Chocolate quality and conching
The following is a summary of the stated research mentioned above. The content summarized here, including the figures and tables, all belong to the researchers (unless otherwise indicated). The summary attempts to stay as close to the original paper as much as possible with some adjustments in regards to jargon, length, or to focus on bean to bar aspects.
Introduction
A major step for many chocolate manufacturers is conching. This piece of equipment is used to mix, shear, and aerate the chocolate mass while being heated above 40°C. The conche is not only used to alter the flavour of the final product, but also impacts the rheological properties and texture of the chocolate. The conche can be controlled for temperature, time, and the type of shearing (between different machines) which can each differently impact the viscosity, texture, and aroma.
The conche machine is often classified as one of two types:
Conventional systems with various designs
Compact systems such as refiner-conche (ball-mill) or premix-refiner-conche (Netzch and MacIntyre)
The aim of this study is to provide information for process development studies and to develop new perspectives in chocolate science and technology.
Conching Phases And Mechanism
There is a difference of opinion when it comes to how many phases exist within the conching process.
Two phase process
Some research (Di Mattia et al., 2014; Counet et al., 2002; Owusu et al., 2012 and 2013) defined it as a two-phase process. The first phase is called the dry phase. In the dry phase, the moisture level is reduced. Volatile acids such as acetic acid is removed, and the surface of all solid particles are covered with fat. The second phase is called the wet phase. Here a homogenous and paste-like fluid mass is achieved by adding more cocoa butter and emulsifiers.
Three phase Process
The three phases here are termed dry-phase, paste-phase, and fluid-phase. Ground up nibs from the refiner, usually in what is called the “flake” form is put into the conche where it is heated, mixed, and aerated. The main purpose of this phase is to allow evaporation of some volatile acids and water from the cocoa liquor. After this dry-phase the acetic acid levels remain relatively constant, since there is no more free water to act as a vapor carrier to remove it (Dimick & Hoskin, 1981). This mass then enters into the paste-phase. After that, it enters the fluid-phase where the addition of cocoa butter and emulsifiers such as lecithin or polyglycerol polyricinoleate (PGPR) are added. This obviously has a great impact on the rheology of the final chocolate.
It’s worth noting that since lecithin has a hydrophilic nature, it could have a negative impact on water removal. This is why it is added in the last phase. As well, if lecithin is added at the beginning of conching, it may be absorbed by cocoa particles instead of being free, and hence become less effective at acting as a surfactant. The performance of lecithin can also be decreased if it is left at the high temperature of conching for a prolonged time (Wolf, 2017).
The dry phase is very important to reducing moisture and modifying the flow behaviour and overall flavour. The last phase where cocoa butter (and lecithin) is added is less important to overall quality (Fischer, 2010). Too much water within the chocolate will react with the sugar and have a negative impact on flow and melting properties. Bolenz et al. (2003) states that under ideal conditions that the amount of water that should be removed from milk chocolate is 0.1-0.5g/100g.
Conching time is impacted by temperature and mixing speed by way of agitation with heat and aeration (Dimick & Hoskin, 1981). Conching alters taste, flavour, melting characteristics, and texture as a result of shear and heat. Conching at high temperatures has been described as a way in which the cocoa butter can become more fluid, and result in better coating of particles. It’s beneficial to know that Strecker degradation reactions (responsible for some aroma development) are not completed during roasting of cacao, and continue during conching. The heat and mixing allows for Maillard reactions and Strecker degradation reactions (Dimick & Hoskin, 1981). Parameters for conching vary according to product type. In milk chocolate, one may want to avoid Maillard reactions and therefore keep temperatures below 50°C. When using sugar substitutes (sugar alcohols), optimizing conching conditions is important to avoid melting or agglomeration of the particles (Engeseth & Pangan, 2018). Sorbitol and maltitol are very hygroscopic, and isomalt forms agglomerates with high residual moisture.
As particle size of the chocolate is reduced during conching (AKA degradation of aggregates), there is an increase in the total number of cocoa particles. This increases the the frequency of contact between them and impacts chemical and mechanical interactions. This degradation of particles in conching causes rheological changes which can include decrease in plastic viscosity and increase in pseudoplastic behaviour (Vivar-Vera et al., 2014). As well, during conching the surface of the sucrose crystals can become coated with a lipid layer of about 34 nm (Kindlein, Elts, & Briesen, 2018).
