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Coffee is a significant and popular beverage consumed by millions of people worldwide. Total world production is more than five million tonnes per year (Ranken, Kill and Baker, 1997). Green Coffee beans are the second largest commodity traded globally second only to oil (Wintgens, 2009). Its roles in everyday life are varied. From the beverage that people turn to for vitality in the early hours of the day, to a drink that encourages social activity. Its consumers are diverse and so as to be expected are the attributes sort after in the flavour of their coffee.
The coffee that we consume today is the final result of a long process that starts with the cherries picked from the different coffee plants. The cherry of a coffee plant contains two seeds or coffee beans as we know them. The plants that produce the coffee beans belong to the genus Coffea which contains more than ninety different species. Two are commonly known, Coffea arabica and Coffea canephora known as Arabica and Robusta respectively (Ranken, Kill and Baker, 1997). Mainstream cultivation takes place in locations with moderate sunshine, temperatures in the region of 20°C and where soil is rich and fertile. Large producers of coffee include Guatemala, Costa Rica, Ethiopia, Yemen and Kenya (Kummer, 2003). Around 60% of the beans grown today belong to the Arabica plant and are considered to be of higher quality than those from the Robusta which makes up the remaining 40%. This value of 60% has declined from its previous value of about 75% in the nineties. The reason for this is because new technologies can mellow the so-called harsh flavours produced by Robusta, enabling more of it to be roasted (Eade and Sayer, 2006). Work has been carried out to prove the idea of Robusta having a poor flavour wrong and is discussed later on.
There are many steps through which the coffee bean passes from plantation to cup. Coffee roasting is one of the key steps and is of considerable importance when it comes to developing the specific organoleptic properties of flavour, aroma and colour, all of which are the foundation to the quality of the coffee (Hernandez, Heyd and Trystram, 2008). This single step is the focal point of this paper and the aim is to identify, compare and discuss the efforts made in this area for flavour development.
General - To convert the selected blend of green coffee beans into a consumable beverage, three operations must be completed: roasting, grinding and finally brewing. The characteristic flavour and aroma of the coffee is developed solely by roasting (Clarke and Macrae, 1985). Colour, aroma and flavour all being highly important to the quality of coffee (Houessou et al., 2008).
To create these flavours and aromas, a series of chemical changes must take place leading to a change in the chemical composition of the bean (Dutra et al., 2001). The roasting process consists of two stages, first the drying stage where moisture is driven out of the bean. This is followed by the second stage where pyrolytic reactions take place within the bean (Cristo et al., 2006). At around 180°C pyrolysis begins and exothermic chemical reactions raise the bean temperature an extra 20-30°C (Ranken, Kill and Baker, 1997). The colour of the beans changes from its original light green colour to brown or in some cases even black. Brittleness is greatly increased so that it is possible to carry out grinding and extraction (Baggenstoss et al., 2008).
Chemical reactions include the Maillard reaction and Strecker degradation and along with others, these produce many different volatile compounds. A lot of research into coffee roasting has been based on the sensory relevance of these compounds and ultimately identification of the key odorants of coffee (Schenker et al., 2002).
As mentioned in (Flament, 2002), the changes that take place within the bean when roasting at 180-220°C result principally from Caramelization and the Maillard reaction. Caramelization is the pyrolysis of mono-, di-, oligo-, and polysaccharides which ultimately produce either soluble caramel compounds or black particles of high molecular weight. The Maillard reaction involves reducing sugars and amino-acids or peptides of low molecular weight. The reactions form nitrogen and sulphur containing molecules - brown nitrogenous polymers or melanoidins (Ellis, 1959).
The Maillard reaction is an important process to understand in the food industry as its products can be both desirable and undesirable (Fellows, 2000). One of the factors that influence this reaction is temperature and so it is a fundamental process to understand within coffee roasting. There have been various types of research carried out to try and model the reactions. These models are based on reaction kinetics. of this is looked at to identify useful comparisons to coffee.
Physical. -Up until the 1900s coffee beans would have been roasted in the home, using equipment such as frying pans or hand turned cylinders. The difficult part of the roasting process is that heat must be applied quickly and homogeneously. You do not want to scorch the beans but at the same time you need enough heat to enable the beans to pyrolize or roast (Sivets and Desrosier, 1979), thus roasting is very much a time-temperature-dependant process. In industry the degree of roast is controlled primarily by appearance of the coffee beans (Ranken, Kill and Baker, 1997). As timing is critical to the roasting process this is where developments need to be made.
Competition amongst big companies creates a constant driving force for the production of bigger and better products; or in this case production of the best tasting coffee. For this reason it is essential to further ones understanding of the roasting process.
