Hydroxypropyl Methylcellulose: The Properties (2)
Interesting Properties: Unique, reversible thermalgelation
"This is a really weird one and counter-intuitive to everything most people understand about gums and gelling agents".
"The bit that is unique is that if you cool it down, it will return to liquid again".
The colder a solution of HPMC gets, the thicker it gets. Quite normal, except perhaps that these gums don’t require heating to dissolve them, which is unusual.
However, once they are dissolved, if you heat a solution, first it thins down a bit (normal for all thickening systems), then as the temperature continues upwards, it solidifies. A little like an egg white, turning from clear and thick to white and solid. This is not unique. Egg white does it! We call it thermalgelation – gels on heating.
The bit that is unique is that if you cool it down, it will return to liquid again. The transition is reversible with temperature. Reversible thermalgelation. Now that is unique.
If you heat it again, it will solidify again. Then return to liquid on cooling…
And on, and on. As many times as you care to do it.
The HPMC solutions’ response to heating is a physical one; a phase change not a chemical one. Phase changes tend to be reversible; chemical changes are permanent. So, a boiled egg stays ‘boiled’ or gelled, because its chemistry is changed by heating. HPMC gels when heated but returns to liquid on cooling because the change is physical not chemical. Think ice, water, steam, water, ice….
As mentioned elsewhere, methylcellulose gives the strongest hot gels (thermalgels) which makes it incredibly useful for making vegan burgers. HPMC generally has a much softer hot gel – useful for stabilising the texture of gluten free bread during baking, stopping potato croquettes from exploding in the fat fryer and stopping pie fillings from baking out. The strength of the thermalgel, whether it is sloppy, like an egg white just changing, or firm enough to cut, depends on what HPMC you are working with, the concentration used, the presence and concentration of other ingredients and the temperature and time of cook.
There are basically, at the moment, three types of gelling characteristics available in HPMC. These are based on the substitution ratio (of HP to M on the backbone) and the total quantity of substitution. These differences affect:
the temperatures at which they dissolve, thermalgel and melt back,
the strength of the thermalgel
and the amount of syneresis (water leakage) you see when they thermalgel.
How are these variations useful?
Humour me whilst I use a few examples of foods to demonstrate this.
Imagine a sauce – for use in, say, a lasagne. It is the bolognaise sauce, with meat, tomato, a bit of starch, oil, seasoning… but it flows out of the lasagne, boiling over the sides of the container when it is cooked. You could thicken it with more starch but the texture would become heavy. Or you could use an HPMC to stop the boil over. As little as 0.3% in the sauce will stop it and make the final lasagne easier to measure out as portions.
If you use one that gels at 67C (F or E types) the gel will be moderately firm and completely stop the boil over. The F will return back to liquid below mouth temperature, which means that the consumer will eat a slightly gelled sauce. Not necessarily attractive but it could be mistaken for a tomato pulp type texture. The E type will turn back to liquid at about 48C, so the consumer will have either a liquid sauce or one that melts in the mouth. One downside of the E types is that they foam very well… so you have to be careful to avoid high shearing them.
Then, imagine a sauce used as a barbecue rib sauce, poured over ribs, then heated. The HPMC will be need to be sticky in the cold to stick to the meat… so a medium to high viscosity, but when it’s heated, the sauce needs to not gel rigidly (so that you can peel it off… not good). It needs to stay in place enough to not fall onto the barbecue coals. This needs a higher gel temperature, higher melt back temperature HPMC, like a K type. Sticky in the cold, then a light soft thermalgel to hold it in place but liquid again (and sticky) at 50C, so when eaten it’s a liquid.
Potato products – made from mashed potatoes, formed into shapes like waffles, croquettes, noisettes (little balls), alphabet shapes etc. are very likely to explode in the fat fryer. Difficult for the manufacturer, downright dangerous for the consumer.
How to make the mash formable to hold its shape in the cold and then not burst apart when deep fried? You could add lots and lots of potato flake or powder but the mash becomes dry, cracks and tastes pretty awful. Alternatively, less than 0.25% of an HPMC would do the job. A K type would give a soft creamy texture to the mash whilst holding it enough to stop it bursting. It would also allow the dry addition of the powder to the warm mash. The sharpness of the edges (definition) might not be brilliant and the hot texture might not be firm enough for some. An F type would give better definition, a firmer hot bite but a less creamy mouthfeel.
This is an area that I’ve worked with quite a bit… it’s not as simple as you might hope. Get in touch if you want to talk.