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Current Challenges of Cold Brew Coffee—Roasting,
Extraction, Flavor Profile, Contamination,
and Food Safety
Raven Kwok 1, Kenny Lee Wee Ting 1, Steffen Schwarz 2, Linda Claassen 3,4
and Dirk W. Lachenmeier 3,*
1
Earthlings Coffee Workshop, Soho East, Sublot 16 Ground Floor, Lot 188, Jalan Wan Alwi Lorong 5, Kuching,
Sarawak 93350, Malaysia; raven@earthlings-coffee.com (R.K.); kenny@earthlings-coffee.com (K.L.W.T.)
2Coffee Consulate, Hans-Thoma-Strasse 20, 68163 Mannheim, Germany; schwarz@coffee-consulate.com
3Chemisches und Veterinäruntersuchungsamt (CVUA) Karlsruhe, Weissenburger Strasse 3,
76187 Karlsruhe, Germany; lindaclaassen@gmx.de
4Department Life Sciences, University of Applied Sciences Albstadt-Sigmaringen,
72488 Sigmaringen, Germany
*Correspondence: Lachenmeier@web.de; Tel.: +49-721-926-5434
Received: 5 July 2020; Accepted: 10 October 2020; Published: 13 October 2020
Abstract:
Cold brew coffee has emerged as a new trend over the last decade. However, “cold brew”
is an extraction style of ground roasted coffee with water at lower than body temperature (typically
8
◦
C or room temperature), rather than a beverage per se. Cold brew extraction poses several
challenges, including the need for specific optimization depending on the multiple influences of coffee
variety and processing, roast degree, grinding, dosage, water composition, turbulence, brew system
(drip, immersion etc.), time and temperature, and their interactions. While cold brew is typically
characterized by a floral sweetness, over-extraction may lead to abundant acidity and bitterness.
To avoid this, an extraction degree of 70% was suggested using shorter time frames (i.e., 2 h at 15
◦
C
with 80 g/L coffee for optimized medium roast profiles). Due to the lack of sterilizing temperatures
during preparation, cold brew is significant in the coffee sector because hygiene and food safety
requirements pose specific challenges. To avoid microbiological contamination and deterioration in
quality, cold brew should be as freshly prepared as possible and shelf-life should be minimized.
Keywords:
coffee; cold brew; nitro cold brew; roasting; extraction; hygiene; risk assessment;
product quality
1. Introduction
The history of cold brew coffee can be traced as far back as the 1600s, with a major invention
being the Toddy cold-brew coffee system in the 1960s [
1
]. However, only recently has cold-brew
become a growing trend across the entire coffee industry [
2
]. For example, cold brew sales grew by a
remarkable 580% in the US in the period between 2011 and 2016 [
3
]. Due to its relative novelty, there is
an absence of in-depth research into this coffee extraction method [
4
]. There is also currently a lack of
internationally accepted standards or definitions about what cold brew is and under what conditions it
is made [
5
]. The demand for high-quality, cold brew coffee may have been driven by the fact that the
segment of iced coffee has been adversely affected in the past “by using old, bitter-tasting, brewed
coffee as the base” or “even worse, many have used coffee extracts” [6].
Several cold brew methods such as drip filtration, full immersion, or cold press are available.
In general, the term “cold brew” describes a method of preparing a beverage in the form of a certain
style of extraction. Cold brew is not necessarily a cold beverage since, unlike iced coffee, cold brew
Challenges 2020,11, 26; doi:10.3390/challe11020026 www.mdpi.com/journal/challenges
Challenges 2020,11, 26 2 of 13
may be served cold or hot. It is important to differentiate between cold brew and iced coffee, which is a
beverage extracted with hot water. Sometimes, mostly in industrial settings, hot brewed beverages are
cooled (the so-called “hot bloom” method [
5
]) and sold as some form of fake cold brew (“called brew”).
This article provides an overview about the current knowledge of cold brew coffee preparation,
along with practical aspects and potential pitfalls specifically related to hygiene and food safety.
Furthermore, open research questions are included, and research challenges are highlighted.
2. Materials and Methods
For the first part of the article (Section 3), electronic searches of literature were conducted
including the databases PubMed, Google Scholar, and Food Science and Technology Abstracts (FSTA).
Search terms used were: (“cold brew” OR “nitro”) AND “coffee”. The abstracts were screened for
relevance regarding aspects of roasting, extraction, flavor profile, contamination, and food safety.
Relevant articles were obtained in full text. The searches were complemented by the literature collection
of the authors. The major results were discussed using several online training sessions with coffee
experts hosted by the professional training centers of Earthlings Coffee Workshop (Malaysia) and
Coffee Consulate (Germany), and research plans were derived from this (Section 4). The feedback of
the experts is also included in a section about open problems and challenges (Section 5).
