Corn starch is the unsung hero in Chinese cooking. It doesn’t add any flavour to the dish per se but it often makes or breaks the dish.
There are two main usages of corn starch in Chinese/Cantonese cooking. The obvious one is sauce/soup thickener. In most cases, you use corn starch when you got the right taste to a sauce but not the right consistency. Of course you could keep boiling it to evaporate more liquid so you get the right consistency. But the more traditional way is to mix equal parts of corn starch and cold/tap water (start with 1-2 TBSP and add more if needed) to make a slurry, then slowly add to the boiling sauce and keep stirring until you get the right consistency. Add a little bit at a time as it thickens up slowly while you are stirring. It’s a lot easier to add more slurry then to try to thin-out gluey sauce.
The less obvious use of corn starch in Chinese/Cantonese cooking is meat tenderizer. When you stir fry meat at high heat, in order to keep the meat tender and juicer, the secret is to add corn starch and water to the marinade before stir-frying. The corn starch, mixed with rest of the marinade, creates a light coating around each sliver of meat to help keep the juice inside the meat and also for better browning. It’s a must-have ingredient that could easily be over-looked and you ended up with a tough plate of meat and wondering what went wrong.