Gel

Did you ever wonder about how a wobbly pudding is wobbly, almost liquid, yet stays on your plate? When you shake it, wonderful waves go across its surface, as if it were a pool. But stop and it is a solid again. This wonderful crossing between a liquid and solid is called a gel. In this episode I hope to make you curious about what they are and how they are made.

In the previous post I discussed some (culinary) properties of a thermodynamic two-phase system, the foam. A general name for these systems is a colloid. A colloidal system consists of a continuous phase (the 'matrix') and a dispersed phase (the 'grains' or 'bubbles'). In a foam the continuous phase is a liquid or solid and the dispersed phase is a gas. One can also have a solid continuous phase and a liquid discontinuous phase. This is called a gel, which hence is a liquid trapped inside a solid matrix.

In the kitchen gels can be used in many places. Usually gels are found as a pudding or aspic. It has to be noted that gelation is different from simple thickening. Thickening agents are not gelling agents per se. Dissolving starch in water will render the water thicker but no gel is formed yet. When the water-starch mix is heated amylose and amylopectin will bond to each other and themselves, trapping water in the process and it will gelate the mixture. When the gel cools water is trapped inside pockets.

There have been some experiments with gelation of soy proteins with (high intensity) ultrasound. So it seems that gelation can occur in several different circumstances. Each very different but still changing the properties of the polymer network of proteins, which is the key.

A few words about destabilisation of gels. Gelatin for example is thermoreversible and once it has been formed must be kept at a low temperature to be stable. Coagulation of the matrix materials will break the bonds formed and the liquid will escape. There are other gels in which the reverse happens. They gelate at high temperatures and destabilise when cooled. Methyl cellulose gel has these properties. It is used  as "inverted hot ice cream". Served warm it is firm and when it cools starts to melt on your plate. I'll definitely have to play with that sometime.

This episode will be a bit more experimental as I will attempt to do some kitchen chemistry making gels. As every gel has it's own properties I won't go into too much detail for each of them but rather try to classify what they can or can't be used for.

We basically use three types of liquids for consumption: water, alcohol and oil. (And anything dissolved in these.) These liquids can be "gelated" with a suitable gelling agent. Not all the gelling agents are proper because the solid phase might dissolve too much in the liquid, rendering us a liquid with some solids in it. In other cases the solid phase will never make it to the continuous phase because the polymer network never forms.

Like foams, gels can be made from polymerisation of either proteins or fatty acids. However proteins usually produce hydrophilic polymers so might not be too useful for gels made of oil (oleogels). In that case one needs a matrix of lipids. Fatty acid esters or fatty acid salts can be used for the gelling agent to provide the solid phase. These fatty acids are usually something like stearate or palmitate.

Here is a list of some hydrogelling agents:
Gelatin/Collagen (long chain protein, gelates with heat, made of collagen from bone tissues)
Lecithin (mixture of fatty acids, glycerides, lipids; It gelates with heat, can be found in egg yolks* and in soybean oil. It also gelates by freezing. Salt and sugar inhibit this gelation when frozen.)
Pectin (ester, splittable in its acid pectate (polygalacturonic acid) (a polyanionic polysacharide) and methanol. Pectin gelates with acid and heat. It is found in plant cells. Its acid also gelates with calcium cations.)
Starch (polysacharide, gelates with heat)
Agar (polysacharide, gelates with heat)
Alginate (anionic polysacharide acid, gelates in contact with calcium cations (or other divalent cations), it is used in so-called molecular gastronomy) I will not talk about it here, unfortunately i couldn't find it anywhere. As soon as i do i will experiment with it.

Oleogelling agents:
Stearate (fatty acid, found cocoa butter)
Palmitate (fatty acid, found coconut oil)

Enough theory, here are the results of some experiments I did.
Hydrogels:

Cooked tapioca starch balls or boba for bubble tea

1.1) Water with starch
Well, the amylose and amylopectin did turn into some kind of a gel. The starch mixture became a bit clearer. But the gel still tastes really "starchy" :).  In addition the polymer network made by the amylopectin is not very strong. Nevertheless it is a gel. Only to be used in stronger tasting liquids, like milk or a broth. Adding acid prevents the polymers from forming. Add acid only after the starch is set. Some measures can be taken to increase the lifetime and rigidity of starch gels. One can also buy starch balls, usually tapioca starch for making so-called bubble tea. The have to be boiled for some time in water to form gel balls. They also come in different colours. Very nice to play with in your food!

Orange juice gelatin

1.2) Water with gelatin
The collagen from the gelatin dissolves in warm water, but we need to take heed not heat up the fluid further as this might damage the collagen molecules. When the solution is left to cool, the collagen forms a network and you get a nice rigid gel. Particular enzymes can break down the polymer network. These are found in pineapple, mango, etc. You have to blanch them before putting them into your gel, otherwise it won't set! An advantage of gelatin is it renders a clear gel opposed to most of the listed gelants, which are still cloudy to a large extent.