3. Cold Brew Extraction—What Do We Know?
3.1. Roasting for Cold Brew Coffee
Roasting plays a big part in determining what flavors go into the cold brew as it is possible to retain
or eliminate various aromas by manipulating different roast levels. Consumer research has shown
that the predominant factors influencing cold brew flavor are the roast level [
7
] and the resting time
after roasting [
8
,
9
]. In general, the authors suggest avoiding excessive dark roasting, so the resultant
coffee avoids unpleasant aromas. Research also found that darker roasts may result in decreased
concentrations of compounds [
10
]. Typical roast profiles for cold brew, filter coffee, and espresso are
compared in Figure 1. The biggest variant is with the espresso roast, while cold brew and filter coffee
are similar. The cold brew is the fastest roast of the three with a more rapid arrival at phase III. The first
phase is more or less the same for all three types. The second, drying phase preserves the acids in
the bean. The cold brew roast is slowed down at the very end of its processing to avoid excessive
formation of Maillard reaction-based aromas.
3.2. Multi-Variate Influences on Cold Brew Extraction
The extraction of cold brew is typically conducted below body temperature, but there is no
universally accepted extraction temperature setting. The concept of “cold” is often culturally relative to
prevailing natural or artificial environments such as countries with cold winters vs. tropical countries,
air conditioning, and refrigeration. The authors, however, believe that temperature settings near or
above body temperature move away from the concept of cold brew, and should be critically assessed
to avoid misleading food information [11].
Cold brew can be extracted by all the three methods used for hot coffee (see comparison of coffee
extraction methods by Angeloni et al. [
12
]). It can be extracted as (i) cold drip (typically with iced
water) in a filtration method, (ii) lixiviation or immersion (grounds in a pot sitting in the water, with or
without turbulence), or (iii) cold press (under or overpressure) [11,13,14].
What exactly does the “cold” mean in cold brew? It may be close to 0
◦
C (i.e., melting ice using
the drip method), at fridge temperature (4–8
◦
C), or at any other temperature below body temperature.
There is a much broader range of temperatures than those used for hot coffee extraction. The cold brew
extraction time is much longer than for hot brews, and largely depends on the selected temperature.
The minimum would be 2 h at 20
◦
C. It is important to adjust the time/temperature equilibrium to not
over or under extract the cold brew.
Challenges 2020,11, 26 3 of 13
Challenges 2020, 11, x FOR PEER REVIEW 3 of 13
Figure 1. Exemplary roasting profiles of cold brew (gray line) compared with filter coffee (dark blue
line) and espresso (light blue line) (FZ-94 sample roaster, Coffee-Tech Engineering, Moshav Mazliach,
Israel).
3.2. Multi-Variate Influences on Cold Brew Extraction
The extraction of cold brew is typically conducted below body temperature, but there is no
universally accepted extraction temperature setting. The concept of “cold” is often culturally relative
to prevailing natural or artificial environments such as countries with cold winters vs. tropical
countries, air conditioning, and refrigeration. The authors, however, believe that temperature settings
near or above body temperature move away from the concept of cold brew, and should be critically
assessed to avoid misleading food information [11].
Cold brew can be extracted by all the three methods used for hot coffee (see comparison of coffee
extraction methods by Angeloni et al. [12]). It can be extracted as (i) cold drip (typically with iced
water) in a filtration method, (ii) lixiviation or immersion (grounds in a pot sitting in the water, with
or without turbulence), or (iii) cold press (under or overpressure) [11,13,14].
What exactly does the “cold” mean in cold brew? It may be close to 0 °C (i.e., melting ice using
the drip method), at fridge temperature (4–8 °C), or at any other temperature below body
temperature. There is a much broader range of temperatures than those used for hot coffee extraction.
The cold brew extraction time is much longer than for hot brews, and largely depends on the selected
temperature. The minimum would be 2 h at 20 °C. It is important to adjust the time/temperature
equilibrium to not over or under extract the cold brew.
The extraction of cold brew coffee depends on several factors including the coffee, roasting,
dosage (brew ratio), water temperature, and water composition such as hardness, turbulence
(increasing the water contact with the coffee grounds by stirring, agitating, or applying ultrasonic
waves), grinding level (particle size and surface, dust), and time (Figure 2). From these parameters,
the roasting profile is particularly significant as it influences the acidity that is extracted out of the
beans. The grinding surface is also vital because the extraction is influenced if the surface is irregular
or rounded.
Figure 1.
Exemplary roasting profiles of cold brew (gray line) compared with filter coffee (dark blue line)
and espresso (light blue line) (FZ-94 sample roaster, Coffee-Tech Engineering, Moshav Mazliach, Israel).
