I wanted to preserve my juices longer. I am using a mango juice powder that includes the ingredients Sugar, Mango Juice, Malic Acid, Natural Flavor. I am dissolving about the powder into 8 oz of purified water. How much citric acid should I use to make it last longer and about how long will it last me? I know this won't be exact but I am just trying to get an idea of how it will work. Thank you so much in advance and please let me know if you have any questions.
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Short answer: Enough citric acid to get the pH below 4.2, then pasteurize. One mM citric acid solution has a pH of 3.2, but it's essentially tribasic with pKa of 5.21, 4.28 and 2.92, so figuring it out on paper will take some work. It might be easier to just add some acid and see what the pH is, repeat as necessary.
References: Tetra Pak White Paper on Pasteurization, Citric Acid wikipedia
How much additional storage time will you get? I cannot answer that, there are multiple other ways foods can spoil, storage conditions are not mentioned (temperature, storage vessel, etc.), and generally without lots of careful work one cannot achieve at home the storage longevity of commercially-produced items.
