Chocolate

Cocoa

Cocoa was cultivated in South America 3000 years ago, and was a favourite food of the Aztecs in Mexico. Columbus and the Spanish Conquistadores brought cocoa beans back to Europe in the early fifteen hundreds, and since then cocoa and chocolate have gone on to become one of our favourite foods.

The cocoa plant is now grown in many tropical areas in the Americas, Africa and Asia, with West Africa and Indonesia being the main sources of cocoa for the rest of the world.

The fruit of the cocoa tree consists of pods of length from 100 to 350 mm and mass of from 200 to 1000 g which contain 30 to 45 beans. The beans consist of a hard shell which is covered in pulp and which contains a nib and a plant germ. The first step in processing the beans is the fermentation of the pulp and this is carried out by the farmers at the plantation.

Fermentation of the beans

Fermentation of the pulp surrounding the cocoa beans is carried out at the cocoa plantation and generally involves stacking the beans or placing them in boxes which allow air to penetrate. Apart from the simple products of fermentation, which include alcohol, acetic acid and finally carbon dioxide the fermentation produces a number of chemicals which are essential in producing the flavours and colours in chocolate. These chemicals diffuse from the fermenting pulp into the beans. The process also modifies bitter polyphenols reducing the bitterness and astringency of the beans.

Chemistry of Fermentation

Cocoa beans go through a number of different stages during fermentation. Here is a summary of the main processes which are important in the subsequent development of chocolate flavours.

Time after start

Main agent

Presence of air

Temp

Process

Other changes

24-36 hours

Yeasts

Anaerobic

~20°C

Sugar ® alcohol

Proteins start to break down into peptides and amino acids. Proteins also react with bitter polyphenols. The beans die during this stage.

48-96 hours

Lactic acid bacteria

Anaerobic

~45°C

Sugar ® Lactic acid

Lactic acid is important in later reactions

4-7 days

Acetic acid bacteria

Aerobic

Much heat evolved ~50°C

Alcohol ® Acetic acid ® CO2

The protein-polyphenol compounds are oxidised, reducing the bitterness of the chocolate.

Drying

After fermentation the farmers will dry the beans at the plantation. The techniques used depend on the weather and on the circumstances at the plantation and can vary from simply spreading the beans out in the sun to using sophisticated commercial drying machinery. The beans are then packed, usually in hessian sacks, and sent to the cocoa processor. Cadbury process all their cocoa product requirements for the Asia Pacific Region in a Singapore plant.

Chemical changes in drying

The chemical changes that take place during drying include:

  • removal of water (from about 65% to about 7%) helps prevent mould growth in the beans

  • volatile (and unpleasantly flavoured) acids such as acetic acid are removed

  • polyphenols become oxidised to produce brown compounds giving chocolate its colour (see the ‘General Chemistry’ section of the ‘Milk, Beer and Wine’ CD);

Roasting

Roasting the beans develops the familiar chocolate colours and flavours through a variety of chemical reactions. The cocoa processor either roasts whole beans, followed by shell removal OR removes the shells and roasts the nibs.  If the second option is chosen the beans are usually dried at 70 – 90oC for about 1 hour to facilitate the removal of the shells by “winnowing”. During the actual roasting process the beans are heated to temperatures of from 110-160°C over 15 minutes to 1 hour. Since the winnowing is now carried out in Singapore, Tasmanian gardeners can no longer get the cocoa bean mulch (which consists largely of the bean shells) for their gardens. Some people may remember this mulch, which was chocolate coloured and hard quite a strong smell.

Chemistry of roasting

The main function of roasting is to convert the beans’ flavor precursors into the compounds that give chocolate its distinctive flavour and aroma. More specifically, chemical changes that take place during roasting include:

  • Maillard Reactions: These are very complex reactions between sugars and amino acids giving rise to compounds called mellanoidins which are responsible for much of the colour and flavour of the final chocolate. Maillard reactions are otherwise known as ‘browning reactions’ and are common in food processing (see the ‘General Chemistry’ section of the “Milk, Beer and Wine” CD.);

  • Astringent polyphenols are broken down, making the chocolate a lot more palatable (see ‘General Chemistry’ in the CD);

  • Many other flavouring compounds are produced including aldehydes, esters and other compounds.

  • Many volatile compounds such as acetic acid and lower mass aldehydes, ketones, alcohols and esters have very unpleasant flavours and are removed during roasting/

Grinding  (Milling)

The roasted nibs are ground between rollers to release the fat and produce a thick, creamy liquid.