The extraction of cold brew coffee depends on several factors including the coffee, roasting,
dosage (brew ratio), water temperature, and water composition such as hardness, turbulence (increasing
the water contact with the coffee grounds by stirring, agitating, or applying ultrasonic waves),
grinding level (particle size and surface, dust), and time (Figure 2). From these parameters, the roasting
profile is particularly significant as it influences the acidity that is extracted out of the beans. The grinding
surface is also vital because the extraction is influenced if the surface is irregular or rounded.
Challenges 2020, 11, x FOR PEER REVIEW 4 of 13
Figure 2. Ishikawa diagram of causal influences regarding cold brew coffee extraction.
3.3. Hazards During Cold Brew Extraction
Microbiological contamination along with typical chemical contaminants such as pesticides,
mycotoxins, acrylamide, and furan must be considered (Figure 3). Cold brew is one the few coffee
beverages that includes some microbiological food safety hazards. Risk is usually minimized by the
roasting process and fresh, hot brewing. The long brewing time of cold brew can facilitate
microbiological activity. Lopez [15] summarized microbiological challenge studies based on personal
communications with the industry showing that various species such as Salmonella, Listeria
monocytogenes, and Escherichia coli may be viable in cold brew for 7–28 days. Specifically, the potential
for Listeria monocytogenes to survive in cold brew coffee was judged as being of concern due to its low
infective dose [15].
Figure 3. Ishikawa diagram of causal influences of extraction on cold brew coffee contamination.
Cold brew producers should be vigilant for any visual or taste changes, which could be an
indication of microbial contamination. The risk of microbial growth is strongly related to the
extraction temperature and is dramatically reduced at very cold temperatures.
The most common organisms that could spoil cold brew are yeasts (leading to alcoholic
fermentation) and lactic or acetic acid bacteria (producing organic acids). For most small-scale
producers (e.g., in their coffee shops), it is impossible to work in a fully sterile environment, so the
product should have limited storage time. Pathogenic microorganisms such as Salmonella or Listeria
must be avoided.
Figure 2. Ishikawa diagram of causal influences regarding cold brew coffee extraction.
Challenges 2020,11, 26 4 of 13
3.3. Hazards during Cold Brew Extraction
Microbiological contamination along with typical chemical contaminants such as pesticides,
mycotoxins, acrylamide, and furan must be considered (Figure 3). Cold brew is one the few coffee
beverages that includes some microbiological food safety hazards. Risk is usually minimized
by the roasting process and fresh, hot brewing. The long brewing time of cold brew can
facilitate microbiological activity. Lopez [
15
] summarized microbiological challenge studies based
on personal communications with the industry showing that various species such as Salmonella,
Listeria monocytogenes, and Escherichia coli may be viable in cold brew for 7–28 days. Specifically,
the potential for Listeria monocytogenes to survive in cold brew coffee was judged as being of concern
due to its low infective dose [15].
Challenges 2020, 11, x FOR PEER REVIEW 4 of 13
Figure 2. Ishikawa diagram of causal influences regarding cold brew coffee extraction.
3.3. Hazards During Cold Brew Extraction
Microbiological contamination along with typical chemical contaminants such as pesticides,
mycotoxins, acrylamide, and furan must be considered (Figure 3). Cold brew is one the few coffee
beverages that includes some microbiological food safety hazards. Risk is usually minimized by the
roasting process and fresh, hot brewing. The long brewing time of cold brew can facilitate
microbiological activity. Lopez [15] summarized microbiological challenge studies based on personal
communications with the industry showing that various species such as Salmonella, Listeria
monocytogenes, and Escherichia coli may be viable in cold brew for 7–28 days. Specifically, the potential
for Listeria monocytogenes to survive in cold brew coffee was judged as being of concern due to its low
infective dose [15].
Figure 3. Ishikawa diagram of causal influences of extraction on cold brew coffee contamination.
Cold brew producers should be vigilant for any visual or taste changes, which could be an
indication of microbial contamination. The risk of microbial growth is strongly related to the
extraction temperature and is dramatically reduced at very cold temperatures.
The most common organisms that could spoil cold brew are yeasts (leading to alcoholic
fermentation) and lactic or acetic acid bacteria (producing organic acids). For most small-scale
producers (e.g., in their coffee shops), it is impossible to work in a fully sterile environment, so the
product should have limited storage time. Pathogenic microorganisms such as Salmonella or Listeria
must be avoided.
Figure 3. Ishikawa diagram of causal influences of extraction on cold brew coffee contamination.
Cold brew producers should be vigilant for any visual or taste changes, which could be an
indication of microbial contamination. The risk of microbial growth is strongly related to the extraction
temperature and is dramatically reduced at very cold temperatures.
The most common organisms that could spoil cold brew are yeasts (leading to alcoholic
fermentation) and lactic or acetic acid bacteria (producing organic acids). For most small-scale
producers (e.g., in their coffee shops), it is impossible to work in a fully sterile environment, so the
product should have limited storage time. Pathogenic microorganisms such as Salmonella or Listeria
must be avoided.
