the beer to maturate over the yeast, the acetaldehyde concentration will decrease and this flavor
will disappear. Another off flavor we can have is from chlorophenols. Chlorophenols
give a very medicine-y flavor. It's often from using bleach as our sanitation solution
in too high a concentration. It is the same compound that gives us cork tint in wines.
Dimethyl sulfide, DMS, is also something we need to watch. Dimethyl sulfide is something
that will be generated during the brewing process, but dimethyl sulfide is the same
compound that is used in the U.S. to give the smell of natural gas. Turns out natural
gas does not have a smell to itself, and so they add dimethyl sulfide so that it has a
very characteristic smell that we recognize as natural gas, well, the smell they add to
natural gas. Of course, you don't want that in your beer. It's very easy to get rid of
dimethyl sulfide. You just need to have a very vigorous boil, but you do have to be
careful because if you extend a very vigorous boil for a long time, it can cause other problems
with other taste components. Again, flavor and aroma in beer is very complex.
Now the next two come from actually storage. If we think about the stale beer flavor, this
actually comes from oxidation of compounds in the solution. The one that gives us a most
characteristic stale beer flavor is trans-2-nonenal. This is an aldehyde compound that occurs from
oxidation of a larger alcohol and gives us a characteristic stale beer. The way to keep
this from occurring is to limit the amount of oxygen in our headspace, but some oxidation
is always going to occur, so therefore the shelf life of the beer will be affected by
this compound. The other notable storage problem with off flavors is the light strike compound
3-methyl-2-butene-1-thiol. This mercaptan compound occurs from the interaction of light
with the hops compounds, especially humulone. So if we get this compound to occur, it means
that light has contacted. Of course, that means if we can keep light away from a beer,
it's going to keep this skunkiness away. So when we have a skunky beer most often that'll
occur when we have light contacting the beer. This occurs very quickly. In a clear bottle
it can actually occur in seconds. So therefore, if we have an opaque bottle or a can or a
keg, we can keep the light strike down to a minimum. Again, the flavor and aromas of
beer are very complex. What we want to do to get a specific style of beer is to understand
what flavor components are coming from the malt, and the type of malt we're using, the
hops, and again the type of hops, and then our yeast strain. All of these add to the
complexity and richness of the flavors of beer.