Holding Temperature
The shelf life of green, unroasted coffee is usually expressed in years – for example, new crop, current crop and past crop. Once it has been roasted, the shelf life is measured in days. The stale flavors in coffee can become detectable as early as 4 days after roast, or as late as 14 days after roast.
That said, during no other phase in the development of coffee’s flavor (from the harvesting of the cherries to the roasting of the beans) does the degradation of the flavor occur as rapidly as it does once the coffee has been brewed.
After a coffee has been brewed, its freshness is measured in minutes. The change becomes noticeable 15 minutes after brewing; after 30 minutes the beverage is no longer considered acceptable. After 60 minutes, the flavor will deteriorate to a point of being highly objectionable.
To brew and serve great coffee, you must create a stable environment for the volatile aromatic and fragile taste compounds that make up coffee’s delicate flavor. This is accomplished by holding the coffee in a manner that reduces the rate at which the volatile aromatic compounds are driven out of the brew, reduces the rate at which the non-volatile taste compounds change within the brew, and reduces the rate at which the water molecules evaporate and thereby change the solubles concentration i the brew. This is generally accomplished by maintaining a holding temperature between 175 and 185 degrees Fahrenheit and in a closed and insulated container.
There are three primary factors that cause undesired flavor change in holding coffee.
1. Coffee’s highly volatile aromatic substances constantly migrate from and to the brewed coffee’s surface
Virtually all compounds in coffee’s aroma have a boiling point well below that of water, which means they’ll state in a gaseous state and leave the surface of the brew as soon as they reach it.Though we know coffee’s highly volatile aromatic substances can escape the coffee brew during holding, there is one important exception that should be noted. Mercaptans are sulfur-containing organic compounds that, although small in number, contribute significantly to pleasurable aspects of coffee’s aroma. Studies have shown that the quantity of mercaptans actually increases at temperatures between 175 and 195 degrees Fahrenheit. But beyond 60 minutes of holding time, these aromatic-flavor compounds are more likely to decompose rather than form.
2. The elevated brewing temperature causes coffee’s non-volatile liquid compounds to chemically change
Coffee non-volatile flavoring compounds are composed of sugars, proteins, mineral oxides, acids, trigonelline, caffeine and phenolic compounds. The degree and rate of change relate to the chemical stability of each of these compounds.
The most distinctive change is an increase in the coffee’s acidity. This occurs as the chlorogenic acid (which constitutes some 15 percent of the solubles in brewed coffee) breaks down during the holding period into caffeic and quinic acid. This easily recognized taste change causes the coffee to become acerbic – increasingly sour and bitter. Since chlorogenic acid accounts for such a large portion of the solubles in brewed coffee, holding coffee at a temperature where the chlorogenic acid remains stable is a key consideration. Studies have shown it is most stable when held at temperatures between 175 and 185 degrees Fahrenheit for fewer than 60 minutes. When held at a lower temperature, or for a longer period, chlorogenic acid decreases in concentration and creates the by-products of quinic and caffeic acids.
Applied heat also induces chemical changes. To maintain the stability of the majority of non-volatile compounds, we avoid placing the holding vessel on a source of direct heat (e.g., a burner), and thus we use insulated containers.
3. Water molecules vaporize and leave the brew, cause the remaining flavoring compounds to further concentrate
As the flavoring materials become more concentrated, the strength of the brew increases and causes a perceptible change in taste. With the simultaneous change in the type of soluble material present in the brew, such as chlorogenic acid breaking into caffeic and quinic acid, this increased concentration level create a highly objectionable taste.
The rate of evaporation is a function of the beverage temperature, how much of the brew’s surface is exposed to air, and the pressure over the exposed surface area.
Serving Temperature
Serving temperatures (as opposed to tasting temperatures, discussed in a moment) preferred for hot beverages (of all types) range from 155 to 175 degrees Fahrenheit for the average consumer. On a psychological level, the preferred temperature relates to a person’s tactile pain threshold; that is to say, the point at which the kinetic energy of the fluid triggers and initial response on the free nerve endings of the tongue.
The amount and composition of aromatic compounds release from the fluid is related to the temperature of the brew. There are many important components of the coffee’s aroma that have boiling points above 150 degrees Fahrenheit; therefore, you cannot perceive them except at elevated beverage temperatures.
Temperature has a direct bearing on the ability to perceive different tastes, primarily reducing your ability to perceive either sweet or salty taste sensations. As the temperature changes, the perceived taste characteristics of the coffee beverage also change. For the average person, the most pleasing balance in the basic tastes (sweet, salty, sour and bitter) occurs when the beverage is served at temperatures above 160 degrees Fahrenheit.
Tasting Temperature
Though coffee is ideally served at a range of 155 to 175 degrees Fahrenheit, the ideal tasting temperature is actually 130 to 155 degrees. So what accounts for the disparity? The temperature of the coffee decreases slightly upon exiting the holding container and going into the cup or vessel. That vessel also absorbs some of the heat, the rate of which depends on the material, density and initial temperature of the vessel. Then, the temperature of the surface of the cup (from which you’ll be drinking) decreases at a rate much faster than the body of the fluid (because the surface fluid is attempting to reach an equilibrium with the air above it). Finally, once the coffee is sipped into the mouth, it undergoes a last temperature drop as it is sprayed across the tongue. It is this, the tasting temperature, that is most pleasing at a range of 130 to 155 degrees.