When filling cans, bottles, or kegs, the use of additives such as ascorbic acid or preservatives may
increase microbiological stability and shelf life, as well as heat sterilization or pasteurization (see [5]).
However, all these methods are expected to negatively impact the flavor.
Heat-induced coffee contaminants formed during roasting, such as furan or acrylamide [
16
],
were found at similar levels in cold brew when compared with hot brew [
17
]. While acrylamide
may increase with prolonged extraction times, furan may decrease due to volatility [
18
]. Similarly,
other chemical contaminants of coffee such as pesticides or mycotoxins are expected to occur in cold
brew when the raw beans are contaminated, but this is not a problem specific to cold brew. Therefore,
we believe that microbiological contamination is the exception to the rule that needs specific mitigation
measures during the cold brew manufacture.
3.4. Extraction Degree
Considering consumer preference in Malaysia and Germany [
11
,
19
], 70% extraction (i.e., 70% in
relation to the total extractable/soluble amount) may be an optimal starting point for a cold brew coffee
recipe to achieve a balanced product (Figure 4). This number does not yet have a scientific basis but
Challenges 2020,11, 26 5 of 13
stems from experience considering all aspects of the product, including the risk of microbiological
contamination and the flavor. In other words, the optimum extraction results in the best flavor while
avoiding contamination. Increased agitation will result in a faster process; cooler temperatures will
take longer. Not all the soluble constituents should be extracted, but just enough to get a good taste
while minimizing the risk of microbiological contamination. This can also happen in hot extractions
such as espresso, where over-extraction will lead to an unpleasant taste [20].
Challenges 2020, 11, x FOR PEER REVIEW 5 of 13
When filling cans, bottles, or kegs, the use of additives such as ascorbic acid or preservatives
may increase microbiological stability and shelf life, as well as heat sterilization or pasteurization (see
[5]). However, all these methods are expected to negatively impact the flavor.
Heat-induced coffee contaminants formed during roasting, such as furan or acrylamide [16],
were found at similar levels in cold brew when compared with hot brew [17]. While acrylamide may
increase with prolonged extraction times, furan may decrease due to volatility [18]. Similarly, other
chemical contaminants of coffee such as pesticides or mycotoxins are expected to occur in cold brew
when the raw beans are contaminated, but this is not a problem specific to cold brew. Therefore, we
believe that microbiological contamination is the exception to the rule that needs specific mitigation
measures during the cold brew manufacture.
3.4. Extraction Degree
Considering consumer preference in Malaysia and Germany [11,19], 70% extraction (i.e., 70% in
relation to the total extractable/soluble amount) may be an optimal starting point for a cold brew
coffee recipe to achieve a balanced product (Figure 4). This number does not yet have a scientific basis
but stems from experience considering all aspects of the product, including the risk of microbiological
contamination and the flavor. In other words, the optimum extraction results in the best flavor while
avoiding contamination. Increased agitation will result in a faster process; cooler temperatures will
take longer. Not all the soluble constituents should be extracted, but just enough to get a good taste
while minimizing the risk of microbiological contamination. This can also happen in hot extractions
such as espresso, where over-extraction will lead to an unpleasant taste [20].
Figure 4. Model considerations about cold brew extraction.
Figure 4. Model considerations about cold brew extraction.
Some empirical research confirms the authors’ suggestion of 70% extraction. Cordoba et al. [
21
]
reported higher scores in their sensorial evaluation when cold brews were prepared using the shortest
time (at 75–86% total dissolved solids compared with a longer extraction time). More research is
clearly necessary.
The highest possible extraction, namely 100%, is not desirable in the specialty coffee field. It is
better to waste a little bit of coffee, especially the part with the bad flavors. There might be an optimum
point, certainly less than 100%, where all the “good” flavors are in and the “bad” flavors are excluded.
The equipment and factors mentioned above must all be taken into account to ensure a consistently
high-quality product.
3.5. Flavor and Taste Profile of Cold Brew Coffee
There are considerable aromatic differences between hot and cold brew coffee [
22
]. For example,
the Coffee Consulate Aroma Wheel [
23
] shows some grayed out areas for cold brew (Figure 5). Those are
the oil-bound aroma groups (i.e., herbs, spices, and nuts), which are suppressed by extraction with
cold water. Fruit, floral, and vegetal aromas are more common; also, light roast, and some mineral
Challenges 2020,11, 26 6 of 13
and chemical aromas in cases of very dark roasts. Especially in the drip method, with melting water
just above 0
◦
C, the product is characterized by fine, very fruity, and floral aromas. As a variation,
some people may extract cold brew with cold milk instead of cold water, which may increase the
lipophilic and more polar compounds and flavors. Compounds such as caffeine, which are very water
soluble, are typically found in similar levels in cold and hot brew [24].
