“Some people are worth melting for.”

As anyone who knows me will tell you, I like food and I like cooking. I especially like being creative with flavours, textures, and presentation, particularly making things look something else / not food / similar. (The time I made Olaf from Frozen in olive oil ice cream with a carrot sorbet centre springs to mind here. It (he?) tasted good – but it will be a while before I try coating sorbet in ice cream again!)

One thing I am rather fond of is jelly, and as you might imagine, I like to make it from scratch. There are two main gelling agents you can get hold of very easily: gelatine (which comes as ‘leaves’ or powder), and agar (or agar-agar, which comes as flakes or powder; vege-gel is very similar, but made with carrageenan, which is ever so slightly different chemically). These are not exactly the same, and do not give exactly the same result, but essentially they both dissolve into a liquid which, on cooling, will then set to a wobbly(ish) solid with no discernible change in taste or smell.

Gelatine is a hydrated form of a protein called collagen, which is found in a lot of connective tissues in the body. Collagen is extracted from animals (usually bones and skin, where it is found in higher quantities), and aside from being used to make gelatine, is also in a lot of beauty products (particularly the plumping and de-wrinkling ones). The protein chains in gelatine are long, and sort of like bits of string. To start with, they exist as a triple helix, with three strands all twisted round each other. (DNA is a double helix – this is a similar shape, but with three strands instead of two.) When they are hot, both they and the water molecules they are in solution with have plenty of energy and move around quite a lot. The strands become all mixed up and not in any particular pattern. As the solution cools, the molecules lose energy and move less and less. Eventually, they are moving sufficiently little that inter-molecular bonds (bonds – or forces – between molecules; these are transient, and much weaker than the intra-molecular bonds between atoms within a molecule) can reform between the strands, and between the strands of protein and the water molecules. The cooler the solution becomes, the less the molecules can move, and the more rigid the structure becomes. This also leads to pockets forming which trap the liquid of the solution (including any sugar, colour, and flavouring added), forming the jelly. The inter-molecular bonds are not very strong, and it doesn’t take much energy to break them – human body temperature is enough! Once they are broken, the jelly returns to its liquid state (until you cool it again).

gelatine collagen gellification jelly protein

The changes in gelatine as you heat and then cool it to give a jelly.

Agar, on the other hand, is a polysaccharide (a long chain of sugars all joined together, from Greek poly- meaning many, and Latin saccharon meaning ‘sugar’) called agarose, which is extracted from seaweed. This makes it an ideal choice for vegetarians and vegans, who can’t have gelatine. Agar gives a much firmer, almost brittle jelly, which can be heated to about 85° before it begins to liquefy again. Although this means it doesn’t have the ‘melt in the mouth’ texture of a gelatine-set jelly, it does open up a whole world of hot jellies. This ability to be heated comes from the high number of hydrogen bonds (as inter-molecular bonds go, hydrogen bonds are particularly strong), which also contribute to the jelly having a much more rigid structure. As with gelatine, on heating, agar takes the form of lots of strands, however when you cool it, it behaves very differently. As the strands cool, the hydrogen bonds force the strands into pairs forming a double helix. These in turn form hydrogen bonds between them to create clumps of helices, which in turn form larger groups. The whole ensemble is held together with many hydrogen bonds at every stage, leading to a much more rigid structure. Because there are more inter-molecular bonds, and more order to the arrangement of molecules, jellies set with agar are much less elastic than jellies set with gelatine – they have less of a wobble to them.

agar polymer jelly gellification polysaccharide gel

The changes which happen in agar as you heat and cool it.

Another factor to consider is entropy. Entropy is a measure of the amount of ‘disorder’ in or of something. Things which have lots of disorder are entropically favoured. A very simple analogy is the state of your bedroom: without effort being put in [by you tidying it], your bedroom will gravitate towards a naturally chaotic and disordered state. However, entropy isn’t the only factor to consider. We also need to think about energy, which is determined by the bonds present. Things can be entropically favoured but energetically disfavoured, and vice versa – it’s the balance between these two which determines quite a lot in chemistry (most often whether a reaction will proceed or not, and at what temperatures). In the case of gelatine, there are sufficiently few bonds holding the molecules together that it doesn’t take much for entropy to dominate, and the jelly to become a liquid. Because of the much higher number of hydrogen bonds in the agar jelly, until you get to much higher temperatures, the energetics dominate, and the jelly stays set. To go back to the bedroom analogy, your parent/s nagging you can be like the hydrogen bonds in the jelly – if they nag you a lot about tidying your room, your room is likely tidier because it is easier for you to tidy it a bit than put up with being nagged about it! [N.b. This is a very much simplified discussion of this really important concept in chemistry – I’ll talk more on it later.]

