Nitro Cold Brew – Technical Note Matthew R. Hartings, Associate Professor of Chemistry, Author of Chemistry in Your Kitchen

Department of Chemistry, American University, 4400 Massachusetts Ave, NW, Washington, DC 20016, USA [email protected]

ABSTRACT: Nitro cold brew coffee has become a marketing and sales hit for coffee shops globally. The coffee’s novelty, ease of preparation, attractive textural and visual properties all play into consumer preferences. While there is certainly trade research studies that inform how some of the largest coffee producers approach making and preparing nitro cold brew coffee, there have been no published research articles that can inform smaller producers and purveyors of coffee. The work in this technical report was sponsored by the Parker-Hannifin Corporation who produces generators that can be used by food retailers as a critical component of selling nitro cold brew coffee. This report details several critical aspects of dispensing nitro cold brew coffee (texture, foam quality and stability, and flavor) and compares the use of nitrogen to air in serving coffee. The report also makes some forward-looking recommendations for procedures that individual retailers might want to explore to make their nitro cold brew coffee stand out in the market.

Nitro cold brew coffee is coffee that is stored cold and dis- generator is ideal for use in coffee shops in lieu of pressurized pensed using high pressure nitrogen, similar to Guinness nitrogen tanks. Proper use of pressurized gas tanks requires Draught or Boddington Pub Ale beers. Just as in these beers, training and on-site infrastructure. When performing safety nitro cold brew boasts a creamy texture that is primarily due to training with the chemistry majors at American University, the the nitrogen-based foam that forms on top of the dispensed cof- students are made aware that the pressurized gas tanks are some fee. The stability and quality of this foam is critical for long- of the most hazardous objects in the chemistry building. The lasting, consumer enjoyment of any nitro cold brew product. As simplicity of the Parker-Hannifin generators is that they do not seen through the analysis in this report, a quality, stable nitrogen require the same level of precaution and safe handling. While a source is critical for optimal dispensing of nitro cold brew cof- nitrogen tank stores 2200 psig of pressure, The Parker generator fee. While many bars are equipped to handle the high-pressure stores less than 125 psig. Additionally, nitrogen tanks need to nitrogen (and carbon dioxide) tanks that their beers require, the be replaced once they are empty. A forgotten order for new coffee industry (and especially many small coffee shops) do not tanks can put a halt on selling nitro cold brew until the filled have the proper infrastructure or worker training in place for tanks are delivered. The nitrogen generators from Parker Han- handling high pressure tanks. nifin run in the background and produce high quality nitrogen from the air. They act like an everlasting tank of nitrogen that never goes empty. While Parker nitrogen generators are cer- tainly convenient, the research and results outlined in this report are valid with the use of either a nitrogen generator or a pres- surized nitrogen tank. The goal of this report is to detail the protocols used in pre- paring nitro cold brew coffee and measuring relevant properties of the coffee. Many of the analyses used here (with the excep- tion of gas chromatography-mass spectrometry, GC-MS) can be performed in-house by any purveyor of nitro cold brew coffee as they refine and optimize their product. The report ends by making several suggestions of straightforward variations that coffee shops could take to make their particular version of nitro cold brew stand out from the crowd. Methodology Figure 1. Image of nitro cold brew coffee dispensed from a pres- Coffee was stored and dispensed from a Komos double tap surized keg. kegerator with 5-gallon, ball lock Cornelius kegs. The kegs The Parker-Hannifin Corporation, who supported the work in were connected through tubes to the pressurized gas or the dis- this report, supplied a beverage dispense nitrogen generator that pensing tap using screw-end, ball lock disconnects. The gas was used for pressurization and dispensing the coffee. This source was connected to the kegs through a gas pressure regu- lator set to 50 psi. Note: When connecting high pressure tanks, different high pressure regulators must be used for nitrogen and inherent that a beverage continue to be able to deliver foam to air. The Parker generator can use a standard air regulator, com- the customer as they continue to drink it. That is, the coffee monly available. should retain some foam within its