Challenges 2020, 11, x FOR PEER REVIEW 6 of 13
Some empirical research confirms the authors’ suggestion of 70% extraction. Cordoba et al. [21]
reported higher scores in their sensorial evaluation when cold brews were prepared using the
shortest time (at 75–86% total dissolved solids compared with a longer extraction time). More
research is clearly necessary.
The highest possible extraction, namely 100%, is not desirable in the specialty coffee field. It is
better to waste a little bit of coffee, especially the part with the bad flavors. There might be an
optimum point, certainly less than 100%, where all the “good” flavors are in and the “bad” flavors
are excluded. The equipment and factors mentioned above must all be taken into account to ensure
a consistently high-quality product.
3.5. Flavor and Taste Profile of Cold Brew Coffee
There are considerable aromatic differences between hot and cold brew coffee [22]. For example,
the Coffee Consulate Aroma Wheel [23] shows some grayed out areas for cold brew (Figure 5). Those
are the oil-bound aroma groups (i.e., herbs, spices, and nuts), which are suppressed by extraction
with cold water. Fruit, floral, and vegetal aromas are more common; also, light roast, and some
mineral and chemical aromas in cases of very dark roasts. Especially in the drip method, with melting
water just above 0 °C, the product is characterized by fine, very fruity, and floral aromas. As a
variation, some people may extract cold brew with cold milk instead of cold water, which may
increase the lipophilic and more polar compounds and flavors. Compounds such as caffeine, which
are very water soluble, are typically found in similar levels in cold and hot brew [24].
Figure 5. Coffee Consulate aroma wheel. Aroma groups that are reduced due to cold brewing are
grayed out.
The difference in flavor profile becomes obvious when a cold brew and a standard hot filter
coffee extraction of exactly the same coffee variety (oeiras) are compared using the Coffee Consulate
flavor profile by a calibrated taste panel (n = 3) (Figure 6) [10]. The cold brew profile is more intensive
in orange/lemon/cucumber flavors, while filter coffee is more intensive in peach, spice, and
microbiological characteristics. Cold brew has slightly less body and is more refreshing.
Figure 5.
Coffee Consulate aroma wheel. Aroma groups that are reduced due to cold brewing are
grayed out.
The difference in flavor profile becomes obvious when a cold brew and a standard hot filter coffee
extraction of exactly the same coffee variety (oeiras) are compared using the Coffee Consulate flavor
profile by a calibrated taste panel (n=3) (Figure 6) [
10
]. The cold brew profile is more intensive in
orange/lemon/cucumber flavors, while filter coffee is more intensive in peach, spice, and microbiological
characteristics. Cold brew has slightly less body and is more refreshing.
Challenges 2020,11, 26 7 of 13
Challenges 2020, 11, x FOR PEER REVIEW 7 of 13
(a)
(b)
Figure 6. Flavor profiles of a cold brew (a) compared with a filter coffee (b) of the same coffee.
Figure 6. Flavor profiles of a cold brew (a) compared with a filter coffee (b) of the same coffee.
Challenges 2020,11, 26 8 of 13
4. Cold Brew—Research Plans
Some initial organoleptic experiments with cold brew were initiated during the Intergastra 2020
trade fair (15–19 February 2020, Stuttgart, Germany), which included the Stuttgart Coffee Summit,
one of the largest professional coffee trade exhibitions worldwide. The visitors at the Coffee Consulate
booth were asked to participate in several ranking order and triangle tests according to ISO 8587 and
ISO 4120 methodologies (n=60 for ranking order tests and n=25 for triangle tests). The statistical
evaluation of the results is ongoing and will be published later, but some initial trends were that cold
brew made with Coffea arabica beans was preferred over the one with Coffea canephora. There was also a
tendency of pulped natural processed Arabica being preferred over fully washed Arabica, potentially
due to the higher sweetness of the resulting brew. Another trial showed that consumers significantly
preferred cold brew over cooled down hot brew, when the same type of coffee beans were prepared in
both instances.
There is not much literature about how cold brew is actually prepared in common practice.
Therefore, a questionnaire has been developed [
11
]. Besides some demographic data and experience
with cold brew, the questionnaire investigates the major parameters such as dosage, water composition,
brewing temperature, brewing time, grinding degree, coffee variety, roasting degree, and serving
styles. The questionnaire was launched during online training in April 2020 and sent to all participants,
but also distributed on the Facebook pages of the affiliated institutions and other social network
channels. The results are currently under evaluation. However, some initial interesting findings of the
first 49 participants as of 22 April 2020, which mostly encompass the participants of several online
training sessions, are shortly summarized below.
There was an almost equal distribution between the different cold brew systems (i.e., drip method,
commercial systems, French press, mixing in various containers, etc.) with a slight preference (34%) of
immersion in containers and filtration afterwards. The applied brew ratios were similarly diverse,
with a majority of participants preferring 80–100 g/L. For water quality, soft or medium hard water
is preferred. The most preferred extraction temperature is 8
◦
C followed by 20
◦
C (see Figure 7).