·         Cocoa mass is the first product produced by grinding the nib. It contains both solid materials and fats. When cocoa mass is heated (40°C) it liquefies to cocoa liquor. Note that the terms cocoa mass and cocoa liquor are usually used interchangeably.  Mass / liquor is liquid when first produced as the heat generated in grinding the nibs causes the fat (cocoa butter) to liquefy. Cocoa liquor is an important ingredient in dairy milk and dark chocolates.

·         If cocoa liquor is pressed in powerful hydraulic presses it is separated into cocoa butter (the fats) and presscake (the solids). When ground into a powder, the presscake is called cocoa powder. Cocoa butter is the main ingredient in high quality chocolate while cocoa powder is used in drinks and in compounded chocolate.

The fats in cocoa

The fats in cocoa, like most fats, are triglycerides. These consist of a tri-ester of glycerol and three straight chain organic acids. Oleic acid, stearic acid, and palmitic acid account for more than 95% of the fatty acids in cocoa butter. The concentration of fat in the liquor is too high for making cocoa powder and too low for making eating chocolate. This is why eating chocolate is generally made from a mixture of the cocoa liquor and some of the cocoa butter, while the presscake used to make cocoa powder has very little fat.

Dutching the liquor

Adding alkali (potassium carbonate) to liquor in a process called ‘Dutching’ reduces the bitterness of the liquor and intensifies the dark colour that is due to the tannins in the chocolate. The chemistry is explained in the polyphenols notes in the ‘General Chemistry’ section of the ‘Milk, Beer and Wine’ CD, but here is a summary.

Phenols contain an OH group and undergo an equilibrium reaction with alkali:

            <rest of phenol>-OH  +  OH  « <rest of phenol>-O  +  H2O

Compounds with the –O functional group are known as phenoxides. Phenoxides are the main colouring agent in solutions of polyphenols so that adding an alkali to a phenol and so driving the equilibrium to the right will enhance the brown colouring of the phenols in the chocolate. The harsh tannin polyphenols in the chocolate are also more soluble in alkaline solution and will dissolve away reducing any unpleasant bitterness in the chocolate.

Chocolate

The main ingredients in chocolate

The main ingredients used in chocolate are cocoa mass, cocoa butter, sugar and milk.

·         The major components of milk chocolate are sugar, cocoa mass, cocoa butter and dry matter from milk (whole milk powder, skim milk powder or chocolate crumb). 

·         Dark chocolate is made without matter from milk.

·         White chocolate is made from sugar cocoa butter and milk solids

Milk Products in Chocolate

Dairy ingredients in chocolate give flavour, texture, colour add nutritional value and assist with fat bloom prevention. A variety of dairy ingredients can be prepared for use in chocolate manufacture

Ingredient

Composition %

Fat

Lactose

Protein

Minerals

Moisture

Fluid (whole) milk

4.0

4.9

3.4

0.7

87.0

Evaporated milk

10.0

11.8

9.5

2.0

65.2

Sweetened condensed whole milk

9.3

11.4

41.0*

9.3

2.0

27.0

Skim milk powder

0.8

52.3

35.9

8.0

3.0

Whole milk powder

27.5

38.2

26.4

5.9

2.0

Anhydrous milk fat

99.8

Trace

Trace

Trace

0.2

* Sucrose

Cadbury in Tasmania obtains condensed milk from its plant at Burnie. The condensed milk will be mixed and heated with sugar and cocoa liquor to form ‘crumb’ which can provide fuller flavours than chocolate made with milk powder for example.

Sugars used in chocolate

Sugar is the sweetening agent in chocolate.  Different types of sweeteners are used when making chocolate with special characteristics, e.g. chocolate for diabetics.

Some possible sweeteners

  • Sucrose is commonly used in chocolate making.  It is a disaccharide that can be obtained from sugar beet or sugar cane.  To obtain the required purity (~99.8% sucrose) other mineral and organic material is removed by the addition of excess lime to precipitate them out of solution.  The excess calcium hydroxide in solution is then removed by bubbling carbon dioxide through the solution, forming calcium carbonate. Filtration and then evaporation then give the crystalline sugar.

  • Lactose is also known as milk sugar as it is about 4.5% of dairy milk.  Lactose has very little sweetening power compared to sucrose.  Lactose is a disaccharide.

  • Glucose (also known as dextrose) and fructose are monosaccharides.  They are found in many fruits and are often called fruit sugars.  They can be can be obtained from the decomposition of sucrose by either addition of acids or by the action of enzymes.  Produced in this way the mixture will contain equal parts of glucose and fructose and is known as invert sugar. Invert sugar is a syrup which is usually used in things like cream centres rather than in chocolate itself.