Detail of chemical changes from food industries manual pg 370
There is a considerable amount of literature published on coffee roasting (Robbins and Fryer, 2003) however; making use of mathematical modelling has not yet been widely adapted in this area. Very little literature data is available on the formation rates of volatile compounds and the influence of different roasting conditions on the development of aroma compounds (Schenker et al., 2002). Many mathematical models have been developed for processes similar to coffee roasting, such as barley, hazelnut and sesame seed roasting. These models are based on heat and mass transfer and are looked at briefly to compare and identify areas that may be applicable to coffee roasting and thus possibly enable optimised roasting conditions to be found.
Many other efforts have been made to propose optimised roasting conditions but these are mainly based on analytical chemistry methods, producing empirical models rather than predictive models based on theory. These have also been studied and are discussed later on.
One of the main reactions that takes place during coffee roasting is the Maillard
Introduction to Coffee Chemistry
Sugars, proteins, free amino acids, chlorogenic acids and trigonelline are the principal flavour precursor compounds in green coffee. Maillard type reactions play a central role, among many other chemical transformations taking place during roasting of coffee. Maillard derived aroma compounds such as thiols, diketones and pyrazines. (Poisson et al., 2009)
May need to include in intro:
Coff chem. -Special attention to aroma
Types of roasts
Detail of Chemical changes and aroma development during the roasting process
Types of roasters and their applications
Key developments that have been made in understanding the Maillard reactions are mainly based on reaction kinetics. During food processing, the changes that take place are primarily chemical and physical and these changes progress at certain rates and with certain kinetics. Kinetic modelling enables one to describe these changes quantitatively and also produce basic reaction mechanisms; both are highly important if it is desired to control the process (Tijskens, Hertog and Nicolaï, 2001).
There are many challenges when it comes to applying kinetic modelling to food systems. Firstly it is often very difficult determine the correct mechanism that takes place for a series of reactions. Secondly attempts to simplify the models without excluding necessary details and including unnecessary details is also not easy. Another typical problem of applying kinetics to food systems is compartmentalization i.e. reactants may be physically separated due to location within different cells. When the food is processed, reactants may come together due to cell damage and the reaction may proceed (Tijskens, Hertog and Nicolaï, 2001).
There have been many research papers published analysing the Maillard reactions that take place within different foodstuffs however it has not been studied in great depth if at all during the roasting of coffee. In this section, some of the developments are looked at and discussed to identify possible routes for developments within coffee roasting.
The process of the Maillard reaction or non-enzymatic browning is a cascade of complex reactions. As stated by deMan (1999), there are five key steps in the formation of melanoidins:
The production of an N-substituted glycosylamine from an aldose or ketose reacting with a primary amino group of an amino acid, peptide, or protein.
Rearrangement of the glycosylamine by an Amadori rearrangement type of reaction to yield an aldoseamine or ketoseamine.
A second rearrangement of the ketoseamine with a second mole of aldose to form a diketoseamine, or the reaction of an aldoseamine with a second mole of amino acid to yield a diamino sugar.
Degradation of the amino sugars with the loss of one or more molecules of water to give amino or nonamino compounds.
Condensation of the compounds formed in step 4 with each other or with amino compounds to form brown pigments and polymers.
Note 'the Amadori rearrangement of the glycosylamines involves the presence of an acid catalyst and leads to the formation of ketoseamine or 1-amino-1-deoxyketose' (deMan, 1999).
Having an understanding of the chemical changes that takes place during the roasting process enables one to identify where previous work on Maillard reactions could possibly be applied. According to Ranken, Kill and Baker, 1997 the following events occur. Most reactions that take place within the bean do so under pressures of up to 8 bar; these are at high temperatures and fundamentally in the absence of oxygen. The pressure is achievable because of the oil within the coffee. As it melts during roasting, it is forced to the surface of the bean by the pressure supplied by carbon dioxide and water vapour production. It acts as a hydraulic seal, thus trapping many of the volatiles that may have otherwise escaped. Decomposition of the proteins produces different forms of amines. From those amino acids that contain sulphur, mercaptans and dimethyl sulphide are formed, both of which are said to be organoleptically important compounds. Decomposition of the carbohydrates produces volatile aldehydes, carbon dioxide and organic acids. Furfural is produced from the partial decomposition of pentosans. Most of the sucrose become camaralized or following hydrolysis becomes involed in Maillard type reactionswith protein and amino acids. Nearlly all of the chlorogenic acids decompose producing non-volatile lactones and volatile simple phenols. These contribute to smokey flavours, particularly in dark roasts. Approximatley 10% of chlorogenic acid content is lost for every 1% loss of dry matter.
Maillard reactions are examples of parallel and consecutive reactions that take place in food. For these reactions to be fully understood, multiresponse analysis has been developed where one is able to analyse more than one reactant or product at the same time.
Review of mathematical models developments for roast coffee
Models developed within the food industry for similar applications
Review of research using analytical chemistry