Final answer:
Jams and jellies are less prone to spoilage due to high sugar content creating high osmotic pressure, which dehydrates bacteria and inhibits their growth. The reduced availability of water, or low water activity, also helps to prevent spoilage, although some molds and yeasts can still grow in these conditions. Preservation techniques like these are crucial for maintaining food safety and freshness.
Step-by-step explanation:
Fruit preserves such as jams and jellies are relatively inhospitable to bacterial and fungal growth due to the high concentrations of sugar they contain. This sugar increases the osmotic pressure, which dehydrates any bacteria that were not eliminated in the cooking process. The removal of water as a result of the high osmotic pressure prevents bacteria from multiplying in what would otherwise be a nutrient-rich environment for microbial growth. Similarly, in preserved meats, a high concentration of salt is used to achieve an analogous effect by creating a high osmotic pressure environment unfriendly to bacterial growth.
Microorganisms need available water, or water activity (aw), to grow. Highly sugared or salted environments reduce the amount of available water dramatically, drawing water out of the microbial cells and inhibiting their growth. Some microorganisms, like certain molds and yeasts, are more tolerant of these conditions but the preservation effect is still substantial in preventing spoilage.
Overall, these high osmotic pressure conditions, achieved by adding preservatives such as sugar and salt, act to maintain or improve the safety and freshness of food, while also reducing the risk of foodborne illnesses.