All martian colonists will require food, either brought from Earth, or grown in greenhouses on Mars. To ensure a reliable supply of food, food production should exceed the minimum required by a substantial margin. The excess food can be stored for future need through food preservation techniques.
Freezing is the most cost effective preservation method, due to the ambient external temperature. Food could be stored in containers external to the colony, probably underground, where the temperature is constant during the year at -63 to -68°C over most of Mars' surface. In most situations, a simple shade would be enough to prevent heating of surfaced stored food even in summer.
Refrigeration requires the maintenance of food temperature within a narrow range of temperatures. This technique is more complicated than freezing, and requires energy input and control to maintain correct temperatures.
Dehydration requires a method of removing the moisture from food. It is usually done by heating using hot air. Smoking is a method of dehydration that also imparts flavors to the food from the combustion of wood chips. This would be a particluarily luxurious preservation method on Mars.
Freeze Drying combines freezing with dehydration. The low ambient pressure could be harnessed for this preservation method.
Special atmospheres such as nitrogen or noble gasses can retard the growth of aerobic bacteria.
Heat can be used to kill microorganisms.
Salt has been used for millenea to preserve meats. It dehydrates the food and inhibits the growth of microorganisms that could spoil the food or cause sickness.
Fermentation preserves foods such as cheeses, yogurt, and many other foods.
Weak acids, such as lactic acid, can be introduced artificially or naturally through fermentation.
Manufactured chemical preservatives, or artificial preservatives, are used in manufactured foods. They may require more effort to produce than a settlement is willing to spend, especially with the other low-tech options available.
Uses for Excess Foods
Excess food can be traded to other settlements.
Excess food or degraded food can be recycled as fertilizer, or fermented to generate alcohol. As food production creates a large amount of biomass, this biomass can go into soil preparation. For most food products, biomass in the form of inedible leaves, stalks and roots exceeds the edible food mass significantly. Many animals can use biomass as food, and produce food themselves at lower energy costs. However the efficiency of met production using low caloric biomass may be fairly inefficient.
Excess food can be fed to animals such as fish, poultry and generally any meat/protein/carbohydrate producing animal/insect/bacteria.