hydrogen bonding water molecule lone pair electrons

(A) The bonding (covalent) within one molecule of water; there is a shared pair of electrons made up of one electron from hydrogen and one from oxygen between each hydrogen atom and the oxygen atom. (B) The hydrogen bond is the attraction between the slightly negative lone pair of electrons on the oxygen atom and the slightly positive hydrogen atom. (C) Across a number of molecules, the hydrogen bonds form a network. This is why water expands when it freezes – the hydrogen bonds all ‘lock’ into place, rather than being able to fluctuate like in liquid water.

polysaccharide polymer galactose agar jelly hydrogen bonding

Agarose, the polysaccharide polymer which gives agar jelly its structure, is made up of repeat units like this. The high number of -OH groups is what gives rise to the high level of hydrogen bonding.

You can have lots of fun investigating this at home, and seeing what you can set with different agents. Agar can be purchased online very easily, and gelatine is readily available in most supermarkets. Start by following the packet instructions (or mine, below), and then see what happens if you increase the amount of gelling agent, or the amount of liquid.

I started by oiling some moulds. I am using silicone ice cube trays, one in a star shape, and one in a cat shape.

One leaf of gelatine on the scales and ready to go. I didn’t have to weigh it, but I was curious!

My gelatine jelly (red) was made with one leaf of gelatine (the instructions said to use 5 leaves to set 570ml/1 pint of liquid), which I softened in cold water for about 5 minutes. In a small saucepan, I put 100g of cold water (which is the same as 100ml of water), 25g of granulated sugar, a couple of drops of orange flavouring, and some red food colouring. I squeezed the water out of the gelatine and added it to the saucepan, then heated it gently stirring with a small whisk until the gelatine was all melted and well mixed in. I poured it into the moulds, where it took about 3.5 hours to get a light set at room temperature; to make sure it set properly, I transferred it to the fridge overnight just in case. (I have never managed to be that successful with gelatine in the past – but there is a first time for everything!)

0.5g of agar powder, weighed out and ready to go.

My agar jelly (blue) was made with 0.5g of agar powder (the tub says to use between 2g and 10g to set 1kg of liquid; 0.5g of agar to 125g of liquid is on the low end of this range at 4g/kg). As with the gelatine jelly, I put 100g of cold water, 25g of granulated sugar, a couple of drops of orange flavouring, and some blue food colouring in a small saucepan, together with the 0.5g of agar (I have special super-accurate scales for this kind of cooking – they don’t measure large masses, but they do measure small ones very accurately). It helps to let it stand for about 10 minutes, before heating it gently with whisking until the agar is all dissolved. You then need to bring it to the boil and boil it for a couple of minutes, before transferring it into your moulds. It sets very quickly (this one took about half an hour at room temperature – but it takes minutes to set in the fridge) so work swiftly but carefully.

gelatine agar polysaccharide polymer protein gellification hydrogen bonding structure

The gelatine jelly (red) and agar jelly (blue) in the moulds and starting to set.

When I turned them out, the agar jelly definitely held its shape better, although it was softer than jellies I have made with agar in the past, and had a better wobble to it. The gelatine stars, well, I think the picture speaks for them. Interestingly, when I turned out the cat jellies, one of the agar ones tore in half, because the jelly is so much more brittle.

agar gelatine jelly protein polysaccharide polymer

The finished jellies. As you can see, the gelatine stars didn’t turn out of the mould that well. Then again, one of the agar cats was sufficiently brittle/inelastic that it broke in half when I tried to get it out.

For reasons which are probably best not asked about, early next year I will be making a number of edible jelly insects. It’s important to have a jelly with a firm enough set that the insects hold their shape (including legs!), and that they can take a fairly high level of detail from the mould. However, I also want them to taste good, including having a nice texture in the mouth. Investigate and report back on which agent you think I should use, and in what proportions!


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