To prevent open pine nuts from turning black. Oxalic acid is an acidic substance that can neutralize the tannic acid in open pine nuts, thereby reducing the black matter produced by the decomposition of tannic acid and maintaining the original color of the pine nuts. It can also make the pine nuts crisper and taste better. Open pine nuts contain a substance called tannic acid. When the pine nuts are fried at high temperature and pressure, the tannic acid is easily decomposed by heat to produce a black substance, which causes the pine nuts to turn black and affects their appearance and taste.