Content

The fourth chapter brings us to the science of Chocolate and Ice Cream.

Chocolate

Chocolate, one of the popular desserts around the world. Made from cocoa bean, it undergoes a long preparation process from harvest to products in the form of cocoa powder or chocolate.

There are 3 general categories to chocolate: dark, milk and white.

• Dark chocolate has a composition consisting mainly of cocoa particles, cocoa butter and sugar.
• In milk chocolate, significant amounts of cocoa-bean particles are substituted with dried milk proteins and sugar.
• White chocolate has no cocoa in its composition and is made of cocoa butter, milk solids and sugar.

Often you would see chocolate advertised at “70% chocolate”, what they are trying to say is it’s 70% cocoa butter and particles and the remaining 30% is sugar.

Cocoa

Fermentation

So what is cocoa? Cocoa powder and cocoa butter are obtained through the fermentation of cocoa beans. The initial fermentation by lactic acid bacteria produces alcohol, which undergoes another fermentation with acetic acid bacteria that converts alcohol to acetic acid. Acetic acid produced penetrates into the beans and etches holes within the cells, allowing astringent phenolic compounds to react with other molecules such as proteins. This would form less astringent complex compounds, making it more appealing to taste.

Roasting

Dried fermented cocoa beans are less astringent and more flavourful than unfermented beans. After drying, cocoa beans are roasted for 30-60 minutes at 120-160 degC, allowing the Maillard reaction to occur. The abundance of amino acids and sugar present within the cocoa beans generates the rich flavour chocolate has, thus note that the difference in roasting temperature range between cocoa roasting and general Maillard reactions.

Grinding and Refining

After roasting, the cocoa nibs (inner part of the beans) are separated from the bean, and passes through steel rollers to produce a thick, dark fluid called cocoa liquor. Cocoa liquor comprises of 2 parts: cocoa butter and cocoa particles. Refining (also the final grinding process) grinds the particles down to 0.02 to 0.03 mm. (We humans are able to detect solids down to 15 micrometres (which is 1000 smaller than millimetres (mm)!)

The cocoa liquor is pressed against a fine filter that retains the cocoa particles, allowing cocoa butter to pass through. This separation allows for different uses of each component. The retained cocoa particles will stack up into a ‘cocoa cake’ which can be smashed into cocoa powder, which is used in desserts, drinks and more. Cocoa butter can be used for white chocolate, skincare and many other products.

Chocolate Polymorphs

There are 6 different chocolate polymorphs. Polymorphs can be loosely defined as the different forms/structures of the same thing. In this case, all 6 polymorphs can be obtained from the same batch of chocolate, but the preferred form is Form V. Why? Form V has a melting point of 32-34 deg C, has a pretty, glossy shine. It is also the most stable form when stored at room temperature, but still susceptible to changes if not stored properly.

Tempering

Tempering is the physical process where a chocolatier uses a scrape to continuously apply a shearing action on the melted chocolate sitting on a cold marble slab. Does that sound familiar? You might have seen it before:

As the marble surface is colder than the melted chocolate, it will lose heat through conduction. The chocolate in contact with the surface will become cooler, and start to pack together and assemble in an orderly fashion. The action of moving the melted chocolate and mixing it continuously is shearing, where a force is applied perpendicular to the cross sectional area. Now here comes the art and experience of chocolatiers. Insufficient stirring would produce too few seed crystals, and over-stirring would  produce too large seed crystals. Both will result in less than desired product, either melting too easily or coarse and crumbly.

Alternatively you can introduce seed crystals into the melted chocolate, allowing the randomly order chocolate to organise in the same packing as the seed crystals.

Fat Blooming

When chocolate bars are not stored properly, and allowed to melt and freeze again, a layer of white powder can be found on its surface. This white layer is actually cocoa butter! Cocoa butter would melt with the unstable crystals and reform on the surface. If it was properly tempered with no unstable crystals of other forms, this is unlikely to happen. The best temperature to store chocolate is 15-18 degC without any fluctuations for melting and recrystallization to occur. So it’s still safe to eat my chocolate with that white powder layer!

Crystallization

Now you might ask, what’s up with ‘seed crystals’?

Seed crystals are neatly packed solid particles of a certain component. This acts as the starting point for others to build upon, similar to constructing a building starting with a blueprint. Similarly, seed crystals provides the base for the melted chocolate to build upon. By providing the necessary mixing and time, the bulk (most of the chocolate) can follow the packing order of the seed crystal, effectively transforming it into the desired form.

Now with this chemistry concept, it’s a good time to move on to ice cream!

Ice Cream

Similarly, this is how ice crystals are packed when it is being cooled and eventually freeze. Remember being shown the ‘magic trick’ of instant freezing water after knocking it? Here’s a reminder:

Due to the lack of impurities or ‘seed crystals’, there is no nucleation/starting sites for water molecule to start organising and packing. When it is disrupted by hitting it on a surface in the right conditions, water molecules will come together and form an ice nucleating seed crystal for ‘everyone else to follow’. As everyone knows the freezing point of water is 0 degC, but if there is no impurities, it can remain liquid below 0 degC and we call this state ‘supercooled’.

The rate of cooling or freezing process of the ice also affects the size of the ice crystals. The shorter the time of cooling, the less time for the ice crystals to grow, resulting in smaller crystals. The smaller the ice crystals, the smoother the ice cream.