The average brewing time was 16 h (standard deviation 10 h, maximum 49 h). Medium roast with
coarse grinding degree was preferred. Following the brewing of the cold brew, the average storage
time was one day (median 0.6 days, standard deviation 1.4 days, maximum seven days).
Challenges 2020, 11, x FOR PEER REVIEW 8 of 13
4. Cold Brew—Research Plans
Some initial organoleptic experiments with cold brew were initiated during the Intergastra 2020
trade fair (15–19 February 2020, Stuttgart, Germany), which included the Stuttgart Coffee Summit,
one of the largest professional coffee trade exhibitions worldwide. The visitors at the Coffee
Consulate booth were asked to participate in several ranking order and triangle tests according to
ISO 8587 and ISO 4120 methodologies (n = 60 for ranking order tests and n = 25 for triangle tests). The
statistical evaluation of the results is ongoing and will be published later, but some initial trends were
that cold brew made with Coffea arabica beans was preferred over the one with Coffea canephora. There
was also a tendency of pulped natural processed Arabica being preferred over fully washed Arabica,
potentially due to the higher sweetness of the resulting brew. Another trial showed that consumers
significantly preferred cold brew over cooled down hot brew, when the same type of coffee beans
were prepared in both instances.
There is not much literature about how cold brew is actually prepared in common practice.
Therefore, a questionnaire has been developed [11]. Besides some demographic data and experience
with cold brew, the questionnaire investigates the major parameters such as dosage, water
composition, brewing temperature, brewing time, grinding degree, coffee variety, roasting degree,
and serving styles. The questionnaire was launched during online training in April 2020 and sent to
all participants, but also distributed on the Facebook pages of the affiliated institutions and other
social network channels. The results are currently under evaluation. However, some initial
interesting findings of the first 49 participants as of 22 April 2020, which mostly encompass the
participants of several online training sessions, are shortly summarized below.
There was an almost equal distribution between the different cold brew systems (i.e., drip
method, commercial systems, French press, mixing in various containers, etc.) with a slight
preference (34%) of immersion in containers and filtration afterwards. The applied brew ratios were
similarly diverse, with a majority of participants preferring 80–100 g/L. For water quality, soft or
medium hard water is preferred. The most preferred extraction temperature is 8 °C followed by 20 °C
(see Figure 7). The average brewing time was 16 h (standard deviation 10 h, maximum 49 h). Medium
roast with coarse grinding degree was preferred. Following the brewing of the cold brew, the average
storage time was one day (median 0.6 days, standard deviation 1.4 days, maximum seven days).
Figure 7. Initial survey result of cold brew extraction temperatures.
Figure 7. Initial survey result of cold brew extraction temperatures.
Challenges 2020,11, 26 9 of 13
5. Open Problems and Challenges about Cold Brew Coffee
5.1. Should Cold Brew Be Served with Ice?
The ice will lower the temperature and may also dilute the aromas. Cold temperatures will also
suppress the optimal taste perception. Therefore, a good cold brew does not need ice. However,
obviously, if consumers like ice, such as in whisky-like cocktails, there is no reason not to offer it.
5.2. Is Cold Brew Extracted at Room Temperature Still a “Cold Brew”?
There is no available definition or regulatory standard on cold brew coffee. According to the
authors’ own judgement, every brew made at temperatures below body temperature may be considered
as “cold” brew. Cold does not necessarily imply “ice cold”. We, however, no longer consider hot
brewed (>65
◦
C) coffees that are just cooled down, as cold brew. If cold brews are brewed at room
temperature (which could reach levels above 30
◦
C in some regions of the world), it must obviously
be considered that the extraction will be much faster. This could even be advantageous regarding
microbiological contamination problems (e.g., comparing 18 h at 8
◦
C vs. 2 h at 20
◦
C). However,
systematic research into this issue is lacking so far. Shelf life should be longer at fridge temperature
than at room temperature.
5.3. What Would Your Suggestion Be for Cold Brew Extraction Conditions?
Based on the experience at Coffee Consulate, an amount of 80 g/L coffee is suggested for 2 h at
initially 15
◦
C (tap water temperature in Europe) then stored in a fridge [
19
]. We use a 5 L food-grade
plastic container with the aliquot of medium fine coffee (Catuai, pulped natural, or natural) and water
(4.8
◦
dH German hardness), then we stir once after 1 h of lixiviation. Finally, the product is filtered
using a standard paper filter. We believe that this method is simple and stable, making a consistent
product as proven by various coffee shops that use this recipe.
From a sensorial standpoint, nothing may be gained by prolonging the extraction time to 4 h
or even longer [
19
]. Increasing the extraction time may only increase the bitterness of the beverage.
