Q: I have an older cookbook that calls for ascorbic acid to be added to sugar syrup when freezing peaches. The local grocer doesn't have it and never heard of it. How come it's not available anymore?

Ascorbic acid is used in recipes to reduce or prevent enzymatic browning of foods, particularly fruits.  It should stil be available in most grocery or health food stores.  If you can't find it in either of those two places, you can substitute the same amount of  Citric acid.  Citric acid works in a different way than Ascorbic acid, but has nearly the same effect on browning.  It may, however make the fruit slightly more sour, so you may need to experiment a bit with the sugar content.  Citric acid can be found in the canning section of your grocery store or at your pharmacist's.

Alternately, you could try using lemon or orange juice at about 2 tablespoons per 4 cups of syrup.  Either of these, however, will affect the final flavor somewhat.

Also, Ascorbic acid is the chemical name for Vitamin C, so you can buy plain Vitamin C tablets and crush them up.  Vitamin C tablets usually contain other ingredients such as cellulose or rose hips, among other things, so look for the purest form you can find.

If I was using Vitamin C in place of Ascorbic Acid in order to prevent browning, I would try soaking the fruit in a solution of about 1 tablespoon of crushed Vitamin C tablets per gallon of water, then draining them and packing them in the straight sugar syrup.  That way, the other ingredients added to the tablets are less likely to have a long term effect on the finished product.

Ascorbic acid may also contribute to the acidity of foods when it is used in canning.  Always use caution and follow canning directions exactly.  If a canning recipe calls for ascorbic acid and you can't find it, look for alternate reliable recipes for which you can find the ingredients.