1. ● BREWING BIOCHEMISTRY
www.ibd.org.uk58 z Brewer and Distiller International November 2015
By Aaron Golston
With modern barley varieties
and improved malting
operations, the protein rest
during mashing may become
a relic of the past.
Protein rests were introduced in
the mashing process in order to to
further degrade the protein matrix in
the malted barley’s endosperm. This
was to allow better access for enzymes
to the starches contained within the
interstitial cavities and to ensure there
would be sufficient free amino nitrogen
(FAN) for yeast health, growth and rep-
lication. Historically, protein rests were
necessary because the malt was often
unevenly modified, contained high
protein levels or crop-to-crop water
sensitivity variations. Now with newer
barley varieties and improved malting
operations, this practice may need to
be reevaluated.
Malt modification
To help clarify why protein rests may
no longer be needed, it would help
to revisit the topic of modification.
Modification is an all-encompassing
term that maltsters and brewers used
to describe the enzyme development,
endosperm breakdown and protein
solubilisation, in the malted barley
kernel. The level of modification is
characterized by the friability of the
malt, the fine/coarse (F/C) difference
and the soluble nitrogen ratio (SNR).
The friability is the ability of the malt
to be easily ground for the purposes
of mashing and it is measured us-
ing a friability meter. The friability
of the base malt should be around
90%. The F/C difference is the differ-
ence between the amount of extract
obtained from a congress mash with
finely-ground malt and one with
coarsely-ground malt. These two tests
are standardized by the ASBC and EBC
for easy comparison between brands
and types of malt. The F/C difference
should be less than 1.5% and ideally
around 0.6%. The SNR is a measure
of how much of the protein from the
barley’s endosperm is degraded to the
point of being soluble in the wort. The
higher the SNR, the higher the level of
modification of the malt. The SNR is
usually around 41 – 46% for pale base
malt.2
An important factor to consider in
recipe development is how well the
base malt is modified. In the United
States, a typical two-row pale malt
is going to meet the aforementioned
specifications. Friability, F/C differ-
ence and FAN levels are all sufficient
as the malt arrives from the maltster
to allow the protein rest to be skipped
and many brewers have eliminated the
protein rest as sufficient FAN levels
can be reached without it. In addi-
tion to the time and energy savings
from skipping the protein rest, some
research done at Carlsberg indicates
there may be another important rea-
son for its omission, namely Strecker
aldehyde formation1
. Most brewers
recognize that the term Strecker alde-
hyde and understand the correlation
between these compounds and the
potential negative impacts on flavour
stability in the final product.
Strecker aldehyde formation
The Strecker degradation was discov-
ered in 1862 by Adolph Strecker but
his work had nothing to do with beer
or food, in fact he only showed that the
degradation of alanine with alloxan
yielded CO2
and acetaldehyde. Work
published in 1927 by Neuberg and Ko-
bel described the destruction of amino
acids by some carbonyl compounds.
Schönberg, Moubasher, and Mostafa
later showed, in 1948, the degradation
of FAN into aldehydes after interaction
iStock.com/leezsnow
Protein rests – are
they still relevant?
Figure 1: Step mash with protein rest (blue line with circles) and step mash without protein test
(red line with circles)
Temp(°C)
80
75
70
65
60
55
50
45
0 20 40 60 80 100 120
Time (min)

2. BREWING BIOCHEMISTRY l
www.ibd.org.uk Brewer and Distiller International November 2015 z 59
with carbonyl compounds.3
Strecker aldehydes form when FAN
and dicarbonyl compounds, such as
α-deoxyosones or Maillard reaction
intermediates react (Figure 3). Thus,
the more FAN available, the more
Strecker aldehydes that can form. With
a protein rest, proteases degrade the
protein matrix of the endosperm and
increases FAN (Figure 4). Maillard
reaction intermediates created during
the boil or already present from the
raw ingredients, can react with abun-
dance of FAN leading to the formation
of Strecker aldehydes.
The work done at Carlsberg never
mentioned protein rests explicitly but it
correlated low mash in temperatures
(48-60°C) with high-protein malts
leading to increased levels of Strecker
aldehydes in the wort and in the aging
of beer. Increased FAN levels were
observed after fermentation when an
all malt grist was used in beer produc-
tion. However, when a malt/adjunct
beer was produced at the same plato,
almost no FAN remained after fermen-
tation. This may or may not be relevant
to all brewers but that is dependent on
the type of beer being produced. If all
malt beers are being made, then there
likely is sufficient FAN levels available
in the base malt as it arrives from the
maltster to sustain the yeast. Further
degredation of the proteins in the
endosperm will only create more FAN
that can participate in staling reac-
tions through the Strecker degradation
pathway.