head after every drink. To make the coffee, 12 ounces of French Roast and In general, for this experiment, around 18 mL of coffee (~ 0.5 20 ounces of Starbucks Pike Place were ground on the coarsest ounces) was dispensed into a clean glass. The coffee was poured setting using a Capresso Infinite Grinder. The coffee grounds into a second container. The mass of coffee in both containers were steeped at 39 °F (4 °C) for 24 hours in a 5-gallon food was measured. The amount of mass remaining in the original grade container with 1.4 gallons of water. This cold brew container is indicative of cling. For measuring this property, method produces a 2X concentrate. The coffee was filtered two different scenarios were compared. In the first scenario, through a 13” non-woven filter cone in a 12” chinois strainer. coffee whose foam had not fully formed was poured. Cold brew After straining, the volume of coffee was doubled through the coffee is much like Guinness in that it takes time for the foam addition of water. The coffee was added to the keg. The keg was to develop as the dissolved nitrogen gas comes out of solution. connected to the tap and to the gas source and pressurized. The This process leads to the distinctive formation that is observed coffee properties were monitored each day for 1 week after in both Guinness Draught and nitro cold brew coffee (see Figure pressurization. Other than flavor degeneration in the air-kegged 2). In this scenario (pre-foam), there is very little foam. There- system, there was very little variation in the other coffee prop- fore, there should be less cling to the glass. In the second sce- erties over time. nario, the foam was fully allowed to form before pouring. The There are several measurements described in this report. For results from this test are shown in Table 1. each measurement, results found using compressed nitrogen Table 1. Foam and Viscosity Data were compared with results found for using compressed air. Volume (Estimates of the solubilities of gasses found in the pressurized fraction of Gas Coffee foam in Foam cling Line Solubility coffee at 39 °F (4 °C) are shown in Table 1.) pressurized coffee (g retained/ g spread test (mole Several tests were performed to assay the quality and quantity with (%) coffee) (cm/g) fraction) 10-5 of foam produced. The volume of foam produced (per volume Before foam: 2.6 ± 0.7 Nitrogen 12.8±0.9 0.26 ± 0.02 N2: 5.9 of coffee) was measured using a 25 mL graduated cylinder and After foam: a digital camera. Foam cling, the amount of foam retained by a 4.1 ± 0.5 glass after a pour, was analyzed using a mass balance. Foam Before foam: 3.2 ± 0.2 N2: 4.6 stability was assessed by monitoring foam volume after the in- Air 11.4±0.7 0.26 ± 0.01 After foam: O2: 2.5 itial pour as a function of time. 4.2 ± 0.5 A line spread test was performed to assess any differences in Ar: 0.12 viscosity. For this test, a sheet was printed with concentric cir- CO2: 0.12 cles with increasing diameters. This sheet was placed beneath a clear sheet of plastic. A tube with an internal diameter of 2 cm was placed on top of the plastic over the center ring on the sheet. A known mass of coffee was poured into the tube. The tube was lifted, and the spread of coffee was measured. Finally, a flavor analysis was performed on the coffee. A 100 m PDMS SPME fiber was incubated in the headspace above a sample of coffee for 30 minutes at 140 °F (60 °C). In that time, the fiber absorbed the volatile compounds from the coffee. The adsorbants on the fiber were analyzed using a Shimadzu GC- MS. Analytes were identified using a database from the Na- tional Institute of Standards and Technology. Results Foam production. Foam is an integral part of the experience of drinking beverages.1 Beer is the most prominent case in point, but there have been several studies of the science of es- presso foams (crema) as well. There are several macromole- cules that are responsible for the retention of gas within bubbles in these drinks. In both cases, some proteins and extracted pol- Figure 2. Illustration of the foam formation in nitro cold brew cof- ysaccharides enable foam formation and retention. For these fee studies, foam volume per total coffee volume was assessed. What is observed from this data is that, for both air and nitro- For these measurements, it is helpful to have a ruler to cali- gen, there is more mass remaining in the original container for brate the sizes observed in the image (see Figure 1). Table 1 the scenario after foam formation than before foam formation. details the results of these measurements, which find that there The nitrogen and air data show no discernable difference be- are no observable differences in the amount of foam initially tween each other for this measurement. produced