Some empirical evidence confirms this opinion: Caffeine and 3-chlorogenic acid reached equilibrium
(i.e., 100% extraction, compare Figure 4) between 6 and 7 h, instead of 10 to 24 h outlined in some
typical cold brew methods [
25
]. Cold drips were recognized as more bitter and temperature was found
to increase the concentration of several compounds [
13
]. We are currently in the process of adjusting
our coffee analytical methods [
26
–
28
] to cold brews aimed at studying and optimizing the multivariate
influences and their interactions on cold extraction.
5.4. Is It Possible to Make Cold Brew with Beans from the Asia-Pacific Region?
These beans are often underrated. The advantage of the Indonesian and South-East Asian
(e.g., Myanmar, Thailand, Philippines, and Malaysia) beans is the uniqueness in very floral and spicy
coffees. To make a cold brew out of these beans really needs consideration of roast profile and extraction
method. Cold brew prepared with beans from Indonesia (Arabica Bali Kintamani coffee) showed a
fruity flavor with intense sweetness [
29
]. Otherwise, no reports on the sensorial data of South East
Asian (SEA) coffees in cold brew are currently available.
5.5. What Would Be the Optimum Shelf Life of a Cold Brew Coffee?
Optimally, the cold brew would sell on the same day of preparation. The authors are of the
strong belief that cold brew should be made fresh everyday (this actually appears to be in divergence
with current practices, see survey result above). When stored for longer, some changes in product
quality can be expected, such as raised acidity and greater ethanol content, also decreases in sugar
content/sweetness due to yeast or bacteria activity. For example, So et al. [
30
] have shown that pH
Challenges 2020,11, 26 10 of 13
declines and total acidity increases during cold brew storage for eight weeks, with greater effects for
storage at 20 ◦C than at 4 ◦C.
In addition to microbiological changes, the product will also go rancid and stale due to an oxidative
process. This oxidative process will not be as fast as for hot brewed coffee, which has a much higher
lipid content. Anecdotally, if cold brew is stored under nitrogen pressure (e.g., in kegs), oxidation can
be avoided and shelf-life extended [
31
,
32
]. Some studies claim that the shelf-life of cold brew is limited
not by microbial stability, but rather by deterioration in sensory attributes [33].
Currently, for artisanal producers, we would suggest a maximum storage time of two days.
Taste is currently the only guidance available on site to determine how long a cold brew may be stored.
Otherwise, microbiological laboratory testing for shelf life analyses might be conducted.
To give an analogy with filter coffee, this product would turn stale and be thrown away after
one to two hours. We do not believe that there is a reason to demand week-long storage times
for cold brew, which basically has the same underlying costs (similar brewing ratio) as filter coffee.
We know from experience that this also greatly increases customer satisfaction. It is a shame that the
segment of cold brew, at least in Central Europe, appears currently dominated at an industrial level by
“called brews” (i.e., hot extracted fakes) and on the artisanal level by over-extracted and over-stored
products, which are sometimes extremely acidic and bitter, which is adverse to the goal of achieving a
loyal, returning customer.
5.6. Is It Possible to Prepare Cold Brew as Concentrate and Dilute before Serving?
If a high-quality coffee is used to prepare a concentrate, there should be no reason that it should
taste bad. This would be the equivalent of high gravity mashing on beer brewing. Increasing the
ground coffee to water ratio may simplify the extraction and storage of the brew, that can be diluted
just before consumption. Most commercially available concentrates, however, appear to be based on
lower grade coffees and bad roasts as well as on hot extraction, using advanced equipment such as
evaporation systems. Currently, we would not advise making concentrates in smaller coffee shops.
There is also no research available on how concentration influences flavor.
5.7. What Are Typical Customer’s Complaints Against Cold Brew?
It is too bitter, because bitterness is the sign that something is turning toxic. The next complaint
would be too sour or too acidic (meaning that it tastes rotten). The acidity may arise from over-extraction
and microbiological spoilage (lactic characteristics). These defects can be avoided when over-extraction
and over-storage is avoided.
5.8. What Are the Risks of Handling Cold Brew Coffee?
Cold brew has a pH value greater than 4.6 (typically 4.9–6.0 [
25
,
34
,
35
]), which is a low acidity and
does not effectively suppress microbial growth [
5
]. Therefore, foodborne pathogens (i.e., human illness
causing microorganisms) or spoilage organisms, which mainly affect flavor, may develop in cold
brew [5], see also Section 3.3. above.
Some initial experiments showed that the growth of pathogens may be inhibited by compounds in
the coffee, but this must be confirmed for each individual preparation [
5
]. Conversely, spoilage organisms
such as molds and yeasts appear not to be inhibited and may increase quickly during storage leading
to fermentation [
5
]. Most sensitive products are ready-to-drink beverages, especially those under
anaerobic conditions (for example when filled under pressure with nitrogen [
34
]). These products
must be closely controlled to mitigate the risk of Clostridium botulinum (botulism), by either pH control
or thermal processing [
36
]. In fact, the product “Death Wish” nitro cold brew was recalled from the
market in September 2017, because it was determined that its production process could lead to the
growth and production of botulinum toxin in this low-acid food commercialized in reduced oxygen
packaging [37,38].
