Sorbic Acid.

Only the undissociated form of sorbic acid crosses microbial membranes, and its share rises sharply as pH falls toward and below its pKa near 4.8. Citric acid holds a liquid formula in that lower pH window, so the same amount of sorbate does far more work.

Glycerin binds free water and lowers water activity, which narrows the conditions microbes can grow in. A weak acid preservative then has less microbial load to hold back, long standing practice in syrups and liquid supplements.

Sorbic acid has a pKa near the low-4 range, so the fraction present as the active undissociated molecule rises as the surrounding pH falls. Ascorbic acid used as an acidulant or antioxidant lowers the pH of a liquid formula and therefore shifts more of the sorbate present into that active form. The relationship is physical chemistry, not a biological interaction.

Alkalising agents raise formula pH, which pushes sorbic acid toward its dissociated sorbate anion. The anion does not cross microbial membranes the way the undissociated acid does, so antimicrobial activity falls off as pH rises. Effervescent and buffered products are the usual place this shows up.

Sodium chloride lowers water activity, and lower water activity slows microbial growth on its own. In a salted or high-solids matrix a given level of sorbic acid holds spoilage organisms in check that it would not hold in a dilute one. The two act by unrelated routes, which is why they are combined.

A readily fermentable sugar raises the microbial load a preservative has to hold, and fermentative yeasts such as Zygosaccharomyces are the classic sorbate-tolerant spoilage organisms in high-sugar products. A sugar-rich matrix therefore raises the preservative demand rather than lowering it. The flag in the co-occurrence index points the same direction as the microbiology.