The time of boiling depends on the degree of acidity, the consistency of the product, the method of preparation, and other factors. Vegetables, including some low-acid tomatoes, and meats, which are relatively low in acid, must be heated to temperatures higher than that of boiling water at atmospheric pressure. This method involves the use of a pressure canner. Because bacterial spores that may be present are more resistant to heat under conditions of low acidity, the time of heating necessary to destroy them at the temperature of boiling water would likely be several hours.