This time they add methyl and hydroxypropyl groups to the multi-glucose backbone. The reaction occurs where there are OH (hydroxyl) groups. A hydroxypropyl group means three carbon atoms with hydrogen (H) and oxygen (O) attached. In the case of the hydroxypropyl group, the end of the chain contains an OH group (the hydroxy bit); this is important. The quantity of the possible reaction sites on the cellulose backbone that are used (substituted) varies with the suffix types, (remember E, F and K). There is a range of substitution levels permitted in food additive legislation and these suffixes identify different proportions of the HP to M within that range.
However, due to that OH on the end of the hydroxypropyl group, things start getting much more complicated when the manufacturers add hydroxypropyl groups to a methylcellulose. Methyl groups are ‘self-capping’. That means once you have one attached to the OH in the glucose, no others can join it. However, hydroxypropyl groups can form chains because they end in an OH. In HPMC these can either end in a hydroxypropyl group or in a methyl group (which will cap it). So, for a given % addition of hydroxypropyl in HPMC, you could have lots of single HP units, a mix of single, double, triple or even longer chains, with or without methyl groups on the end, and of course the added variable of 3 potential reaction sites on each glucose in the backbone.
How does this affect its performance? Honestly, I don’t think enough research has been done to investigate that, but, to me, this goes a long way towards explaining what has been called the idiosyncratic nature of HPMC. If you develop a product with one HPMC from a particular company, it will not be easy to replace it with another supposedly identical one from a different company or even from a different manufacturing site within the same company. They can be sufficiently different to cause a lot of product quality issues. This doesn’t mean you shouldn’t work with it for fear of not being able to change supplier, but it is something you should be aware of. Again, I’m not going to get much more chemical than this. If you want more details, contact me.
Celluloses & Science
Starting at the top, what is cellulose anyway?
Cellulose is the building block of living plants. It’s made from extremely long chains of the same simple sugar, glucose (you might know it as dextrose). If you chop up the chains to their individual links, you get glucose. Nothing else. It is however, extremely difficult to do that. Bacteria can do it, which is why herbivores host bacteria in their guts to break the cellulose down into the glucose all our bodies use as our energy source. When we humans eat cellulose in whatever form (lettuce, apple, celery… methylcellulose) we can’t digest it and it goes through our bodies as a non-digestible fibre. This actually helps with the smooth working of our guts.
The cellulose used to make methylcellulose and hydroxypropyl methylcellulose comes from living plants; trees to be precise. A regular, consistent, sustainable source; the trees that are cut down to make the cellulose pulp are grown specially and replaced.
Why mess about with it?
Unfortunately, for the food industry, cellulose is pretty boring – it doesn’t do much apart from absorb water like a sponge. It doesn’t dissolve, thicken, form gels or emulsify… it basically just sits there. In order to make it do something useful, we have to add a bit of chemistry to the mix.
Thee manufacturers of methylcellulose take the cellulose and add methyl groups to the multi-glucose backbone. Methyl group in chemical terms means a carbon atom with 3 hydrogen atoms attached. In the case of methylcellulose about 30-33% of the possible reaction sites are used.
Once the methyl groups are on board, the cellulose starts to do some really interesting and really useful stuff. Things that the plain cellulose does not do. I’m not going to get much more chemical than this. If you want more details, contact me.