The microorganisms that give us our Easter treats

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More than 3 million tonnes of chocolate are produced annually worldwide, and at this time of year much of it will be made into Easter eggs. Most people know that chocolate is produced from cocoa beans, which are the seeds of fruits from the Theobroma cacao tree. What is less known is that cocoa cannot be produced without the involvement of hundreds of species of fungi and bacteria.

Cocoa fruit being picked.

The seeds within the Theobroma fruits are the starting source of cocoa. The bitter tasting seeds are encased within a sticky mucilaginous pulp which is not a component of chocolate, but is essential in the processes which transform the Theobroma seeds into cocoa beans. The Theobroma fruits are harvested, cracked open, spread outdoors in boxes and covered with banana or other leaves. Since the fruits are open to nature, they are quickly colonised by microorganisms living on the fruit surface, or in the air or rain. The first to arrive are sugar loving yeasts such as Saccharomyces who ferment the sugars in the pulp and release as waste products alcohol. The rising alcohol levels eventually kill the yeast, freeing the remaining pulp sugars for the next microscopic species to colonise, the lactic acid bacteria. When they metabolise sugars, as the name suggests, lactic bacteria such as Streptococcus species release lactic acid, which in combination with alcohol from the yeast, help break down the Theobroma pulp, which in turn helps to release the seeds. By this stage the seed pods have been converted into a mass of acidic bubbling slime, which is stirred to aerate it. This creates conditions for colonisation by the most important bacteria in chocolate production, the acetic acid bacteria, Acetobacter and Gluconobacter. These highly acid tolerant microbes convert the alcohols produced by the yeast to acetic acid, in a chemical reaction which is heat generating, thereby speeding the Theobroma pulp breakdown. Acetic acid is able to enter the Theobroma seed, which destroys the embryo and in so doing releases the chemical compounds essential for chocolate flavour development.  The naturally intense bitter flavour of the seed is also reduced by this treatment.

Cocoa beans in a freshly cut cocoa pod. Credit: Aude, via Wikipedia

After this microbial treatment, which takes around 5 days, the fermented Theobroma seeds are harvested, dried and roasted, which further develops the chocolate favour, and transforms the seeds into what we recognise as cocoa beans. To produce chocolate, the Theobroma seed shell is removed, releasing the cocoa nibs which are ground, and heated causing the mixture to become liquid. When cooled, the cocoa liquid can be processed into cocoa solids and cocoa butter. Dark or bitter chocolate contains cocoa solids and cocoa butter, and little or no sugar. Milk chocolate contains cocoa solids, cocoa butter or vegetable oil, milk and sugar. White chocolate contains no cocoa solids and is made using cocoa butter, sugar and milk.   Most Easter eggs are made using milk or white chocolate.

Because of the global popularity of chocolate, chemical analysis of cocoa and chocolate has given us much information about individual flavour component contributions. However, it is not possible chemically to reproduce the characteristic chocolate flavour spectrum that microbial fermentation gives to cocoa beans. This means without the hundreds of microbial species that ferment the Theobroma seed pods, there would be no cocoa beans, and no chocolate Easter eggs!

Microbiology and the hot cross bun

The yeast that play a major role in cocoa bean production also give us another Easter

treat, the hot cross bun. This is a leavened (raised) sweet bread roll that is spiced, studded with mixed dried fruit and marked on top with a cross. Hot cross buns are traditionally eaten at Easter on Good Friday in the UK, Australia, New Zealand, Canada, South Africa and some parts of North America.  The history of eating hot cross at Easter goes back to the 14^th century, but our understanding of the importance of microorganisms in making these seasonal treats is much more recent.  Hot cross buns have a pleasant sponge like texture due to the presence of a microscopic fungus, the yeast Saccharomyces cerevisiae.  When given sugars, Saccharomyces via fermentation generates bubbles of carbon dioxide which cause the dough to rise, and the hot cross bun to have a light, open texture. Yeast through release of their enzymes contribute to hot cross bun flavour, and during baking add nutritionally important B vitamins.  Close relatives of Saccharomyces (brewers and wine yeast) also play leading roles in the production of the wines, beers and lagers drunk at Easter. Yeast also give us flavouring preservatives such as vinegar, and of course we eat them directly as marmite!

Dr Primrose Freestone is Senior Lecturer in Clinical Microbiology in the Department of Infection, Immunity and Inflammation.