Changes in frying oil Types of reaction

The oil is subject to three types of reaction during deep frying:

  • hydrolytic reactions;
  • oxidation reactions;
  • pyrolysis of oxidation products.

Triacylglycerols in frying oil are hydrolysed by steam produced from water in the fried product when it is in contact with the hot frying oil. As the two reacting partners are not miscible, the reaction is relatively slow, resulting in the formation of diacylglycerols and free fatty acids. Diacylglycerols are more polar and therefore their contact with water vapour is better; monoacylglycerols and free fatty acids are formed by further hydrolysis. Monoacylglycerols are rapidly hydrolysed into fatty acid and glycerol. Under deep frying conditions, glycerol is dehydrated into acrolein, which is very volatile and its vapours irritate the eyes and mucosa.

The rate of oxidation reactions depends on the concentration of oxygen. Oxygen present in the original frying oil is rapidly consumed, usually before the temperature of oil reaches the frying temperature. Additional oxygen can enter frying oil only through diffusion from air (Fujisaki et al, 2000). When contact with air is moderate the oxidation of the frying oil is slow. It is consumed for the destruction of natural antioxidants, and only when they are destroyed, tria-cylglycerols are oxidised, too. Hydroperoxides are formed as primary reaction products, but they are very unstable at high temperature so that their content rarely exceeds 1%.

Some components present in fried food affect the oxidation rate of frying oil (Pokorny, 1998). The oxidation rate could be reduced by addition of antioxidants even when they are less efficient than under storage conditions. Most synthetic antioxidants, such as BHT and BHA, are too volatile under frying so that they have only moderate activity. Gallates are more efficient in frying oils. Currently it is considered preferable to use natural antioxidants. Tocopherols are present in most frying oils, and their addition is efficient (Gordon and Kourimska, 1995). Ascorbyl palmitate, citric acid and its esters are useful as synergists. Rosemary and sage resins were also found to be active in frying oils (Che Man and Tan, 1999). Oxidation reactions can be inhibited by polysiloxanes, which form a very thin layer on the surface of the frying oil, preventing the access of oxygen (Ohta et al, 1988). Because they are not resorbed in the intestines they are considered safe for human consumption.

The third group of reactions are secondary reactions of hydroperoxides. They are decomposed in three ways during frying:

  • Decomposition into nonvolatile products with the same number of carbon atoms, such as epoxides, ketones or hydroxylic compounds. When the con centration of these products (known as polar products) exceeds 25-27%, the frying oil has to be replaced by fresh oil. At still higher levels of polar products, foaming takes place, which increases the contact area of oil with air, and thus the rate of oxidation.
  • Decomposition into volatile low-molecular weight compounds, such as aldehydes, alcohols, ketones or hydrocarbons. Some products possess a typical fried flavour, e.g. 2,4-decadienals or unsaturated lactones. They are formed from linoleic acid bound in frying oil.
  • Decomposition into high molecular weight compounds, usually dimers or trimers with fatty acid chains bonded by C-C, C-O-C or C-O-O-C bonds. The content of polymers is a good indicator of the degree of frying oil degradation. When their content reaches 10%, used oil should be replaced by fresh oil.

Several methods are used for monitoring oil degradation during frying (Wu and Nawar, 1986). Used oil can be analysed with use of HPLC (for polar compounds) or HPSEC (for polymers); this is best done in combination with column chromatography (Sanchez-Muniz et al, 1993). Among other methods, the spectrophotometry, determination of permittivity (dielectric constant), specific gravity or different colour tests can be used (Xu, 2000).

Frying oil can be used for a longer time if it is purified from insoluble particles and polar substances by using a suitable adsorbent, such as magnesium silicate (Perkins and Lamboni, 1998). Commercial products for this pupose are available (Gertz et al, 2000). Their combination with antioxidants is recommended (Kochhar, 2000). Membrane processes have been proposed for purification of frying oil (Miyagi et al, 2001).

12.2.2 Choice of frying oil

Frying oil should contain some bound linoleic acid to generate a fried flavour (Warner et al, 1997). Some oils, such as soybean, sunflower or rapeseed oils are rich in linoleic acid, but are rather unstable under frying conditions and should be replaced very often by fresh oil, which is expensive (Gertz et al, 2000). Low polyunsaturated oils, such as olive oil, are highly priced. Hydrogenated vegetable oils are more stable but are objectionable because of the content of trans-unsaturated fatty acids. Pork lard is an excellent frying medium from the standpoint of sensory value, but there are objections because of its high content of saturated fatty acids and of cholesterol. The best choice are high-oleic low-polyenoic modified vegetable oils, such as fractionated palm oil, i.e. the palm olein fraction (Che Man and Hussin, 1998), modified soybean, sunflower, rape-seed, peanut, and even linseed oil. If they contain 3-10% linoleic acid, they still produce an attractive fried flavour and are sufficiently stable on frying. A problem is their availability on the market.