Balsamic vinegar with pectin

1.3) Water with pectin and acid
Used in making jams. Pectin is naturally found in fruits, but in lower quantities. The pectin gelates in the presence of acids. In reality few fruits can be gelated naturally by this process. In making jam extra pectin is added.
Pectin de-esterifies in the presence of the enzyme pectinesterase turning it into pectate and methanol. The pectate is a gelant especially in its calcium salt, calcium pectate. (Kiwi's contain pectinesterase inhibitors, to prevent the pectate from forming.) Here we used pectin in vinegar to form a gel. Also here it's best to warm it "au bain-marie".









1.4) Water with egg white
Cooking egg whites it turns into a smooth white gel. Heat leads to denaturing and polymerisation of the proteins in the egg white. This can also be achieved with alcohol. Put a bit of alcohol with the white  to break the hydrogen bonds. Adding acids will further block any ionic residues, and denature the egg even further.

Egg yolks with whisky (heated)

1.5) Water with lecithin (egg yolks)
Egg yolks gelate by heat and by freezing. An obvious gel is a custard which is made with egg yolks and milk. More pure egg yolk gels possess less hardness at lower pH. (So it's not a good idea to use with acids, or add after the gel is formed.) Yolks also gelate with the addition of alcohol, a fact known from making advocaat. It works best adding heating and beating the yolks first (to denature the proteins) and after that adding the alcohol. In my example I added alcohol too fast and the proteins curdled yielding small domains of gel. It tasted pretty good though :) - So for next time I'll have to add alcohol very slowly to heated yolks. Apparently the structure improves over time. You can keep it several months, due to the alcohol content.

1.6) Whole egg gels. Apparently whole eggs are even better at making gels. The proteins from the yolk and the albumin interact with each other to provide a good polymer network. This makes one wonder why it doesn't apply to foaming of egg whites with some yolk in them.

Oleogels:
For example oil with stearate or with palmitate.
These are kind of tricky I must admit. Since who would like to eat a gel made of mainly oil ;) These are mainly used in cosmetics, skin creams and similar products. There exist also an intemediate form: hydro-oleogel. An emulsifier as monoalcylglycerol is used to bind the water to the lipids. In principle it could be used to make a gel from an oil water mixture. Instead of a mayonaise we could make a "mayogel". In any case it wouldn't be very healthy. To be tried...



There could be a problem with gelling alcohols: During heating of the gelating agent the alcohol evaporates. Secondly the alcohol can dissolve too much of the solid phase to be able to make a gel. Furthermore the stability of hydrogels depends also on enzymes or acids destabilising the solid matrix containing the liquid cells. It would be nice that alcohol used in the gelation proces is also absorbed in the pockets. For advocaat this is surely the case, but for other cases i'm not so sure.

Many gels (or colloids) can have interesting properties of their own. Some of them exhibit non-newtonian behaviour. When a shear force is applied they can either lower viscosity (pseudoplastic) or display an increase in viscosity (dilatant). These are difficult to predict mind you. An example of a pseudoplastic "gel" is ketchup, you try to shake it and then it flows freely all of a sudden. (Obviously over your clothes.)

Some more culinary gels:
Custard: Lecithin from the egg yolk and milk combine to make a gel.
Zabaglione: Instead of advocaat, use egg yolk and marsala to make a gel! (You can also foam it up, rendering a three phase system! A "foamified" gel?)
Adding horseradish to ketchup gelates it even further. It could be that the horse radish reacts with the pectin from the tomatoes to form a rigid gel. (Although horse radish is not very acidic, so this is to be investigated.)

Honey gel by pectin

Honey gel by agar-agar. 

A honey gel, one by adding pectin and one by adding agar. In principle this is just a hydrogel, but with very little water in it. So it has to be seen how much of the network actually forms. One can add extra water of course. Pectin didn't make the network stiff enough, so we have a slush. The gelatin didn't completely dissolve in the honey, it probably needs more water, so no picture. The only one that worked properly was agar-agar. Interesting texture, a bit grainy, and honey flavoured! Not bad though. One does have to take care the temperature isn't too high. Here it's also recommended to use au bain-marie.

In any case let your imagination run wild with this information, and go and search for new food gels! The certainly add very interesting textures to our dishes, like the following: Balsamic vinegar gel and strawberries. I can really recommend it.

The balsamic vinegar gel layered with a strawberry, one could add some mint sauce to top it off

Interested readers have probably already found that we can have 9 different combinations of Solid/Liquid/Gas (S/L/G) in a two phase system with one continuous and one dispersed phase. You'll be wondering about the other combinations! We described G in S, G in L, and L in S. Remaining are G in G, L in L, S in S, S in L, L in G and S in G. Let's leave the G in G and S in S out (as all gasses mix, and for a solid in a solid I don't know any interesting example). An aerosol is a solid in a gaseous continuous phase. Most commonly an fine spray is pictured, in which the solid particles are so tiny they do not float down. Of course for the kitchen the particles need not float, and any grainy substance interspersed by air is in principle an aerosol. We will encounter some interesting (and edible) ones in a future episode. The remaining ones are a solid or liquid dispersed phase in a liquid continuous phase, or an (aqua)sol and emulsion respectively. An aquasol can be found as tiny gold flakes in alcohol for example ("Goldschläger"). Emulsions will be the topic of a next entry.


* Lecithin was originally extracted from egg yolks (λέκιθος : egg yolk in greek - "lekithos")