Final answer:
Yeast should not come into direct contact with flavored products containing sugar not intended for fermentation, unsanitized surfaces or equipment, or raw dairy products due to the risk of infection or contamination.
Step-by-step explanation:
Yeast should never come into direct contact with substances that can inhibit its growth or lead to contamination. For example, it is important to avoid direct contact between yeast and products with high sugar content not meant for fermentation, such as the sugar found in flavored condoms, which can encourage yeast infections if they come into contact with the yeast naturally present in the human body. Additionally, unsanitized hands, skin, lab surfaces, or any unsterile equipment can introduce unwanted microorganisms to cultures of yeast, disrupting the fermentation process or leading to contamination. In the case of food production, specifically dairy, pasteurization is key because raw or unpasteurized dairy products can contain deadly microorganisms that should not come in contact with yeast—or be consumed by humans.