  • Sugar alcohols (polyalcohols) are generally produced from sugars.  Their sweetening power and lower calorific value make then suitable for use as sugar substitutes in diabetic products.

Mixing and Refining

Milk chocolate results from the mixing cocoa mass, milk powder, sugar, cocoa butter and sometimes other products generally called cocoa butter equivalent CBE.  CBE may be used to improve flow characteristics and/or to reduce costs (cocoa butter is very expensive).  If chocolate is made from crumb (milk solids, sugar and cocoa mass) then the crumb is mixed with cocoa butter. Milk powder is the main commercial alternative to crumb, but this does not have the same rich flavours as crumb.

Chocolate Crumb

Chocolate crumb is made from concentrated milk, sugar and cocoa liquor. Chocolate crumb is manufactured by first mixing sweetened condensed milk with sucrose and water and heating the mixture to 74 °C.  Cocoa mass is added and mixed until completely blended.  The mixture is continuously fed into an evaporator where is it held for 3 to 4 minutes at 125°C. The important Maillard reaction takes place at this stage.  The crumb is then dried to a moisture content of about 1%.  This can then be stored and used to minimise the impact of seasonal variation in milk supply.

The important chemical reactions that take place during the crumbing process include:

  • Milk protein and lactose undergo Maillard reactions that produce caramel flavours.   This is an important consideration in the use of ‘crumb’ in chocolate production because the crumb flavours are richer than the flavours that are obtained with other starting ingredients.

  • The breakdown of milk fats by enzyme action produces small amounts of free fatty acids that can impart a buttery and cheesy flavour. 

Alternatives to cocoa butter

Cocoa butter equivalent (CBE) is a fat with the same proportions of fatty acids (mainly oleic, stearic and palmitic) as chocolate and is claimed by the manufacturers to be a cheaper substitute for cocoa butter. CBE is made from a mixture of oils obtained from other plants including palm oil and various nut oils.

Coconut butter replacers (CBR) consist of a mixture of oils which tend to have a slightly higher melting point than cocoa butter and do not require tempering (see the section on Tempering chocolate). This makes them very suitable for moulded chocolate and other chocolate products where mechanical stability is important and it would be a difficult to go through the tempering process. CBR fats have a similar balance of fatty acids to cocoa butter (mainly 16 to 18 C atoms in the fatty acids) and can be mixed with cocoa butter.

Cocoa butter substitutes (CBS) are different from cocoa butter in that they contain a large proportion of lauric acid with 12 carbon atoms. This gives them properties which are quite different from cocoa butter so that the two should not be mixed. The lauric acid tends to have a ‘soapy’ taste, but CBS fats also have a sharper melting point than cocoa butter and do not require tempering when they have been melted. They are sold as ‘compound chocolate’ and are used both commercially and at home to produce moulded chocolate confectionary.

Conching

Conching gives chocolate its ‘velvety smooth’ texture and also smoother, more mellow flavours. Conching is the mechanical working (stirring and shearing) of the refined chocolate flake at a temperature of about 45 – 70 °C, depending on the type of machine and the duration of conching.  Depending on the process and desired product characteristics conching can last from several hours to 4 days and in general the longer the period of conching the smoother is the final chocolate.

Chemistry during conching

1        Reduction in moisture content

·         Water content is reduced from about 1.6% to about 0.6 – 0.8%

2        Removal of unwanted flavour components.

·         Partial removal of low-boiling aldehydes and acetic acid, assisted by the removal of moisture

3        Flavour development

·         Removal of unwanted volatiles.

·         Additional Maillard reaction between free amino acids and reducing sugars  Reaction is slow at conche temperatures but is assisted by the shearing force of the conche, which allows interaction particles are forced together.

·         Some of the astringent polyphenols in the chocolate may be removed by combining with proteins. This would give the chocolate a less astringent, more mellow flavour.

4        Homogenising

·         Agglomerates of particles are broken down by shearing force

·         Particles of chocolate mass (which are astringent) and sugar (which on its own is unpleasantly sweet) are coated cocoa butter. The effect of this is to give softer and smoother flavours as well as a smoother texture.

