What it actually is
Most hydrocolloids and starches are long chains whose behaviour depends on charge. Lower the pH and that charge changes, so chains that were repelling each other stop doing so. Worse, acid catalyses hydrolysis of the chain itself, and heat accelerates it. The material is not simply performing less well; it is being taken apart.
Why it matters
A sauce that reached the right viscosity in the vessel and came out of the retort thin has usually not been underdosed. It has been hydrolysed. Because the loss happens during the heat step rather than during mixing, it is invisible until the product is finished.
What changes it
pH, temperature, and above all the time spent at temperature while acidic. The material chosen decides how much of that it can take: xanthan survives conditions that destroy guar, and a cross linked starch survives conditions that destroy a native one.
