Thanks for your help.

oxidation of fat is faster at very low water activity which decreases with slight increase in water activity due to dilution of fat. Further moisture presence increase oxidation of fat and at high water activity fat oxidation rate is high

Thank you Anil

Dear Yakoub,

Here is my little understanding of the issue:

First, Water activity is defined as the ratio of the vapour pressure of water in a food item (P) to the vapour pressure of pure water (PO) at the same temperature. So mathematically, we have

Aw = P/P0

Where aw = water activity; P = partial pressure of water in food material; P0 = vapour pressure of pure water at the same temperature.

As for Lipid oxidation it is one of the major causes of food spoilage and there are three main mechanisms by which lipid oxidation take place:

1. Oxidation via a self-catalytic mechanism is the main reaction which takes place in oil becoming rancid. This is called the oxidative deterioration of lipids or ‘auto-oxidation’. The auto-oxidation follows a free radical mechanism and can be visualised to be consisting of three stages as follows.

a) Initiation

In the first step of auto-oxidation process called initiation, hydrogen is removed from the fatty acid chain to yield a free radical. The reaction can be shown as below.

RH R˙ + H˙ ( R and H are free radicals)

b) Propagation

Once a free radical is formed, it combines with oxygen to form a peroxy free radical which can remove hydrogen from another unsaturated molecule yielding a peroxide and a new free radical. This is called ‘propagation reaction’ and may repeat up to several thousand times in a kind of chain reaction

R˙ + O2 RO2˙ ۟2 + RH ROOH + R˙ RO

(peroxide)

c) Termination

The propagating chain reactions are terminated through a reaction between the free radicals to yield non-active products:

R˙ + R˙ R – R

R˙ + RO2 RO2R

nRO2 (RO2)n

2. Lipolysis: Rancidity in presence of enzymes, heat and moisture causes the hydrolysis of ester bonds in lipids. This is called lipolysis. These resulting peroxides then breakdown to yields low molecular weight aldehydes and ketones which vaporize to give the peculiar off flavour. The flavour threshold for these is as low as 1ppb. Lipolysis occurs during deep fat frying due to large amount of water released from the food and the high temperature. Release of short chain fatty acids by hydrolysis is responsible for the development of an undesirable rancid flavour in raw milk. This is called hydrolytic rancidity. However, controlled and selective lipolysis is used in the manufacture of food items as yogurt and bread.

NB: Therefore, I think that if water activity has to play a role, it should be at this level i.e either low or high aw can affect the enzymatic hydrolysis.

3. Thermal Decomposition leading to fat deterioration is a result of high temperature heating of fats in the presence of oxygen. This deterioration is due to the oxidative reactions of saturated and unsaturated fatty acids and interaction of nutrients among themselves. The compounds formed are cyclic and ayclic dimers, long chain alkanes, aldehydes, ketones etc.

Hope this can also contribute to your understanding!

Regards!

Difo

Thanks dr Difo. But where is the relation between wa and lipid oxydation?!

Dear yakoub,

What I meant by my explanation above is that, to my little understanding, the only way Aw is related to lipid oxidation is at the enzymatic reaction level. Because as you may also know, it has been established that Most enzymes are inactivated at Aw below 0.85. as such, Oxidation of lipids are know to slow down as water content increases. Other examples of reactions influenced by Aw hydrolysis of protopectin (splitting and demethylation of pectin) and autocatalytic hydrolysis of fats and pheophytinization reactions.

Except if what you want is a mathematical expression showing the relationship, i hope this can complement my previous explanation and help you. Otherwise, you may have other explanations from other experts.

Regards!

Difo