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Chemistry

Ginger (Zingiber officinale)

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gingerbreadol

Gingerol, a molecule responsible for the smell of fresh ginger, made from gingerbread*

Week 2 of the 2016 Great British Bake Off was Biscuit Week, culminating in a classic showstopper challenge which put the culinary and architectural skills of the bakers to the test as they constructed story-telling gingerbread scenes. Ginger (Zingiber officinale) is a popular ingredient all over the world in sweet and savoury cooking, so let’s take the chance to delve a little deeper into the chemistry going on in the bakers’ ovens.

1.    The picture above shows the skeletal formula of 4-gingerol made from fingers of iced gingerbread. Gingerols are a family of volatile oils which give fresh ginger its scent and which react during cooking to give compounds with similar flavours and smells.

The root of the ginger plant, zingiber officinale, is an aromatic and flavoursome ingredient often found in kitchens. It can be bought as the fresh root, a dried powder or pickled in syrup. In its various forms ginger appears in recipes ranging from curries and stir-fries characteristic of Asian cooking to traditional sweets like candied ginger, cakes and gingerbread. The plant originated in southern Asia and the word “ginger” is thought to have its origin in the Dravidian languages of this region.
European cuisine boasts a huge spread of variations on the theme of gingerbread, from the soft Dutch cake peperkoek (“pepper cookie”) to the thin, brittle biscuits found in Scandinavia such as the Norwegian pepperkaker. Many countries use gingerbread as a base for edible Christmas decorations, including the famous gingerbread men introduced by Queen Elizabeth I to entertain foreign guests. In the fairy tale of Hansel and Gretel the hungry children stumble upon a house made of gingerbread – it is unclear whether the Brothers Grimm took inspiration from the work of German bakers or whether the edible cottage was their own invention.

gingerol and capsaicin

Molecular structures of 6-gingerol (above) and capsaicin (below)

2.    Fresh ginger gets its distinctive aroma principally from the volatile compounds called

gingerols. This family of compound are termed “β-hydroxy-ketones” because they have a hydroxyl (-OH) group situated two carbon atoms along from the carbonyl (C=O) group. The diagram shows the similarity between 6-gingerol and capsaicin, the molecule responsible for the heat of chillis, which goes some way to explaining the gently fiery taste of ginger.

The other gingerols present in ginger differ only by the length of the hydrocarbon “tail” of the molecule – this part does not significantly influence the reactivity of the molecules so their behaviour can be described collectively.

shogaol and zingerone

Structures of shogaol (above) and zingerone (below)

However, this is far from the whole story of the flavour and smell of ginger – two other molecules, shogaols and zingerone, are also very important here. Shogaols are considerably more pungent – a term used to describe chilli heat – than gingerol; shogaols have 160,000 Scoville heat units to gingerol’s 60,000. Zingerone is more closely related to vanillin (a common additive to sweet foods) and imparts the sweeter ginger flavour found in cakes. The compounds are all closely related, as shown by the reaction scheme below. Shogaols are made by elimination of a water molecule from gingerols, and zingerones are the products of a retro-aldol reaction of the β-hydroxy-ketone group.
The aldol reaction is a synthetically useful way to couple molecules together which was discovered by Alexander Borodin, an influential organic chemist and promoter of women’s education, but certainly most famous as a composer of the romantic era. The reaction occurs in two main steps; firstly a molecule of the ketone rearranges to an enolate – an alkene with a deprotonated hydroxyl group – and secondly this enol attacks another molecule of the ketone. This is interesting because the same starting material can react as both the nucleophile (reacts with positively charged centres) and the electrophile (reacts with negatively charged centres). As shown by the equilibrium arrows in the scheme below, the reaction is reversible, so when ginger is cooked the gingerol can undergo the backwards reaction to give an aldehyde and a ketone.

reaction scheme2

Reaction mechanisms of the conversions of gingerol to shogaol and zingerone

The dehydration reaction occurs during the drying processes used to preserve powdered root ginger, which is why ground ginger has a stronger smell than fresh ginger. The retro-aldol process which gives the sweeter zingerones occurs during cooking, hence the more gentle flavour of ginger cakes and biscuits. This simple relationship between the “reaction conditions”, the chemical composition and the flavour of the ginger has enabled chefs and bakers over the centuries to develop recipes which deliver an array of flavours from one simple ingredient.

Contributors: Harry Morgan (research, writing, baking, pictures), Anna Morgan (icing)

*Yes, there is an H missing. It’s gingerbread, not a modelling kit, after all.

One thought on “Ginger (Zingiber officinale)

  1. very informative, well written!

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