The Misconception of Unset Jelly

As an experimental cook, the conventional fruit which are set in jelly on top of a cake are not good enough. So, fuelled by the desire to find the most original combination of fruit that could adorn a cake creation whilst smothered in jelly, I managed to find a few unusual possibilities, until I met my nemesis: fruit, which no matter how long the jelly sat in the fridge, prevented it from setting.

This post looks at why, and the slight misconception which surrounds the idea that some fruit prevent jelly from setting.

An Introduction to Gelatine

What is gelatine?

Gelatine is a mild tasting protein that is derived from collagen in animal tissue, which in turn is a hard, insoluble, fibrous protein [1]. Collagen is the connective tissue protein that provides strength to muscles and tendons and resiliency to an animal’s skin and bones, meaning that in humans it makes up one third of the total protein content [1]. Furthermore, since it is a structural protein, collagen is found in many parts of an animal’s body as it helps to maintain the structure and shape.

Where does gelatine come from?

Most gelatine is manufactured from pig skin because around 30% of its weight is collagen [2]. Firstly, the pig skin is soaked in dilute hydrochloric acid for roughly 24 hours. This step leads to the unravelling of the crosslinking protein bonds in collagen, resulting in the free protein chains then being extracted. These are filtered, purified and dried into sheets or granules (powder) that contain around 90% gelatine, 8% water and the remaining 2% is salts and glucose [2].

How does gelatine work?

Gelatine has claimed the prize of being different to all other proteins typically used in a kitchen setting, partially because it is the only protein that has the power to thicken liquids. This is why gelatine thickened sauces are ‘crystal clear and syrupy’ rather than opaque and creamy like sauces which use starch or flour as the thickening agent [2]. Gelatine’s unique properties arise from that fact its response to heat is not one that is usually demonstrated by proteins. Normally, food proteins respond to heat by unravelling (meaning they lose their tertiary and potentially secondary structures), and then bonding to one another to coagulate into a firm, solid mass. This is demonstrated by an egg frying since the albumin (liquid protein of the white) firms up into a solid mass of egg white as it is heated. However, gelatine proteins do not readily form bonds with one another, meaning that although heat initially causes them to unravel and disperse like any other protein, the gelatine proteins never form new bonds. This results in the liquid that they are dispersed in remaining as a fluid. Furthermore, because gelatine proteins are also long and stringy, they tend to become interwoven and this leads to the hot liquid in which they are suspended to thicken, although not completely solidify when warm. As the gelatine gradually cools down, the protein strands line up next to each other and twist into long ‘ropes’ and turn the liquid into a firm gel [2].

Plot Twist from an Innocent Addition

Learning from my past experiences, the following fruit should not be added if you wish for your jelly to achieve its intended state:

  • Pineapple
  • Kiwi
  • Figs
  • Papaya
  • Pawpaw
  • Mango
  • Guava
  • Ginger root

There are, as always, some exceptions to this rule, however I will come onto this later.

An unexpected culprit

You may have noticed that some of the aforementioned fruit are quite acidic, such as kiwi, and for me, initially, this was why the jelly did not set. However, it was as I did more research that I realised this was not the case.

As far as I am aware, when we study enzymes at school, whether at GCSE or A Level, enzymes seem to take on the role of the keys to existence, the Gods of all things bright and biological, or words to that effect. Or an effect slightly less exaggerated.

And yes, I do not dare disagree with the importance of their function as biological catalysts, whether it be in the baby food industry, slimming food industry or even saving the world by being a part of biological washing powders which require a lower temperature.

However, enzymes are the reason behind your cake masterpiece having a ‘soggy top’ as a result of the jelly not setting, which would lead to a piercing ice-blue stare from Paul Hollywood himself, an ‘It’s a little informal’ comment from the Queen of baking, Mary Berry, and may even provoke Shakespeare’s ‘God has given you one face, and you make yourself another’ exclamation.

Why are enzymes the downfall of jelly?

The listed fruit contain enzymes, in particular, proteases. Pineapple, kiwi, papaya, pawpaw and mango all contain actinidin, papaya and pawpaw also contain papain, pineapple also has bromelain, figs contain ficain and ginger contains zingibain.

The reason that jelly sets is because the collagen proteins in the gelatine form a tangled mesh as a result of being interwoven, meaning that water molecules are trapped, as well as other components of the liquid, and this provides the gelatine its semisolid state when it cools [3].

gelatine-structure

Fig 1. The long gelatine molecules as seen in set jelly [4].

The proteases in the fruit act on gelatine protein, and this can be thought of as the proteases acting as scissors and ‘cutting up’ the long strands of gelatine protein into smaller pieces, so that they can no longer interweave and create a network to trap water and other liquid molecules, meaning the jelly does not set [4]. This is shown in figure 2 and figure 3.

enzymes-acting-on-gelatine

Fig 2. Scissors representing enzymes (proteases) acting on gelatine [4].

shorter-gelatine-molecules

Fig 3. Shorter gelatine molecules after protease action [4].

In addition, it is important to note that pineapple and kiwi contain far more proteases than the other listed fruit. The reason for this difference is unknown, however it may be linked to the idea of repelling pests. As a basic concept, animals and bacteria are made up of proteins meaning that essentially the high levels of proteases in the fruit will digest any of the pests trying to feed on the fruit [4].

Moreover, I should address the exception that I mentioned earlier in the post. The fruit has to be fresh in order for it to ‘rain unset jelly on your cake parade’. For example, canned pineapple will not ruin your showstopper. This is because during the canning process, the pineapple is heated to kill bacteria so that the pineapple can be in the can for a long period of time and not decay [4]. This process also denatures the enzymes which means that they no longer act on the gelatine protein and prevent the jelly form setting. The high heat causes the bonds in the protein of the enzyme to vibrate meaning the bonds break (hydrogen bonds break first). Since hydrogen bonds are an essential part of the tertiary structure of the protein (which create a fibrous chain or globular chain) and the secondary (which is responsible for the protein either being an alpha helix or beta pleated sheet), the two structures of the enzyme are lost. This results in the active site of the enzyme no longer being complementary to the substrate (the gelatine protein molecule), so that no enzyme-substrate complexes form, meaning the gelatine is not catabolised. As a result, the jelly sets in its normal fashion.

Overall, for me the misconception lies in the fact that the fruit which should not be added fresh are rather acidic, especially kiwi. This always seemed to mean that it is the acidic conditions that the prevent the jelly from setting rather than the presence of certain enzymes.

References

[1]   http://www.medicalnewstoday.com/articles/262881.php

[2] http://www.finecooking.com/item/63379/the-science-of-gelatin

[3] https://www.scientificamerican.com/article/bring-science-home-fruit-gelatin/

[4] http://www.thenakedscientists.com/HTML/experiments/exp/science-of-fruit-jellies/

Other websites used

http://chemistry.about.com/od/foodcookingchemistry/a/foods-that-ruin-jell-o.htm

https://en.wikipedia.org/wiki/Actinidain

 

 

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