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Ascorbic acid would not be the best choice to lower pH in this instance. Attempting to lower pH to make the environment inhospitable to bacteria growth is the approach you are attempting. It is only 1 aspect of preservation. A better approach for your dog biscuit would be to lower the water activity. This can be drying out more, or if you want the treats to be soft, use a humectant like glycerin. Dogs can taste sweet, and most have a sweet tooth, but they do not have the enzymes in their saliva to ward of caries in their teeth, and this can go in a bad direction quickly. Also, dogs are very susceptible to diabetes. As much as they love sweet things, it is not good for them. Simple sugar like in carrots and tomatoes is apparently okay in moderation.
If you want to stick with trying ascorbic, the addition is measured in parts per million (ppm) for pure ascorbic acid. Most of the time it is sold diluted as either 5% or 10% ascorbic. Do not confuse this with citric acid – not the same thing at all. As I recall GMP limits for ascorbic are 90 ppm on the flour. (you can check the FDA CFR on this for human food – I am not sure about pet food) Mostly it is used in yeast dough to provide oxidation for fermentation. There is no legal limit, but it will taste very bitter if you go too high.
As an example; 90 ppm on 10KG flour = (10,000 / 1,000,000) * 90 = 0.9g of pure ascorbic – if it is 10% ascorbic, then 9g.
I hope this is helpful to you.