Many years ago we used a white powder in water to soak our freshly cut fries to prevent discoloration. I believe it was called "stay white" powder, can you help me find where to buy some?

The white dry powder you are referring to that allowed potatoes, apples and many other cut vegetables to not discolor was often referred to as “Stay-White” and contained sulfites. Many people were found to be allergic to this product, the FDA revoked the use in 1986. Acidulates, or chemicals that have acidity can be used to control browning. Citric acid works pretty well; lemon juice is an example. White wine vinegar is another choice, dilute in a solution of 1 gallon of water with 1 tablespoon of lemon juice or vinegar. You also need to reduce oxygen availability, immerse the cut potatoes in water as the potatoes will brown when exposed to air.