Citric acid, lemon juice, pineapple juice, grapefruit juice and white vinegar will act as preservatives.

Citric acid.

None, freeze the portions and microwave defrost them as needed. It also stops your fridge from being overtaken by dog food, and it keeps your fridge free from cross contamination. If you are talking about homemade dog biscuits, then citric acid is best.

Something acidic or freeze the food.

Freeze the food a thaw before feeding

please do not use the citric acids, ingesting on a daily basis will alter your dogs urinary PH making it hihgly acidic which them can lead to bladder stone formation a form that woud require surgery to remove.

If you do add it is take necessary to purchase ph strips and check the urine to ensure it is staying in the normal range inorder to adjust the amount to know how much is safe to use

If you are preparing home cooked meal also be aware dogs need higher levels of calcium and cooking/heating destroys vitamins so you need to provide supplements

This article explains what and how much is needed
http://www.canine-epilepsy-guardian-angels.com/supplements.html

Natural preservatives and antioxidants – Vitamin C (ascorbic acid), Vitamin E (tocopherols), and mixed tocopherols.
http://www.canine-epilepsy-guardian-angels.com/Pet_Food_API.htm

Adding citric acid and glycine to foods could lessen the formation of acrylamide — a carcinogen produced by baking, roasting, frying or toasting starchy foods….
http://www.newstarget.com/019650.html

…essential oils sage and rosemary could slow oxidative spoilage of meat better than synthetic antioxidants butylhydroxyanisole (BHA) and butylhydroxytoluene (BHT)….
http://www.newstarget.com/020721.html

And always keep your pet food refrigerated or frozen too.

Check out this website. They have a list of suggested books for making homemade dog food listed on the right side of the front page under “Food and Health.”

http://www.howl911.com/