Despite the trend towards low-fat food, fried products remain popular. On the other hand, consumers are better educated and want healthy and environmentally friendly ingredients. Retailers are therefore looking at their suppliers to make sure they can meet the requirements of their customers.
Generally, liquid oils are more sensitive to oxidation and therefore they used to be hydrogenated to enhance stability for the catering industry and fastfood sectors. However, considerable amounts of trans fatty acids are formed during hydrogenation, which are undesirable from a nutritional and health perspective. Since also consumers are well-informed about the health risks of trans fats, there is a clear trend to look for alternatives to protect frying oils from oxidation.
Deep frying has been known since ancient history and brings about an attractive change of flavour, texture and colour in the food. However, deep frying also leads to undesired chemical and physical changes that affect both the quality of the deep frying medium and the fried food.
Frying is a complicated, multi-factor process where the frying oil undergoes a complex series of oxidation reactions. This complexity increases further when food is introduced into the hot oil. In fact, fats and oils are mixtures of triglycerides. These nonpolar components can be strongly affected by oxygen, heat and water. Total polar compounds (TPC) are the major decomposition products of triglycerides present in frying oil. Various chemical reactions including oxidation, hydrolysis and polymerisation result in the formation of polar compounds such as free fatty acids, aldehydes, mono-and diglycerides and polymerised triglycerides.
The colour of frying oil will become more intensive and darker and is highly affected by the type of food being fried. Darkening of the oil is caused by Maillard reaction products of amino acids and unsaturated carbonyl compounds. This reaction results in the formation of a variety of heterocyclic compounds which contribute to the flavour of the fried food.
However there is no direct correlation with the colour change and the deterioration of fat. Oil can turn dark quickly, while still having a good oxidative status. On the other hand, chemical parameters such as polar compounds and anisidine value will increase when frying oil becomes rancid.
Figure 1: Correlation of different parameters with oxidative deterioration of the oil
Synthetic antioxidants such as Tertiary Butylhydroquinone (TBHQ) can be used to extend the shelf life of frying oils, but are less effective at deep frying conditions. Furthermore, there is a clear trend towards natural solutions. With this trend in mind, Kemin developed FORTI-FRY™ Liquid, a natural antioxidant consisting of tocopherol rich extract, specialty oils and an innovative emulsifier system to protect the frying life of oil.
In order to prolong the frying life of rapeseed oil, FORTI-FRY Liquid was added at a dosage of 0.25 % and 0.5 % to the vegetable oil and compared with an untreated oil. Furthermore, 0.03 % of mixed tocopherols and 0.02 % (max. legal limit) of the synthetic antioxidant Tertiary Butylhydroquinone (TBHQ) were included as a positive control.
The frying trial consisted of two frying cycles of four hours per day. Eight batches of raw French fries were fried for two minutes each. Every frying cycle consisted of three hours pre-heating at 140 °C and one hour frying at 180 °C. At the end of each cycle, oil samples were taken for analysis with Near Infrared (NIR) Spectroscopy. NIR Spectroscopy is a spectroscopic method that uses the nearinfrared region of the electromagnetic spectrum.
Parameters being measured to determine deterioration of the frying oil were Dimerised and Polymerised Triglycerides (DPTG) and Total Polar Compounds (TPC).
NIR analysis of Total Polar Compounds (TPC) showed that TPC in oil treated with 0.5 % FORTI-FRY were significantly (p<0.05) lower compared with untreated oil or oil treated with single antioxidants. Oil treated with 0.25 % FORTI-FRY showed an overall improvement compared with the untreated oil and single antioxidant treatments, although the difference was not always significant. Both treatments with mixed tocopherols or TBHQ were ineffective and never showed a significant improvement in TPC compared with the untreated oil (Figure 2).
Based on the legal limits for TPC in frying oil (between 24-27 %), the untreated oil had to be discarded after 59 frying batches. With the addition of 0.25 % FORTI-FRY the frying life could be extended to 69 batches, corresponding to a 17% increase.
Figure 2. NIR results of TPC in rapeseed oil during a discontinuous frying process (error bars represent the standard error of the mean). Maximum legal limits are country dependent and range between 24 and 27 %.
NIR analysis of Dimerised and Polymerised Triglycerides (DPTG) showed that formation of DPTG was significantly lower in the FORTI-FRY treatments compared with untreated oil. Furthermore, the 0.5 % treatment significantly (p<0.05) outperformed the single antioxidant treatments. The treatment with 0.25 % FORTI-FRY was overall numerically lower compared with the single antioxidant treatments and significantly lower compared with the untreated oil.
Based on the legal limits for DPTG in frying oil (between 10-16 %), untreated oil had to be discarded after 42 frying batches. With the addition of 0.25 % FORTI-FRY the frying life of the oil could be extended to 53 batches, corresponding to a 25 % increase in shelf life (Figure 3). With the addition of 0.5 % FORTI-FRY the frying life of the oil could be extended to 58 batches, corresponding to a 38 % increase. Both single antioxidant treatments were not effective in delaying the formation of DPTG and could only give a numerical improvement compared with the untreated oil. Based on the legal limits, both single antioxidant treatments extended the frying life with three additional batches, corresponding to an increase of only 7 %.
Figure 3. NIR results of DPTG in rapeseed oil during a discontinuous frying process (error bars represent the standard error of the mean). Maximum legal limits are country dependent and range between 10 and 16 %.
When food is deep fried, a complex process of heat and mass transfer takes place between the oil and the fried food product, in which both the food structure and the medium’s properties change. Furthermore, a number of chemical and physical reactions occur.
The choice of the deep fried medium determines the stability of the frying oil. Natural antioxidants help maintain a good quality of the deep frying medium and the fried food by delaying degradation.
The 0.5% dosage of FORTI-FRY could prolong the frying life to 75 batches, corresponding to a 27 % increase. FORTI-FRY Liquid, a powerful combination of mixed tocopherols, specialty oils and innovative emulsifier system was effective in stabilising frying oil during a discontinuous frying process. Depending on the used quality parameter, the frying life of rapeseed oil could be increased with 25 to 37 %. Single antioxidants like mixed tocopherols and TBHQ were ineffective in stabilising the frying oil during a discontinuous frying process.
FORTI-FRY significantly prolongs the lifetime of the oil by preventing undesirable off-flavours and decreasing foaming. The emulsifiers also support a sufficient heat transfer manifested in even browning of the food.