Food Science and Processing

FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 12 : Composition, Nutritive Value and Processing of Nuts and Oil Seeds

Utilization of fats and oils in food preparation

1. Smoking point
• Temperature at which smoke comes continuously from the surface of fat.
• Development of free fatty acids by hydrolysis of fat during frying causes a decrease in the smoking point.
• Fat that has had repeated or prolonged use will begin to smoke at a temperature too low for frying.
• Suspended matter such as flour or batter particles lowers smoking point
• Greater the surface of fat exposed the lower is the smoke point.
• Fats heated in shallow wide pans with slightly sloping sides begin to smoke at low temperature than those heated in smaller pans, with vertical sides.
• Smoking point of soya, cotton seed, pea nut and corn oils is 2300C.
• The smoking point of hydrogenated fats is 221-2320C.
• Shortenings containing monoglyceride as an emulsifier smoke at a lower temperature of about 1760C.


2. Changes in fat on heating
• There are four main ways in which oil decomposes and all of these lead to a diminution of smoke point and operating temperature and lowers cooking quality, if the oils are kept for a long time.


3. Pyrolysis (thermal break down)
• The action of heating oil , whether or not it is in contact with food, causes a break down in its molecular structure
• When fat begins to smoke, its chemical break down begins and free fatty acids and acrolein are formed from glycerol.
• If the fat is allowed to smoke for any periods of time , the acrolein causes irritation of eyes and nostrils.


4. Oxidation
• Reaction of hot oil surface with oxygen in air causes molecular break down and development of rancidity.


5. Hydrolysis
• As water is boiled in large quantities from food during frying, some of this water will tend to decompose the oil.


6. Reaction with food residues
• Food fragments left in oil after cooking undergo chemical reaction particularly if the oil is heated without removing residues. Straining oil after use reduces this type of deterioration.
  
  
  

Decomposition mechanism in TG

1. Fat or oil used as medium of cooking
• Used in shallow and deep fat frying.
• Better cooking medium than air or water as it is having greater specific heat. Temperature of fats and oil is about 2000C higher than that of H2O.
• Pan frying to cook dosa, chapatti, omelettes, cutlets
• Deep fat frying to cook vadas, cutlets, bajjis and pakodas.
• A satisfactory fats has high decomposition point, high smoking point with no undesirable odor.


2. Fat absorption
• Fat absorption should be minimum from the point of palatability and ease or rapidity of digestion.

Factors affecting fat absorption are

1. Foods containing high sugar and fat absorb more oil
2. Addition of egg cause increased absorption of fat
3. Doughnuts made from soft wheat flours absorb more fat than made from strong flours
1. Gluten present in soft wheat flours increases fat absorption
4. Porous surfaces like bread or fermented foods absorbs more fat
5. Surfaces with more cracks absorbs more fat
   Ex. Adding sodium bicarbonate to bajji causes porousness in food and results in high fat absorption.
6. Greater the surface areas of the food products, the more will be fat absorption
7. Higher moisture content leads to enhanced fat absorption
   Ex. Pooris and vadas

Temperature of fat and the length of time of heating

1. Longer the time the food is cooked with fat, higher will be fat absorption
2. Using the repeated heated oils lowers smoking point
3. Higher the free fatty acid content, the higher will be the oil absorption.
4. Presence of finely divided material in fat accumulated during repeated frying lowers smoking point

The frying life of fat can be improved by

1. Filtering after use,
2. Storing in a refrigerator
3. Low surface of fat exposed lowers smoking point . This indicates the importance of using deep rather than shallow pan for frying.