The changes include loss of vitamins and minerals, tough texture, and loss of flavor and color. The brief heating also reduces the number of number of microorganisms on the surface of the food and enhances the color of green vegetables. Blanching sufficiently wilts vegetables like spinach and chard so they pack better. Green peppers, onions, and herbs do not have to be blanched. Always choose plastic or glass containers, plastic bags or wraps that are labeled for use in the freezer.
Some foods can be safety processed in a water bath canner.
For others you will need a pressure canner. Tomatoes and figs can also be safely processed in a water bath canner if lemon juice, vinegar or other acidifying ingredient is added. Pressure canners can reach temperatures well above the boiling point of water. Only when foods reach these temperatures can you be sure that the Clostridium botulinum bacteria spores that cause botulism are destroyed. If the spores are still present after improper processing, they become active bacteria.