The flavour stability of lager beers
has been studied by many people over
the years, as the flavour profile is
very delicate and off-notes are easily
detected. Even though the research
has focused on lagers, the Strecker
degradation occurs in all beers and
many other foodstuffs. The extent to
which Strecker aldehydes will impact
the flavour profile of a specific beer
would have to be determined using
advanced analytical instruments, such
as GC/MS or LC/MS, and validated with
a trained sensory panel and flavour
stability programme. The compounds
formed during the Strecker degrada-
tion in beer depends on the individual
amino acid composition of that beer.
Some of the compounds that can form
are 2-methylpropanol (from valine),
2-methylbutanal (from isoleucine),
3-methylbutanal (from leucine), me-
thional (from methionine) and phenyl
acetaldehyde (from phenyl alanine)4
– and generally they are not consid-
ered to be positive aroma attributes of
most beers and have been shown to
increase as beer ages. They have been
described as grainy, varnish or fruity;
malty, cherry, almond or chocolate;
almond, apple-like or malty; cooked
potatoes or worty; and flowery or
roses, respectively, as reported by
Saison et al5
.
Other authors have advocated skip-
ping protein rests for the sake of foam
stability when using well-modified
malts6
but it was not until the work
done at Carlsberg1
that low mash
in temperatures and Strecker alde-
hyde formation had been correlated.
Therefore, eliminating excessive FAN
production by skipping the protein rest
during the mashing process should
be advantageous to the shelf life and
flavour/aroma profile of most beers, if
flavour stability is an issue.
About the author:
Aaron Golston earned his Master’s
from UCDavis in Charlie Bamforth’s
lab, after which he worked for Mill-
erCoors as an Analytical and Micro-
biological Specialist at their Irwindale
Brewery. In 2013, he joined Lagunitas
Brewing Company as the Quality Man-
ager for their new Chicago Brewery. In
2015, he completed his IBD Diploma in
Brewing.
References
Bech, L. (2012). Significance of Amino Ac-
ids for the Production of Strecker Aldehydes
and Potential Ways of Reducing These Stal-
ing Compounds in Beer.
O’Rourke, T. (2002). Malt Specifications &
Brewing Performance. The Brewer Inter-
national. 2.10: 27-30.
Schonberg, A. and Moubacher, R. (1952).
The Strecker Degradation of α-Amino
Acids. Chemical Reviews 50.2:261-277.
Malfliet, S., Opstaele, F.,Clippeleer, J.,
Syryn, E., Goiris, K., Cooman, L., and
Aerts, G. (2015). Flavour Instability of Pale
Lager Beers: Determination of Analytical
Markers in Relation to Sensory Age-
ing. Journal of the Institute of Brewing.
114.2:180-192.
Saison, D., De Schutter, D. P., Uyttenhove,
B., Delvaux, F. and Delvaux, F. R. (2009).
Contribution of Staling Compounds to the
Aged Flavour of Lager Beer by Studying
Their Flavour Thresholds. Food Chemistry
114: 1206-1215.
Kunze, W. (2004). Wort Production. Tech-
nology Brewing and Malting. 3rd ed. VLB,
Berlin.
Moisture, % 4.9
Fine Grind,As is, % 77.4
Fine Grind, Dry Basis, % 81.4
F/C Difference 0.5
Course Grind,As is, % 76.9
Coarse Grind, Dry Basis, % 80.9
Colour, °L 1.99
Diastatic Power, °Litner 149
Alpha Amylase, DU 71.1
Total Protein, % 11.58
Soluble Protein, % 5.26
S/T Ratio 45.4
Viscosity, cP 1.45
Beta Glucan, mg/L 92
FAN, mg/L 194
pH 6.03
Figure 2:Typical 2 – row pale malt analysis (2015)
+ + +
amino acid Strecker aldehydedi-carbonyl compound carbon dioxide amino ketone
O
O O O
O O
O
OH
NH2
H2N
C
R R R H R R
R
Figure 3: Strecker degradation reaction
Proteases
Protein rest
Gly Leu Met Val Ile Pro Pro Phe Gly Pro
Met
Pro
Val Phe Gly Pro
Ile Pro
Leu
Gly
Figure 4: Protease activity during protein rest