Triglycerides, glycolipids, and other minor components are added to the chocolate at the end of conching. Lecithin covers the sugar crystals, and adding it in the wet-phase of conching as soon as possible can help at reducing viscosity. Deagglomeration leads to a decrease in temperature values, and is indicative of melting behaviour (Glicerina et al., 2013). As well, gaps in the structure increase and are filled with cocoa butter, which has an impact on the flow properties. Middendorf et al. (2015 & 21016) found that cocoa butter did coat the surface of sugar crystals, but that the surface coating had low homogeneity (didn’t coat it entirely or evenly).
Influence Of Conching On The Chocolate Flavour
The perception of aroma including volatile and non-volatile flavour components, as well as the effects the oil phase has on the release of aroma are factors to be considered. The complex chocolate aroma is a result of the presence and levels of hydrocarbons, alcohols, ketones, aldehydes, furanones, acids, esters, and pyrazines (Braga et al., 2018), with a total of over 500 volatile and non-volatile components (Da Veiga Moreira et al., 2018).
In its raw state, cacao is very bitter. It is only after fermentation, drying, and roasting that it begins to take on its more characteristic flavour. Both the presence and levels of precursors are important to the formation of the overall flavour. Conching plays an important role in overall flavour development as it removes undesirable volatile compounds and moisture, forms certain desirable flavours, and also impacts particle size regulation. The moldy, woody/smoky off-flavour elements and fruity aromas are reduced during conching (Owusu et al., 2013). As well, some volatile compounds are removed as a result of oxidative and carbonyl reactions catalyzed by heat and aeration during conching (Dimick & Hoskin, 1981). Acidic notes are reduced due to a decrease in volatile acid concentrations during roasting. However, non-volatile acids such as lactic, succinic, tartaric, citric, and oxalic acid are not affected by roasting.
There is a relationship between temperature and time, and increase in shear force during conching with flavour development (Ziegleder, 2004). Maillard reactions occur during roasting, and impact the formation and levels of aldehydes and pyrazines which contribute a great deal to chocolate flavour. These reactions can also occur during conching, and strong caramel flavours can be produced with high temperature conching. Pantillon (1995) reported that lactose underwent caramelization in milk chocolate via Maillard reactions.
Small peptides and free amino acids can be associated with cocoa-specific flavour precursors. Differences between cocoa related aroma precursors and protein profiles were determined for various fermented cocoa hybrids grown in Brazil (Da Veiga Moreira et al., 2018). This means that the differences in peptide and free amino acid profiles of various cacao may be taken into consideration when deciding on parameters of the conching process.
There are also losses in favorable compounds during conching as well. Pyrazines are a very important group when it comes to chocolate. tetramethylpyrazine is sometimes noted as most critical chocolate aroma component. Pyrazine levels vary depending on weather conditions, maturity, and variety factors in cocoa. They have also been found to decrease with long conching time (Fischer et al., 2010). Owusu et al. (2012) reported that 2,3,4,5 tetramethyl pyrazine, 2,5 dimethyl pyrazine, dimethyl trisulfide, and linalool concentrations decrased with long conching time. Another study determined the amounts of compounds with the highest concentrations found in un-conched dark chocolate decreased when this chocolate was conched. The compounds that decreased were 2,5 dimethyl pyrazine, 2-ethyl-5methylpyrazine, and 2/3-methylbutanol. However, benzyl acetate and 5-methyl-2-phenyl-2-hexanal compounds increased (Owusu et al., 2013). Amadori compounds which are formed during cocoa drying and roasting may also break down during conching (Heinzler & Eichner, 1991).
It is generally accepted that long conching time is a disadvantage for volatile aromatics due to evaporation effects. Although the concentrations of most aroma compounds is reduced especially in the fat phase, the amount of aroma components remained relatively constant in water-soluble (sugar and protein) and insoluble (cocoa solid) phase. Bolenz et al. (2003) stated that aroma development for milk chocolate was less important than dark at the conching stage, and so the dry conching can be bypassed.
