I make organic and all Natural Vinaigrette's, lately they're fermenting and fizzing. Whats the best preservative to use? Guar gum, potassium sorbate? Don't want added sodium from sodium benzoate.
The best preservative for salad dressing is high acid content; below pH 4.5, most pathogens will not reproduce. However, most salad dressings are above that, and yours certainly is due to the large quantity of water you add.
The next method used is heat; this is how commercial salad dressings are made shelf-stable. I have a bottle of Ken's Caesar in my pantry right now that containts zero preservatives, and is shelf-stable for months or years, through heat treatment. For that to be effective, you must follow safe canning practices and heat filled bottles to at least the temperatures and times listed in the linked article. In fact, you may need higher temperatures and longer times, or even pressure canning, due to the very low acidity of your dressing.
There are chemical preservatives that you can add, but none of them protect your customers from all pathogens. For example, sodium benzoate is often used to prevent yeast and mold, but does nothing to inhibit botulism or salmonella.
You like your microbes because you are making vinagrette (I think). But lately they are too frisky with the bubbling and fizzing and you want them to quit it.
You can kill them with heat. You can can them. Or you may can them. Once they are at the point you like as regards fixx and flavor open the bottles (no heating closed bottles, ever!) and have them sit in the hot water bath just as you would for canning. Canning works by killing microbes. The same procedure will kill the microbes responsible for the fizzing you don't want.
Canning can change flavors but probably not as much as adding enough salt or other substance to chemically suppress the microbes. Once your vinaigrette product is sterilized it should be fizz free.