Are potassium sorbate and sodium benzoate heat stable in food products?
Both potassium sorbate and sodium benzoate are generally heat stable and can be used in various heat-processed foods. However, their effectiveness and stability can vary depending on other factors such as the acidity of the food, the presence of other ingredients, and the specific temperatures and conditions of processing.

Potassium Sorbate: It's stable at high temperatures, but its effectiveness as a preservative diminishes in conditions of high heat. Also, it is most effective in lower pH environments.

Sodium Benzoate: Sodium benzoate is also heat stable, but like potassium sorbate, its effectiveness can diminish under high-temperature conditions. It's most effective in foods with a lower pH, becoming more effective as pH decreases (it's typically used in foods with a pH of 4.5 or lower).

One consideration with heat stability and these additives is that while they can maintain their structure under heat, extended exposure to high temperatures (like in sterilization or certain types of cooking) may potentially decrease their effectiveness as preservatives.

In summary, while both potassium sorbate and sodium benzoate can withstand heating, their preservative qualities may be diminished under certain high-heat conditions and may be less effective in high pH environments. For specific applications, it's best to consult with a food scientist or an expert in food preservation.
Why are potassium sorbate and sodium benzoate only effective in acidic foods?
Potassium sorbate and sodium benzoate are more effective in acidic foods due to their pH-dependent antimicrobial activity. They are primarily active against yeast, mold, and some bacteria, and their efficacy is enhanced in an acidic environment. In acidic conditions, these preservatives are present in their active, undissociated forms, which can penetrate microbial cells and inhibit their growth.
When the pH of a food product is high (more alkaline), potassium sorbate and sodium benzoate are less effective because they are predominantly in their dissociated forms (sorbate and benzoate ions). These ions are less able to penetrate microbial cells and hinder their growth. 

Therefore, in foods with a higher pH, other preservatives or preservation methods may be necessary to ensure the safety and shelf life of the product.
It's worth noting that the specific pH range at which potassium sorbate and sodium benzoate are most effective can vary. Potassium sorbate is generally more effective in mildly acidic to neutral pH ranges (pH 4-6), while sodium benzoate is effective at lower pH levels (pH 2.5-4.5). The optimal pH range for each preservative depends on factors such as the target microorganisms and the specific food product being preserved.