Influence Of Conching On The Chocolate Flow Behaviour
The chocolate flow behaviour (the way chocolate moves when melted) are important both for the product quality itself and for the process (how it’s used). The rheological properties have an impact on viscosity, consistency, and mouth feel. It affects how the chocolate behaves in the mouth, which may also cause changes in aroma perception. Deformations and production losses during molding are also directly related with rheological properties.
Rheological properties are affected by two main factors:
1. Formulation: fat concentrations, fat type, emulsifier concentration
2. Process: refining, conching, and tempering
The function of conching is associated with particle size and homogenous distribution. The conching process causes the solid particles to be coated with the fat phase, which has an impact on flow properties.
The purpose of adding fat and lecithin in the conching process is to increase the coating level of the sugar and cocoa particles. This will decrease the viscosity (make it more fluid) because it reduces the inter-particle interaction (they will move past each other more easily with that extra coating of fat). For example, viscosity values in milk chocolate samples varied between 10 and 20 PA s after refining, and were determined at 0-10 after conching and tempering (Glicerina et al., 2015). As well, maintaining high temperatures during conching appears to improve viscosity (Bolenz et al., 2003). When lecithin and remaining cocoa butter is added to the final part of conching, more particles are lubricated. Therefore the mobility of particles increases, and the interaction between them decreases.
Removing water from the chocolate during conching also has an impact on its rheology. Removing water will decrease viscosity. It’s suggested that 1% cocoa butter should be added to reduce the effect of 0.3% moisture on viscosity (Saputro et al., 2019; Wolf, 2017). In cases where enough water does not evaporate, PGPR (an emulsifier/surfactant) can keep the limited amount of water under control and limit its impact on the rheological properties.
Large gaps between solid aggregates can cause some cocoa butter to be immobilized. Too much immobilized cocoa butter will have a significant impact on rheological properties. The structural changes of agglomerated particles during conching impact the resistance to melting. As well, the presence of lecithin and increase in fat may lead to a decrease in the temperature associated with the beginning of the melting of chocolate (T-onset) and the temperature of complete melting (T-end) (Glicerina et al., 2013).
Influence Of Conching On The Texture
Hardness of chocolate is a textural quality, and is said to be correlated with the type and amount of fat used, sugar type, particle size distribution, tempering conditions, and temperature of conching. When particles of cocoa and sugar are coated during conching, the promotion of key textural qualities can be achieved such as “snap”. The smoothness of the chocolate is achieved by breaking down agglomerates and coating them with cocoa butter. Chocolate products with different textural properties can be achieved with different time and temperature combinations when conching (Konar, 2013, Owusu et al., 2013, 2012).
Texture does not just stand alone, but has an impact on the flavour perception of the chocolate as well. The perception of flavour can be changed or masked as a result of various textural and melting properties. Therefore, texture development should be considered when conching.
Conching And Chocolate Safety
Conching is a stage where relatively high temperatures are used. At this stage, all possible microbial contamination from raw material should be eliminated. Due to low moisture content (<1.5%) and water activity (0.25-0.50), chocolate can be defined as a microbiologically stable product.
Salmonella can be encountered with thermal resistance, and is an important safety problem to consider (Nascimento et al., 2012). In a study by Krapf and Ganteinbein-Demarchi (2010), various samples of chocolate mass and cocoa butter were inoculated with Salmonella spp. and then conched at various temperatures (50-90 °C). Results suggested that only heat treatment was not sufficient for Salmonella spp. because of inadequate inactivation. They suggested ultrasound-assisted conching to ensure adequate reduction of Salmonella spp. Nascimento et al. (2012) investigated the effect of cocoa roasting and conching of milk chocolate on Salmonella inactivation, and found that there was a rapid death rate in the first 180 minutes of conching, and a slow inactivation afterwards. They stated Salmonella resistance changes according to type and content of the mixture, process temperature, and initial load.
Another food safety issue that may be encountered during conching is heavy metal. A ball-mill conche has the possibility of heavy metal migration from the metal balls to the chocolate. Iron (Fe) migration to chocolate may occur due to long term conching (Kruszewski & Obiedzinski, 2018).