·         The chocolate is worked until it is a uniformly liquid product that has the required flow characteristics.  The chocolate may be made less viscous by the addition of an emulsifier such as lecithin

Tempering

At this stage the ‘chocolate’ is a runny liquid. The characteristics of chocolate on the other hand are: stability (ability to retain the same appearance and shape over time), glossy surface, smooth mouth feel, and snap (the sound a thin piece of well-tempered chocolate makes when it is broken in two). Tempering is the process of converting the liquid into solid chocolate by changing the crystal structure of the fats in the chocolate and is carried out by very carefully heating and cooling the chocolate.  The time taken to temper chocolate varies considerably, depending on whether the chocolate is to be use to make solid bars of chocolate or is used to make coated products. 

The chemistry of tempering

The triglyceride molecules in fats have three long ‘arms’ consisting of the fatty acid chains. In the liquid form, these three arms are free to move about at random. As the chocolate is cooled, the arms can pack together in a number of different ways to produce different types of solid crystals. Rapid cooling will produce ‘untidy’ structures in which the molecules are loosely packed. These crystalline forms tend to change over time, making the solid unstable. Carefully controlled cooling over a longer time allows the molecules to pack together in the most efficient way, so that the final crystalline solid is dense, stable and has a relatively high melting point. When liquids cool and crystallise, they tend to crystallise around ‘seed’ crystals so that an important part of tempering is to ensure that at some point in the process, the mixture contains only liquid chocolate and some solid ‘seed’ crystals of the correct type.

One important property of CBR and CBS fats (see the previous page “Mixing and Refining”) is that they do not require tempering and so ‘chocolate’ made using these fats is easier to use in moulds. Cheaper Easter eggs for example are made with these products so that the manufacture does not have to go through a long tempering process. Higher quality Easter eggs on the other hand are made with ordinary chocolate which is tempered giving a slightly more expensive, but to many a nicer product.

The process of tempering

Tempering chocolate (or other fats such as fats used in pastry) involves keeping the chocolate at a range of carefully controlled temperatures for the correct amount of time. The steps are:

  1. heating the chocolate to 50°C (dark chocolate) or 45°C (milk chocolate) to melt all fat crystals;

  2. cooling of the chocolate to point of crystallisation (about 30°C);

  3. allowing the temperature to fall to about 27°C so that the chocolate can begin to crystallise (solidify). A mixture of fat crystals will form, both stable and some unstable. 

  4. The temperature is now carefully raised to about 30 – 32°C to melt out only the unstable crystal forms.

  5. At this stage, the only crystals left in the chocolate will be the ‘good’ crystals which give the chocolate its familiar texture. As the chocolate cools, these act as ‘seeds’ so that the remainder of the liquid chocolate will crystallise around these seeds into the correct form.

Producing the final chocolate products

Chocolate Blocks

  • Depositing
    Rows of chocolate block moulds travel along a conveyor belt and a measured amount of chocolate is deposited into each one from a large hopper above the moulding line. Extra ingredients, such as fruit and nuts, are added when other varieties of Cadbury Dairy Milk blocks are made.

  • Vibrating
    The moulds are vibrated on the belt to make sure the chocolate inside is level and that there are no air bubbles.

  • De-moulding
    The moulds pass through a long cooling tunnel where the chocolate sets. Once the chocolate is solid, the moulds are tipped upside down and the blocks are knocked out onto a conveyor belt.  After final quality inspection, the blocks are machine sealed in foil, wrapped, and packed ready for distribution.

Chocolate bars

  • Bars – Centring
    First the ingredients that are used to create the centres for each particular chocolate bar are mixed and cooked using steam, then cooled. The cooled centers are then passed though cutters on a conveyor belt and sliced to size.

  • Bars – Bottoming
    The cut pieces travel along a mesh conveyor belt which is awash with chocolate. The mesh belt vibrates back and forward, to ensure that the bottom of the centre is fully coated with chocolate.

  • Bars – Enrobing
    With its bottom coated, the centre then passes through a shower of chocolate, where it is fully enrobed on the top and sides. The coated bar is carried through a long cooling tunnel where the chocolate sets. After final quality inspection, the chocolate bars are machine wrapped and packed ready for distribution.

Chocolate Eggs

  • Depositing
    Half-egg moulds pass under a vat of chocolate where a measured amount of chocolate is deposited into each shell. The moulds are then closed to form an egg shape.

  • Vibrating
    The moulds are vibrated to remove any air bubbles from the chocolate and spun in every direction to evenly coat the inside of the shells. The moulds are gradually cooled until the chocolate is set in the shape of an egg.

  • Eggs – De-moulding
    When the chocolate is set, the moulds are opened and the hollow Easter eggs are carefully tipped out onto a conveyor belt. After final quality inspection, the Easter eggs are wrapped in foil by an egg-shaped wrapping machine and packed ready for distribution.

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