FN9: Food Preservation

Freezing, drying, and pasteurisation are three common preservation methods that work through different mechanisms and have different effects on food quality. Freezing works by reducing temperature to -18°C, at which point microorganisms become dormant and enzymatic activity is drastically slowed. Water freezes, immobilising the food's contents. However, ice crystal formation damages cell walls — slow freezing creates large crystals that rupture cells, causing texture changes (e.g. soft strawberries, tough meat). Fast freezing (blast freezing) minimises this damage. Nutrients are largely preserved, though some water-soluble vitamins may be lost in drip loss on thawing. Drying removes moisture, reducing water activity below the level microorganisms need for growth (below 0.6 for most). Without available water, bacteria, moulds, and yeasts cannot grow. Drying concentrates flavours and nutrients per gram but significantly alters texture — dried foods are hard and require reconstitution. Some vitamin C is lost through oxidation during the drying process. Dried foods are lightweight and shelf-stable at room temperature. Pasteurisation uses heat (72°C for 15 seconds) to destroy pathogenic bacteria, making food safe to drink/eat. It extends refrigerated shelf life but does not kill all microorganisms or spores. The moderate heat preserves most nutrients and flavour, though some heat-sensitive vitamins (B1, C) are partially lost. Pasteurised products require continuous refrigeration and have limited shelf life. Each method has trade-offs: freezing best preserves texture and nutrients but requires constant energy; drying creates shelf-stable products but changes texture significantly; pasteurisation ensures safety with minimal quality change but requires refrigeration. The choice depends on the food type, intended use, and available storage.