Picture It… is proud to present the first in a new series of special short posts inspired by the return of the popular BBC TV baking contest, “The Great British Bake Off” and written by our guest author, Harry Morgan, a chemistry undergraduate at New College, Oxford. Each week we hope to embark on a brief journey into the science behind a particular ingredient, technique or recipe highlighted by the bakers in the GBBO tent.
Here at Picture It, we are as excited as anyone about the return of the Great British Bake Off, and in the opening week the technical challenge provided a chance for a quick glance at some kitchen chemistry. It saw the bakers attempting to make Jaffa cakes, a favourite sweet snack since their introduction in 1927, and normally flavoured with orange jelly and chocolate.
1. In the picture above we can see orange jelly resting on a Petri dish and surrounded by beakers and molecular models of limonene (in this form it smells of citrus fruit), the amino acid glycine (a key building block of gelatin, used to make jelly) and water. Jelly is a traditional sweet treat, both on its own or as a component of puddings like trifle, mentioned in writing as far back as 1520 at Henry VIII’s garter feast. In Victorian kitchens jelly was made using boiled animal ingredients, such as calves’ hooves or ground deer antlers, whereas today we are rather more likely to open a packet of gelatin. In fact, the only thing that has changed is the manufacturing arrangement – gelatin is still made by processing the skins, bones and connective tissues of cattle and pigs by a range of physical and chemical methods.
These processes all aim to convert collagen, the main structural protein of animal connective tissues, into its less robust derivative – gelatin. This makes quite a lot of sense as a route to making jelly; by partially degrading the tough tissues in bones and cartilage we can make an edible substance with enough structure to hold its own shape but still be soft enough to mould, and indeed wobble. Purified gelatin often comes out of a packet in the form of a brittle thin sheet (as shown in the photo on the right), or a dry powder which must be soaked in cold water and then dissolved in warm liquid before the mixture cools and then sets. This sequence is necessary because gelatin will not dissolve straight into cold water, but heating it too strongly will cause too much protein degradation and prevent the mixture from setting.
Certain fresh fruits contain enzymes called proteases which are also capable of degrading the proteins giving jelly its structure. For this reason fresh pineapple and kiwi fruit cannot be used in making jelly; however, the heating involved in preserving food in cans denatures (breaks down) these proteases, so tinned pineapple can be added to jelly without trouble. Last Wednesday (24th August 2016) we saw the contestants in the Bake Off tent making orange jelly, some adding orange juice for flavour. Oranges do not contain gelatin-degrading enzymes, but it is crucial to use smooth orange juice as the bits disrupt the gelatin structure so the jelly does not set well.
Gelatin is not the only way to make gels from solutions in water – for a look at how pectin makes jam set, see our post on marmalade.
2. A single collagen molecule, called tropocollagen, has an unusual structure called a “triple helix”. This is formed by taking three long chains of amino acids, called polypeptides, and winding them around each other in a spiral, much like braiding a piece of rope. It is closely related to the familiar double helix structure of DNA, but instead of two strands tropocollagen is made from three. The strands are held together by an intermolecular interaction called hydrogen bonding. Hydrogen bonding is an interaction between an H atom bonded to an element other than carbon, such as oxygen or nitrogen, and an atom capable of donating a pair of electrons.
In the picture of acetic acid (right) the hydrogen bonds are shown as dashed lines. While these intermolecular interactions are perhaps a hundred times weaker than covalent bonds, they are numerous and still strong enough to drive proteins to form complex structures such as the triple helix of tropocollagen. In living tissue these molecules then assemble into aggregates held together by covalent bonds, called “crosslinks”, and more hydrogen bonds. This makes the tissues made up of collagen very physically strong.
The process of degrading collagen to gelatin therefore consists of two parts – the hydrogen bonds and crosslinks between the molecules must be broken, to pull the molecules away from each other, and then some of the peptide bonds making up the backbone of the proteins must also be hydrolysed. This results in a more disordered mixture of shorter chain polypeptides which are not able to aggregate into such strong, fibrous structures as collagen does. However, they are still capable of hydrogen bonding to each other, albeit more weakly, giving jelly its characteristic soft texture.
Contributors: Harry Morgan (writing and photos), Natalie Fey (final edit).