Best Preservative for Gums?

Hi! Which do you think it's the best preservative for gums, specially Guar Gum? I'm mixing some dyes to it, and the viscosity is getting lower 3/6 months laters it's production. 

I think if there's some option I can add to it to make the gum viscosity lasts for at leats 2 years. Any help?
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