Milk is one of the main ingredients in the production of milk chocolate and white chocolate. Milk products are complex materials containing proteins, lactose, fat and emulsifiers. Milk ingredients are critical in delivering the desired properties and sensory profile (taste, texture) to consumers. They have a significant effect on chocolate processing, particularly processing behaviour of the molten chocolate mass e.g. flow properties (Haylock and Dodds, 2009).
Milk chocolate is the most popular type of chocolate and more milk chocolate is bought rather than dark and white chocolate in most countries of the world. It should be considered that different forms of milk should be used in combination with cocoa and sugar to produce milk chocolate and white chocolate (Haylock and Dodds, 2009; Beckett, 2008). Milk can be dried to make different powders. Skim milk powder and full cream milk powder are the most common milk powders used for chocolate manufacturing. These two powders have different flavours, textures and liquid flow properties due to different heat treatments during the drying, and also due to the different state of the fat. In skim milk powder, all the fat is free to have a reaction with the particles and the cocoa butter. Many full cream milk powders have fat tightly bound with the individual particles resulting in less fat to help the flow or to soften the cocoa butter (Beckett, 2008).
2.2.2. Postharvest processing of Cocoa
The key indicators in the processing of cocoa are the post-harvest treatments which result in final cocoa and chocolate products. These treatments include all the primary processes of harvested cocoa pods before the final dried bean is obtained. The processes involve opening the pods to extract the beans, fermentation and finally drying (Kongor et al., 2016). Fermentation and drying are usually carried out on the farm and they play a critical role in the formation of flavour precursors (Fowler, 2009).
2.2.2.1. Fermentation
Fermentation is a key stage in the processing of cocoa beans that result in the formation and development of flavour precursors such as amino acids and reducing sugars (Afoakwa, 2010; Afoakwa et al., 2008; Nair Prabhakaran, 2010). Fermentation is also responsible for the development of the chocolate brown colour (Beckett, 2008) and reduction in bitterness and astringency (Afoakwa et al., 2008). Proper and correct fermentation is essential for producing a good flavour in the final chocolate, as well as to ensure that the bean is killed, thus avoiding germination and alteration of the beans (Gutierrez, 2017; Beckett, 2008).
Fermentation is carried out in different ways from country to country and even from region to region within the same country. The most used fermentation methods include platforms, heap, baskets and boxes (Lehrain and Patterson, 1983; Afoakwa et al., 2008). The duration of fermentation depends on the variety of the beans and on the farmers’ practice (Beckett, 2009). Forastero beans are left to ferment for 5 to 6 days but for Criollo beans, only 1 to 3 days is sufficient (Afoakwa, 2010; Afoakwa et al., 2008).
The fermentation process can be considered in three stages. In the first 24-36 hours after harvesting and opening the pods, the beans and pulp are exposed to different microorganisms. Anaerobic yeasts convert sugars (sucrose, glucose and fructose) into alcohol under conditions of low oxygen and a pH of below 4. Bean death happens on the second day, caused by acetic acid and alcohol (Fowler, 2009; Aprotosoaie et al., 2016). Some yeast start pectin degradation of the pulp cell walls by pectinase, therefore, the pulp turns liquid and is drained away (Aprotosoaie et al., 2016). After 48 t0 96 hours, the yeast activity is prevented by aeration, alcohol concentration and increased pH. Lactic acid bacteria become dominant in this stage (Fowler, 2009; Aprotosoaie et al., 2016; Gutierrez, 2017). They convert sugar and some organic acids into lactic acid. Toward the end of the second stage, acetic acid bacteria become more significant when aeration increases. Acetic acid bacteria are responsible for the oxidation of ethanol to acetic acid (Fowler, 2009; Aprotosoaie et al., 2016). All these processes that occur in the last stage are an exothermic reaction, and these processes heat up the cocoa mass to 45°C to 52°C which is essential for flavour development (Aprotosoaie et al., 2016).
Figure 2.3 illustrates a schematic presentation of the chemical changes and reactions which occur during the fermentation to produce the cocoa flavour precursors (Fowler, 2009).