Bouncy food balls and crunchy crystal prawns could have a common secret.
Sin Kong coffee shop's fish ball noodles.
It all started with tough pork. One evening, I bought a couple of packs of diced pork from a supermarket on the way home from work – normally, I like to cook something for dinner. Regrettably, the meat was really awful – sinewy, tasteless and not unlike boiled tough cardboard in texture. I simply had to throw most of it away.
For the next evening, I sprinkled about a teaspoon of baking soda on top of the second pack and worked that into the meat. Then I left it alone for about an hour and a half before cooking as I already knew how tough the meat was. Using the same recipe as the night before, this time the meat was soft and tender and rather delicious.
So the old trick of applying an alkali to loosen the amino acids in the meat proteins worked again – and I was relieved to be able to enjoy a good dinner.
An unlikely email
Then purely by chance, my editor passed me an email from an Australian chef and culinary lecturer who asked about the use of borax (known in Chinese as “pang sah” or peng sha) in the preparation of those translucent, crunchy “crystal” prawns beloved by Asian gourmets.
I hadn’t known this and originally it sounded unlikely that people would apply a chemical used for laundry detergents, fungicides, herbicides and insecticides on prawns – but I was wrong.
It seems that the West had been using it as a food preservative for almost a century and some Chinese restaurants have indeed been using borax to make crystal prawns for decades.
This practice has now been banned in many countries but apparently still continues unabated in some of the shadier restaurants.
What is also interesting is that borax can also be used to treat rice- and wheat-based products such as “cheong fun” (rice sheet rolls) “kway teow” (flat rice noodles) and “la mian” (hand-pulled wheat noodles).
At this point, it should be noted that borax is listed as “not acutely toxic” by several chemistry databases and it is also highly unlikely that any restaurant would leave significant amounts of the compound as residue in the food that they serve.
The symptoms of borax poisoning are abdominal pain, convulsions, diarrhoea, headaches, nausea, vomiting, skin rashes and renal failure.
So if you have experienced any of those symptoms after a dinner out, then I would suggest that you don’t go back again to the same restaurant, especially if you have had to replace a kidney or two.
By the way, it seems that Indonesia’s Directorate of Consumer Protection also warns of some risk of liver cancer if high amounts of borax are ingested for five years or more. So just being rated as “not acutely toxic” doesn’t mean the stuff isn’t potentially dangerous.
Now that the bad news is out of the way, let’s explore why borax is so effective at making prawn proteins and grain starches so distinctively rubbery and interesting in texture.
Borax is an important boron compound and is known by various chemical names – a few of the common ones are disodium tetraborate decahydrate, sodium tetraborate, disodium tetraborate or simply sodium borate. The chemical representation is Na2[B4O5(OH)4]·8H2O and it is a naturally-occurring inorganic compound, originally mined in Tibet centuries ago, but since then, huge deposits have been found in California, Nevada and Turkey – and now most of the world’s borax comes from those regions.
Har kau (prawn dumplings) from Ee Chinese Cuisine, Eastin Hotel Petaling Jaya.
A Little Dim Sum Place restaurant at Desa Sri Hartamas, KL.
Funky borax
Borax is funky as it is one of the ingredients which can be used to make silly putty – one just has to combine white PVA glue with a borax solution in the right proportions and the result is a crude form of silly putty.
How it works is that PVA glue is normally made up of long chains (or polymers) of polyvinyl acetate molecules which slide easily against each other in a liquid solution.
The borax solution causes these long chains to cross-link to each other with molecules of borax acting as the dabs of glue between individual chains of polyvinyl acetate.
The actual cross-link bonding is done via the hydrogen atoms joined to the boron atom in the middle of the borax molecule (see diagram).
Apart from turning the glue from a liquid into a non-liquid, this cross-linking of PVA chains introduces a springy but malleable texture to the mix of the compounds – and hence we get a form of silly putty.
This isn’t really the silly putty you buy in shops as it is water-based and will dry out eventually. The same chemical process to induce hydrogen-based cross-linking for silly putty can also be used to make synthetic play slime by varying the proportions of PVA glue and borax solution – you can also add some colour dyes to make it more fun.
I don’t need to provide the chemical recipes here because they are easy to find in lots of chemistry-for-fun books.
Why lava flows so slowly
As a little curious diversion, the cross-linking of polymers is one reason why lava flows relatively slowly down from an erupting volcano. One would assume that lava flows slowly because it is dense molten rock – but molten metals are actually denser and flow a lot faster than lava.
So one explanation why lava flows are slower than expected relative to their density is because lava has a lot of silicon-based polymers – and these polymers cross-link due to the presence of gases like hydrogen sulphide and hydrogen disulphide mixed in with it.
The hydrogen atoms around the sulphur in the lava gases bond with the molten silicon-based polymers creating lots of silicon polymer cross-linkages – and this increases the viscosity of the lava substantially, slowing its speed down the mountainside.
I hadn’t realised this before so it’s quite cute what one can learn from researching the impact of borax on prawns.
A bakso stall in Jogjakarta.
Freshly rolled fish balls.
Borax and prawns
Anyway, from the silly putty example above, one can start to deduce why borax causes the proteins in prawns to become like elastomers – this means that the prawns become more elastic and will attempt to recover in size and shape after the application of pressure, unless of course, one is chewing and breaking them down into little pieces while eating them.
If you have read some of the earlier articles, you would know that proteins are natural polymers made up of monomers (little strands) of amino acids. Fundamentally, proteins are polypeptides or complex chains of amino acids bonded by peptide bonds (simple amino acid monomers).
As such, many proteins are therefore not organised as straightforward linear chains – in other words, proteins do not tend to line up like strands of dried spaghetti but actually have shapes, which may be fan-like or circular or some other amorphous forms.
Borax acts on these polymers just as it acts on other polymers and will bind polymers together forming cross-linkages which change the texture and structure of the proteins.
With PVA glue, the polymers are mostly simple chains and the effect of borax was to turn the PVA polymers into a non-liquid and introduce a malleable texture to the compound mix.
With proteins, the polymers are complex chains in many configurations and therefore the cross-linking effect of borax is to turn the proteins into denser, springy clusters, changing them into a form of elastomer.
Basically, the process makes prawn proteins develop that particularly interesting, crunchy, elastic texture so prized by Chinese food gourmets.
Of course, borax will have the same effect on other proteins and hence it can also be used in meat balls and fish paste balls to impart a springy texture.
For the same reason, borax was used in the past to restore rotting meat and seafood to something like its original texture – but thankfully this practice has now been generally banned.
Borax is great on stuff where the protein density is not too high – hence it works best on prawns and fish which have lots of water, but it works less effectively on fresh animal flesh as it may render the meat rather tough (and who likes a chewy, rubbery pork chop?). It works well also with minced meats mixed with some other filler such as starches.
Next: Borax’s other unlikely food friends, sodium hydroxide and lye water.
https://www.thestar.com.my/lifestyle/food/features/2015/01/11/curious-cook-boraxs-bounce-factor