Food Science and Processing

FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 3 : Methods of Food Preparation

Pressure cooking

A relatively small increase in temperature can drastically reduce cooking time and this fact is utilized in pressure cookers. In pressure cooking, escaping steam is trapped and kept under pressure so that the temperature of the boiling water and steam can be raised above 1000C and reduce cooking time.

Boiling for 1 hour at 1000C can be reduced to half an hour at 1100C and further can be reduced to ¼ hour at 1200C. In practice, the reduction in cooking time is limited by the pressure temperature relationship of steam. Above 130-1400C the pressure generated increases enormously for every small increments in temperature. Above 1300C massively constructed pressure vessels are required. Hence 1200C is an effective limit.

The domestic pressure cooker is usually constructed of heavy gauge aluminium and the pressure is regulated by weights placed upon a release valve. A safety valve is always fitted to prevent pressure inadvertently building up beyond the safe and specified level for the vessel construction. A common practice in pressure cooking is to allow a copious flow of steam from the release valve before capping it with the weights. This ensures that all air is removed before the cooking process starts. Even the presence of a small quantity of air with the steam diminishes the heat transfer properties of the steam drastically. Ten percent of volumes of air will almost double the cooking time. Purging of enclosed air is therefore a vital step in using a pressure cooker.

Advantages

• It takes less time to cook.
• Different items may be cooked at the same time.
• Fuel is saved.
• Requires less attention.
• Nutrient, food or flavour loss may be less.
• There is an indication for the completion of cooking.
• There are less chances of scorching or burning.
• Through knowledge of using the equipment is required. Otherwise accidents can happen.
• There may be mixing of flavours.
• Food may be undesirably soft.
• Foods cooked in pressure cooker are rice, dhal, vegetables and meat.