upon pouring when comparing nitrogen to air pres- surized . Foam Stability. The long-term stability of the foam head might be the most important property of the foam in nitro cold Foam Cling and Retention. Foam cling can be thought of as brew. Foam dissipation leads to a vastly different drink experi- the amount of foam that remains on the glass after pouring or ence. The longer the foam is retained in the head, the longer a drinking. Because foam affects customer preferences, it is customer will enjoy their drink and the more likely they are to surface than 10 grams of water. The viscosity of these samples return to purchase another. is due, in part, to how strongly the molecules within the liquid Figure 3 shows the foam dissipation properties of nitrogen- interact with one another. dispensed coffee air-dispensed coffee. In this set of experi- Line spread tests were used assess the relative viscosity of ments, there is a stark contrast between the nitrogen and air nitrogen-dispensed coffee to air-dispensed coffee. An image of samples. Data shown in Table 1 indicate that the foam for- the experimental setup is shown in Figure 4. The resulting data mation properties of both air and nitrogen are similar. However, from these experiments are shown in Table 1. There is very little the air produced coffee foam loses its volume much more rap- difference between the results of the nitrogen- and air-dispensed idly than the nitrogen produced foam. On average, the nitro cof- samples. While this is not too surprising, these measurements fee loses only half of its foam over 45 minutes while the air are discussed in the ‘Outlook’ section for coffee shops who are coffee loses 65% of its foam over that time. Most concerning in looking to make a slightly different nitro cold brew. this graph is the rapid initial losses in the air-dispensed coffee.

Figure 4. Example result of a Line Spread Test. The distance from the inner circle to the edge of the liquid is measured for several points and averaged. The plastic measurement surface sits on top of a calibration sheet with concentric circles of different, known dimensions. Coffee Flavor. One of the benefits of storing coffee under Figure 3. Foam decay over time. (top left) Images of nitrogen-dis- nitrogen is that the flavors have a longer shelf-life. The presence pensed coffee after pouring and after 45 minutes. (top right) Images of oxygen is detrimental to flavor quality. Oxidation of coffee of air-dispensed coffee after pouring and after 45 minutes. (bottom) aromas can lead to off or damaged flavors. For the kegging sys- Graph showing the % of foam left on the coffee as a function of tem, storage under high pressures of oxygen (compressed air) time. The markers correspond to the averages over all of the data should obviously be detrimental to the flavor properties of the taken (gray squares – nitrogen, blue circles – air). The shaded re- cold brew coffee. An illustration of this is shown in Figure 5. gions show the variance from the average over all of the data taken Every peak in these traces shows the presence of some vola- (gray – nitrogen, blue – air). tile compound in the . The retention time (x-axis) The most likely reason for these differences is the bubble and is completely dependent upon the identity of the flavor com- foam forming properties of other gasses (estimated gas concen- pound. That is, a flavor compound that has a retention time of trations are shown in Table 1). In comparing carbonated beer 50 minutes in the instrument for the nitrogen sample will have with nitrogen-dispensed beer, the nitrogen bubbles are much the same retention time in the air sample. The pre-kegging sam- smaller than CO2-bubbles. The mixture of gasses in the coffee ple and nitrogen-kegged sample both show the presence of an obviously leads to a disruption of foam stability. A closer in- aroma compound at around 50 minutes that is not present in the spection of the foam remaining at the end of these experiments air-kegged sample. This indicates that this flavor compound is show that there are large bubbles in the air samples versus changed by being stored under pressurized air. There are multi- smaller bubbles in the nitrogen samples. The foams from these ple examples of compounds present in the pre-kegged and ni- two gasses are very different from one another. trogen-kegged samples that are not present in the air-kegged Coffee Viscosity. Viscosity is related to mouthfeel. Milk has sample. Therefore, it is apparent that long-term contact with ox- a different viscosity than a , which has a different viscosity ygen degrades the flavor compounds in the coffee. Because of than an , which has a different viscosity than drip cof- this reason, it is not recommended that air be used for dispens- fee. One quantitative method for evaluating viscosity is the ‘line ing cold brew. spread test,’ described above.2 During this test, the farther a liq- uid spreads, the lower its viscosity. This is intuitive and holds to personal experience. 10 grams of honey will spread less on a