Challenges 2020,11, 26 11 of 13
We would currently compare the risk of cold brew coffee to other alcohol-free beverages,
for which considerably more experience exists. As cold-brew has a small amount of sugars
(e.g., some initial findings found cold brew higher in reducing sugars than hot brewed coffee [
39
]),
and unlike beer, does not contain alcohol which inhibits bacterial growth, the risk would be similar to,
e.g., other alcohol-free beverages with pH >5. Hence, special care and diligence need to be applied
regarding handling and cleaning of dispensing equipment and coffee lines (e.g., for nitro cold brew
“on tap”). For example, the German norm DIN 6650-6 suggests a cleaning and disinfecting interval of
at least once a day for alcohol-free beverages [40].
Special care is also required when cold brew is filled in kegs needing careful study of the shelf
life [
6
]. Most commercial cold brews in the market also require refrigeration for both storage and
dispensing [
41
]. We recommend to specifically consider cold brew in the Hazard Analysis Critical
Control Points (HACCP) system, which is mandatorily implemented in many countries including the
European Union. To avoid contamination with pathogenic microorganisms, maximum storage times
based on microbiological shelf-life testing should be implemented (including labelling of maximum
duration and compliance controls that the product is removed before this date). Visual controls for
clouding and organoleptic testing at least once a day before service starts should be conducted.
Some further guidelines are specified in the “Cold brew coffee toolkit for industry” published by
the National Coffee Association of U.S.A. [
36
]. The toolkit includes information on challenge studies to
avoid risk from spore-forming bacteria such as Clostridium and guidance on shelf-life testing, storage,
and handling [
36
]. The British Columbia Centre for Disease Control suggests a maximum refrigerated
storage of less than 10 days for products stored at 4
◦
C and below, if no other controls are present [
34
].
The Centre also suggests the following options to control the hazards of spore-forming bacteria for
products intended for longer term storage: (i) Heating and pasteurization, (ii) reducing the pH to 4.6 or
below, (iii) ensuring aseptic processing, (iv) addition of preservatives, and (v) a combination of these
controls [34].
5.9. What Are the Advantages of the Cold Drip Method?
Based on the experience at Earthlings Coffee Workshop, customers like the concentrated, punchy
flavor from ice drip coffee. It is usually served straight or with ice cubes, dependent on the customers’
preference. The ratio is usually 1:5. 300 g of blend and about 1450 g of ice cubes plus 50 g of filtered
water. The time is about 8–12 h. The use of ice cubes is extremely important for the flavor.
6. Conclusions
We are at the very beginning of scientifically understanding what cold brew coffee is. It is important
not to have a pre-mindset into any direction and be really open to experience the entire spectrum
of possibilities with this extraction technology. The cold brew extraction is a highly multivariate
process [
13
], and additional chemical, microbiological, and sensory studies are needed to increase
our understanding. Additionally, the taste profile of cold brew can be influenced by the serving,
e.g., nitrogen infused or so-called nitro cold brew, milk addition, sugar addition, use of ice, or other
ingredients such as alcoholic beverages. The nitro method is specifically interesting because it
considerably changes the look and taste of the beverage. The nitrogen gas percolates through the glass
like a Guinness beer, leaving a creamy head on the top [
6
]. The flavors are dispersed across the tongue
because the bubbles dramatically increase the surface giving a smooth and creamy taste [
6
]. In our
experience, the nitro method also intensifies the sweetness and takes the bitterness out of the product.
Finally, the food safety hazards of cold brew need increased consideration. As suggested by
Lopez [
15
], both regulatory agencies as well as industry should address the issue aiming for standards
and good manufacturing practices reducing the risks of cold brew coffee.
Author Contributions:
Conceptualization, S.S. and D.W.L.; formal analysis, D.W.L.; investigation, S.S. and L.C.;
methodology, S.S., L.C., and D.W.L.; project administration, D.W.L.; supervision, S.S. and D.W.L.; visualization,
Challenges 2020,11, 26 12 of 13
Steffen Schwarz; writing—original draft, D.W.L.; writing—review and editing, R.K., K.L.W.T., S.S., and L.C.
All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments:
This article is based on training sessions by professional coffee training centers in Malaysia
and Germany. The training sessions were initiated by presentations of invited speakers followed by Q&A sessions.
All participants in the online training sessions are thanked for their attendance, questions and suggestions.
Photogem25/Fiverr is thanked for proofreading the revised manuscript version.
Conflicts of Interest: The authors declare no conflict of interest.
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