Influence Of Conching On The Colour PRoperties
The concentration of tannin with oxidation and emulsification can cause changes in colour properties (Count et al., 2022). However, the mechanisms of how this change occurs have not been studied. Properties such as brightness, hue angle, and chroma value are important in terms of consumer evaluation. Prawira and Barringer (2009) reported that long conching time caused low particle size perception and brighter chocolates which were preferred by most consumers in their study.
Bioactive Compounds And Conching
The most important bioactive components for chocolate are phenolic substances, and can be a motivation of choice for some consumers. The phenolic substance in cocoa based products is affected by raw material variety, origin, processing techniques and technologies, as well as process methods and parameters (Di Mattia et al., 2014; Jalil & Ismail, 2008).
The information on changes of phenolic compounds during chocolate processing is limited. Schumacher et al. (2009) reported to change in phenolic levels during conching. Sulistyowati and Misnawi (2008) reported polyphenol and antioxidant activity decreased significantly due to conching temperature. Dimick & Hoskin (1981) reported a rapid lost of simple phenols during conching. Di Mattia et al. (2014) determined changes in the composition of phenolic compounds dependent upon time and conditions of conching. They also stated that amount of phenols were higher in chocolate with short term conching.
More research is required to understand the effects of conching on polyphenol concentration and profile, and the presence of milk-derived polypeptides should be taken into consideration as well.
Disadvantages and innovative conching studies
Conching can be a high cost process due to the high temperatures used and the extended processing times. The time needed depends on the amount of chocolate being conched and the configuration of the conching equipment itself. Normally the conching time ranges from 6-24 hours in more efficient conches. A traditional long conche normally takes about 72 hours (Ziegleder, 2017). Most chocolate makers only use 4-6 hours of conching time, but this can have negative impacts on the quality of the final product.
There are many styles/models of conching machines. If a maker were to change between different conching machines or upgrade as they scale up, some issues will come up and often experimenting and trial runs will need to be conducted. Different machines have a different impact in regards to how volatile compounds are influenced. Larger machines with more powerful motors will produce higher shear rates, and lead to rapid heating and a decrease in conching time. Therefore, the sensitivity of the temperature control and motor power should be taken into consideration. Equipment capacity is another factor to consider when choosing or upgrading conching machines. Small capacity conche systems may be used to reduce process time and production costs. Modifying a recipe may also require modifications to the conche parameters. Changing the amount of cocoa mass, chancing in conching times may be required depending on the moisture removed and the change in the level of unwanted volatiles needed.
In low cost chocolate or compound chocolate, Cocoa butter equivalents (CBE), non-lauric cocoa butter replacers (CBR) and cocoa butter substitutes (CBS) may be found. They are usually used to increase melting resistance. Conching time when these type of fats are included is shorter. Chocolate using CBS can tolerate up to 5% CBS of cocoa butter weight, and CBR can tolerate up to 25%. More than this may result in a eutectic effect and result in bloom development. This is why some manufacturers who use these cocoa butter replacers use cocoa powder instead of cocoa mass, since it contains 10-12% lower fat content compared to cocoa mass. Also, ball-mill or pre-mixer-refiner-conche would be preferred use over conventional conching systems for these sorts of chocolates.
Microstructural changes during conching are important to the flow properties of molten chocolate. If particle size is too small, more cocoa butter is needed. A chocolate with a lower unit cost is possible by optimizing the cocoa butter amount during the conching process (Owusu et al., 2012). For example, conching time can be reduced in dark chocolate by increasing mixing speed and temperature without compromising chocolate quality (Vivar-Vera et al., 2014). The mixing speed is more effective than the temperature increase.
One issue with short-term conching is the water absorbed from the air by the main components of chocolate. During refining, the water comes from the milk powder and cocoa liquor and towards the amorphous surface of the sugar particles, where it forms a solvate layer of sugar dissolved in water which can cause the particles to stick together (Bolenz et al., 2005). Moisture should be removed in the dry-conching phase. Aside from texture and rhelogical properties, melting behaviour and storage generally depends on low moisture content of < 0.1%.