on this liquid. After measuring this value, the vendor can try to match it with their hydrocolloid-modified coffees. Coffee Flavor. Admittedly, the coffee brewing procedure used in this technical note is not ideal for generating depth of flavor. There are many other brewing processes available to professional coffee vendors. One specific example involves hot coffee brewing and dispensing directly into an actively chilled container. In speaking with several about nitro cold brew coffee (and cold brew coffee, in general), many dislike the flavor pro- file of cold-served coffee and inability to craft customer-spe- cific cold brew drinks. Additionally, high temperatures heighten flavors, and low temperatures dampen them. It would appear that the dispensing process of nitro cold brew coffee presents an opportunity for introducing programmed fla- vors into the dispensed coffee. The nitrogen could be passed through a container of aromatics (ground coffee, flour petals, other) on its way to the keg. This extra amount of added flavor is one way that baristas could enhance or customize their cold brew flavor experience. Figure 5. GC-MS traces of volatile flavor components of coffee Conclusions before kegging (top), after kegging in nitrogen (middle), and after Nitrogen cold brew coffee has earned a large market share of kegging in air (bottom). coffee sold by vendors. Retention and expansion of this market Outlook will depend upon vendors modifying their customers’ cold brew experience. This technical note describes some optimal serving This technical report provides an outline for vendors who are parameters and supplies a framework generating a unique cold looking to start selling nitro cold brew coffee and for vendors brew experience. The broad finding is that nitrogen is superior who are looking to expand on their cold brew offerings. There to air for dispensing cold brew coffee. Vendors need to be aware are several critical coffee properties that will affect consumer that use of air will result in flavor degradation and poor foam perception and purchasing: foam stability, coffee viscosity, and quality. Finally, vendors have several options (additives that af- flavor. fect viscosity and foam stability and innovation in the way new Foam Stability and Coffee Viscosity. The molecular compo- flavors are added to the cold brew) for producing unique nitro nents that affect mouthfeel (viscosity) are often the same com- cold brew experiences for their customers. The outlines given ponents that affect foam stability. There are a number of studies in this technical support should give the vendors confidence in that explore the proteins and hydrocolloids (galactomannan and how they approach that innovation. arabinogalactan, for example) that affect mouthfeel and foam References retention in beer and in espresso.1 Vendors who are looking to change these properties would do well to experiment with the 1. a) ‘The Mouthfeel of Beer – A Review’ Langstaff and addition of these types of molecules to their coffee. There are a Lewis J. Inst. Brew. 1993, 99, p 31-37. b) ‘Foamability, Foam number of guides to using hydrocolloids to elicit different food Stability, and Chemical Composition of Espresso Coffee As Af- properties. ‘Texture’ is an open-access guide for the practical fected by the Degree of Roast’ Nunes, et al. J. Agric. Food. use of these molecules.3 There are a number of examples in this Chem. 1997, 45, p 3238-3243. c) ‘Influence of Polysaccharide guide for increasing drink viscosity or foam retention. Once Composition in Foam Stability of Espresso Coffee’ Nunes and samples have been prepared, the following experiments are sug- Coimbra Carbohydrate Polymers 1998, 37, p 283-285. d) gested. Certainly, vendors should perform taste and perception ‘Comparison of Foam Analysis Methods and the Impact of Beer testing themselves to see which samples they like best. Should Components on Foam Stability’ Neugrodda, et al. J. Am. Soc. a vendor want more quantitative values for these, the foam sta- Brew. Chem. 2015, 73, p 170-178. bility measurements described here would be ideal. It is sug- 2. ‘Line-Spread as an Objective Test for Consistency’ gested that the vendor initially decide on a target viscosity. That Grawemeyer and Pfund J. Food Sci. 1943, p 105-108. is, what other liquid (whole milk, skim milk, espresso, etc) has 3. ‘Texture: A Hydrocolloid Recipe Collection’ Lersch 2014 the mouthfeel they are looking for? Perform a line spread test https://blog.khymos.org/recipe-collection/