Food Waste and Catering Waste; Focus on Valorisation of Used Cooking Oil and Recovered Triglycerides - The Economic Utilisation of Food Co-Products - page 90

6.3.1 UCO Physical and Chemical Properties
The process of frying has a significant effect on the physical and chemical
properties of the oil employed. These differences will be affected by the FFA
profile of the parent oil, and by the period of frying undergone by these oils in
addition to the type of process it has undergone.
As a general rule, oils recovered from frying processes will have,

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comparison with the parent virgin oils:

Higher viscosity, with fats being solid at ambient temperature.
Higher free fatty acid content due to the hydrolysis of triglycerides plus
oxidation promoted by the presence of water at high temperatures.
Lower smoke point; An oil or fat subjected to thermal stress will
decompose at a certain temperature (usually 150-180 1C) forming smoke
that will be comprised of fatty acids, glycerol and decomposition products
such as acrolein, hydrocarbons. This is known as the smoke point.
Generally, the smoke point of oil increases as the free fatty acid content
decreases and as the degree of refinement increases. A lower smoke point
will be expected in oils that have been employed in long periods of frying.
Decreasing the smoke point has a strong impact on the quality of food in
frying processing. Also intermittent frying has a markedly greater effect on
oil deterioration than continuous frying, therefore monitoring of oil frying
usage is essential to ensure minimum standards of quality in food
processing. 24
Higher amounts of moisture and impurities, as a result of the frying
process, where ambient moisture and water contained in food are
absorbed by the oil.
Lower resistance to oxidation, resulting from the thermal degradation of
natural antioxidants present in virgin oil, the hydrolysis processes
promoted by the presence of water and the formation of auto-oxidative
species. The stability of the oil can be improved by the use of additives. 25
Higher composition of polar component, such as polymers, aldehydes,
short-chain fatty acids and ketones. These species are formed by the inter-
action of fat/oil with air, food and moisture at high temperature conditions
(150-180 1C). At these conditions, a number of chemical processes will occur
(peroxide formation/oxidation, Maillard reactions, hydrolisis, etc.), leading
to the formation of, amongst others, polymeric polar species responsible for
the formation of sludge and viscosity increase in frying oils.

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Attending to these parameters and drivers, certain recovered oils from frying
will have better optimum properties for use in value-added applications.

6.3.2 Quality of UCOs
The quality of oil or fat used for frying is of paramount importance to ensure
adequate standards for fried food. The standardisation of thermal rancidity