As seen from the graph in the previous section, there is a region of rapid nucleation where new seed crystals are formed. This doesn’t mean that no new crystals are formed after, but rather the preference to grow from the existing ones.

The formation of water to ice is a cooling process, or freezing, where energy/heat is taken out. There are many ways for cooling, from the use of chemicals (liquid nitrogen, dry ice) or the use of machines to remove heat (freezer, anti-griddle). These method have different rates of cooling and produces ice cream of different textures.

Types of ice cream

There are many types of ice cream from sorbet, sherbet to dairy and Turkish. Each and every variation of ice cream is unique, but there are too many intricate details to discuss. Here we shall discuss 2 common types of ice cream:

1. Sorbet
   Sorbet contains no milk or dairy products. It is generally prepared from a thick, flavoured solution. It is then freezed before serving. Effectively, it is flavoured solid ice.
2. Dairy Ice Cream
   This is the most commonly available type of ice cream. It contains dairy/milk or cream and sugar, flavourings, colourings and sweeteners. The mixture is normally stirred before freezing to introduce air into the thick mixture, essentially forming a combination of foam and emulsion (will be explained later).

   Generally, the common ingredients are:
   – Milk (proteins): the main ingredient to stabilize emulsions and foams. Can be whole milk, skimmed milk or condensed milk; each has different fat %.

   – Sugar: the essential sweetener in our ice cream! Examples: glucose/dextrose, fructose and sucrose.

   – Oils & Fats: Helps to stabilize the foam formed that provides the creamy texture. Examples: Milk fats!

   – Water: 60-72% weight/weight of ice cream! The main component that mixes everything together.

   – Emulsifiers: to stabilize the air bubbles in ice cream, giving that aerated texture. Examples: egg yolks and mono/di-glycerides (fatty acids)

   – Stabilizers: reduces the rate of melting, slows down the migration of moisture in ice cream during storage. Examples: Xanthan, gelatin and sodium alginate.

* Fun fact: Total solids in an ice cream refers to everything else other than water!

Coarsening

Coarsening is a process where the size of particles increase at the expense of total number of particles. There are 2 ways this can happen:

1. In emulsions and foams (this will be explained in the next section), it is called coalescence. For ice crystals, it is called accretion. Coalescence and accretion is the process of two or more adjacent particles join to form a larger one, or simply combining.
2. For ice crystals and emulsions, it is called Ostwald ripening. For foams, it is called disproportionation. Ostwald ripening and disproportionation is the transfer process from smaller particles to larger particles by diffusion, or giving and receiving.

Remember when the ice cream becomes ‘freezer-burn’ where it just becomes too icy and chunky? There could be 3 possible reasons:

1. Recrystallization of melted ice cream: the water and milk fats in the ice cream separated when it is melted and recrystallize when freezed, forming large solids that spoils the good texture it once had.
2. Ostwald ripening of ice crystals: the slow natural rearrangement of ice crystals, forming larger ice crystals that gives that coarse mouthfeel when consumed.
3. Condensation of water vapour in the headspace of the ice cream tub: the empty space above the ice cream can be called headspace. When placed back in the freezer, the water vapour condenses on the surface on the surface of the ice cream.

(Tip! I read somewhere that you can avoid #3 if you store your ice cream upside down, effectively condensing the water vapour on the lid instead.)

Workshop

In Week 7, we conducted our own workshop under the guidance of Dr Linda Sellou.

Colloidal Dispersions

There are 4 main types of colloidal dispersions: aerosols, emulsions, foams and sols.

• Aerosols: Liquid dispersed in gas
• Emulsions: Liquid dispersed in liquid
• Foams: Gas dispersed in liquid
• Sols: Solid dispersed in liquid

Chocolate Mousse

Each group was picked their chocolate randomly by drawing lots and every chocolate brand was advertised to contain at least 70% cocoa. We broke our chocolate apart and melted it down using a warm water bath, continuously stirring it.

By introducing air while stirring, air is trapped and dispersed within the melted chocolate while it cooled. This forms a foam (dispersed gas trapped in a continuous liquid medium). This gives a fluffy, aerated texture in the mouth. Due to the use of dark chocolate, it may not be appealing to all as it has a bitter taste.

Ribena Sorbet

Sorbet is not really a colloidal dispersion, but a rapidly cooled frozen solution. We took dry ice (solid carbon dioxide) and crushed them into smaller sizes before adding to a concentrated Ribena solution. Rapidly stirring the concentrate, the water in the Ribena solution freezes and forms small ice crystals. This gives the sorbet a smooth, slushy texture when eaten.

* Tip! Use more ribena syrup if you are trying it out 🙂 A sweeter mixture gives a better tasting sorbet.

I had a blast attending this workshop, trying the various chocolate mousse and Ribena sorbet that different groups made.

Conclusion

The core theme between chocolate and ice cream is the cooling process, which significantly affects the shape, size and/or texture of the final product. I have learnt so much about chocolate and ice cream from their composition, their preparation process, the chemistry behind, even down to their storage conditions.

There’s really too much science that can be discussed in great detail. I hope that I have sufficiently picked out the primary ones that are easily understood!

NEXT: Last chapter, Molecular Gastronomy and the Future of Food!