Water and coffee for roastmasters and baristas
Abstracts from articles by Vladimir Shamaev
Water and coffee
Before considering the interaction of the object we are interested in with water, it would be nice to define what a coffee bean is, what it consists of and what properties it possesses. It is also useful to have an idea about the other participant of the process - water, and of course, about the environment where these two structures interact. Briefly about all partners.
A green and roasted coffee bean can be defined as a heterogeneous, capillary porous, colloidal body of organic origin with a cell structure. A coffee bean is a heterogeneous system, its structure is cellular with a mass of pores of various sizes, including capillary 0.2 ÷ 0.3 μm, and micropores, the diameter of which is within 0.01 μm.
The set of chemical components in coffee tree seeds is the same as in the seeds of other plants. The behavior of water in the biological material - green coffee - is significantly different from the behavior of water in roasted coffee.
The water in a coffee bean exists in the forms of steam, liquid and as part of complex molecular systems. The moisture in the porous colloidal material is usually subdivided into free, absorbed and bound. The porous structure and the presence of biopolymers retain water inside the bean.
The bean remains moist until the air contains water. Saturating the bean with water (sorption) or removing water from the bean (desorption) along with extraction are key processes in coffee production technology. All processes on the surface and inside the coffee bean are influenced by environmental factors.
The environment (atmospheric air) is a gas mixture consisting of inert nitrogen, aggressive oxygen, a small amount of other gases, mainly neutral, and water in the form of steam. Steam passes into the liquid phase at a certain temperature, called the dew point, and falls out in the form of rain, snow, or dew. Atmospheric moisture plays a huge role in the agrarian period of coffee, in the initial processing of seeds, in logistics, heat treatment and even in evaluating the flavor and taste. It should not be forgotten that in the atmosphere there exist countless various microorganisms invisible to the eye, fungal spores, etc. Their vitality depends on the thermodynamic parameters of the environment and has a prevailing effect on the shelf life of the products.
To be able to consider the entire route that coffee and water takes from the farm to a cup with a fragrant drink consistently and carefully, it is necessary to know what chemical substances coffee consists of.
The chemical composition of coffee and its interaction with water
The chemical composition of coffee tree seeds does not differ from the chemical composition of seeds of other plants, the same proteins, fats, carbohydrates, acids, minerals and water. The difference consists in the quantity and quality of these substances.
The moisture indications of the coffee bean and, therefore, its consumptive and technical qualities depend on the form of the substances in the coffee bean. As seen from the table, green coffee mostly contains complex carbohydrates. Some proteins and fats also have the form of polymers. The total weight of biopolymers in the bean is 50÷ 60%. One of the properties of substances from this class is the ability to attach and retain water molecules.
In the process of roasting, complex substances decompose into simple substances. Their ability to bind water decreases substantially or completely disappears. The moisture content of roasted coffee is reduced to 1 ÷ 2%. A few hours after roasting, the moisture content of the bean increases again and reaches 3 ÷ 4%, but this is not so much due to the presence of biopolymers, but thanks to the hygroscopicity of the porous material.
What is free and bound water in coffee?
Free and bound water
All the water that is in the bean falls into three types: free, hygroscopic and bound. This division is relative enough, since some of the water molecules move freely within the bean and can go from one form to another in certain conditions, thus, it can take any of the listed forms. The amount of water in each form is constantly changing in time, and the processes are very slow and depend on the biological and physical characteristics and the chemical composition of the bean, and on the state of the environment as well. The moisture content of the product tends to an equilibrium state with the environment humidity, but changes in the air are faster than in the bean, and this difference impedes the establishment of thermodynamic equilibrium.
Measuring the amount of moisture in one form or another is a difficult task. For these measurements to be possible, special devices have been invented in which the determination of the moisture level and other parameters is carried out in an isolated room with an artificially created environment having stable parameters. Despite all the above-mentioned facts, the forms of the presence of water in the bean have fundamental differences, and the division into free and bound water provides answers to many questions.
Free water has the same properties as pure water, since its binding energy does not differ from the binding energy of ordinary water. It also evaporates easily, passes into the liquid or solid phase at the same temperatures, preserves solvent properties. Free water, not bound by polymers, is available for biochemical, chemical and microbiological reactions. Another name for free water is mechanically bound moisture (physico-mechanical water). Free moisture is extracted from the object by pressing, by centrifugation or by evaporation. During the evaporation, a portion of bound water and a small amount of volatile substances that make up the bean are removed.
Hygroscopic water (absorbed moisture) is the moisture absorbed by the bean from the air. It has a higher potential for binding energy and is more dependent on the system in which it is located. Just like free water, absorbed moisture evaporates at elevated temperatures. The steam in the form of a dense layer on the surface of the bean and the steam that penetrates into the porous structure refer to the hygroscopic moisture. The behavior of water in the gas phase inside the bean is variable. Molecules of water from which steam has formed, join the polymer and leave them, then condense in the micropores, then again turn into steam. Hygroscopic water can be referred to as water in a transitional stage. Its properties partly repeat those of free water and partly of bound water.
Bound water is associated moisture, firmly connected with various components of the bean - proteins, lipids and carbohydrates. Its properties differ significantly from those of pure water. Bound water cannot be a solvent, it does not freeze at low temperatures (-40 ° C). A considerable amount of energy is necessary in order to separate bound water from the substance with which it combines. The removal of bound water leads to a violation of the structure of proteins, fats and carbohydrates with which it is associated, and to a change in the technological characteristics of the bean. As there are a lot of water binding mechanisms, bound water is divided into chemically bound, organically bound, adsorption-bound, microcapillary water, osmotically bound, hydrated water and immobilization water. Chemically bound water has the greatest binding energy. It can be removed from the material only with the help of a chemical reaction or an intensive heat treatment - by calcination.
Binders are introduced in the system to increase the amount of bound water - in food industry these are sugars, soluble salts and some alcohols.
Water in the process of coffee preparation
The water used to make the drink can be characterized in the following way: the water must be crystal clear and have high quality characteristics. Obviously, such a definition is not enough, as water in coffee making performs not only the most important work - extracting substances necessary for forming taste and aroma from the powder of ground coffee, but other tasks as well. In the boiler system of a professional coffee machine, water is a heat agent. Milk is churned with the help of the steam. The role of water is also significant in laboratory studies. The environment humidity affects the assessment of the taste and aroma of the drink.