296 The nutrition handbook for food processors 12.3 Impact of deep frying on nutrients

The following main changes occur in the frying process (Fillion and Henry, 1998):

  • Mass transfer between frying oil and fried food;
  • Thermal decomposition of nutrients and antinutritional substances in fried food;
  • Interaction between fried food components and oxidation products of fried oil (Dobarganes et al, 2000).

12.3.1 Impact of frying on main nutrients

The main change in the food composition during frying is the loss of water and its replacement with frying oil. Most foods (other than nuts) contain water as their major component. In contact with hot frying oil, water is rapidly converted into steam, at least in the surface layer of fried material. The temperature of inner layers does not exceed the boiling point of water so that water losses are only moderate.

Spaces left in fried food after water evaporation are filled with frying oil (Pinthus et al, 1995). This process increases the available energy content of the product and because the energy intake in the diet is too high in many countries, it is desirable to reduce the absorption of frying oil. This may be achieved by drying pieces of food on the surface before immersion into oil (Baumann and Escher, 1995). Another way is to produce a crust on the surface of fried pieces, which prevents water losses and oil uptake, and preserves juiciness in fried material (Ateba and Mittal, 1994). It is possible to cover the surface with batter or various other preparations, such as cellulose derivatives (Priya et al, 1996). The oil absorption can be reduced by half using these procedures. The oil removed by absorption into fried food should be replenished by fresh oil from time to time in order to keep the volume of frying oil constant.

Changes in nutritional value depend not only on the amount of absorbed frying oil, but also on its composition. If fresh edible oil is used, the contents of essential fatty acids and tocopherols in fried food rise. If food is fried in oil used for a longer time, the content of essential fatty acids and tocopherols becomes low, so that the increase in nutritional value, due to absorbed oil, is not significant. On the contrary, such antinutritional products as polar lipids and polymers are absorbed with used frying oil.

If fried food is stored, even under refrigeration, the thin layer of frying oil on the surface is autoxidized, especially in case of oil rich in polyunsaturated fatty acids (Warner et al, 1994). Fried food should be stored either in vacuum or an inert gas or protected by antioxidants.

If fat-rich food is fried, such as bacon, sausages or fat fishes, some fat originally present in food is released into frying oil. Eicosapentaenoic and docosa-hexaenoic acids were detected in oils used for frying fish like sardines (Sanchez-Muniz et al, 1992). Cholesterol may also be extracted into frying oil.

If plant foods are subsequently fried in the same oil, cholesterol or fish fatty acids may be absorbed.

Lipids present in food are decomposed only to a small extent, including high unsaturated fish oils. It is due to short frying time and limited access of oxygen. Based on dry matter content, the concentration of most nutrients is reduced during frying, as the original nutrients are diluted with absorbed frying oil. Starch and non-starch carbohydrates are partially destroyed during frying, and starch-lipid complexes are formed (Thed and Phillips, 1995). The fraction of resistant (undigestible) starch changes during the operation (Parchure and Kulkarni, 1997). Sucrose is hydrolysed into glucose and fructose, which are destroyed by heating, mostly by Maillard or caramelisation reactions.

Proteins are rapidly denaturated in surface layers of food particles, more slowly in inner layers than on the surface. Enzymes get nearly completely deactivated. The availability of proteins in humans is usually reduced by frying (Fukuda et al, 1989), especially on the surface (Pokorny et al, 1992). Some essential amino acids are destroyed, such as lysine or tryptophan (Ribarova et al, 1994). If protein comes into contact with the hot walls of the frying pan above the oil level, it is dehydrated and pyrolysed into polycyclic aromatic compounds (Overvik et al, 1989).

12.3.2 Impact of frying on micronutrients

Vitamins are relatively labile substances. Tocopherols are decomposed by oxidation reactions so that frying oil used for repeated frying contains only traces of tocopherols. Ascorbic acid is also destroyed by mechanisms similar to those of reducing sugars. The Vitamin B complex is also substantially damaged by frying (Kimura et al, 1991; Olds et al, 1993). Carotenes and carotenoid pigments are easily oxidised and polymerised (Speek et al, 1988), which is visible from colour changes.

Mineral components are also affected. Iron and other heavy metals are mostly bound in complexes, which are partially decomposed during frying, and metal ions may contaminate frying oil by decreasing its resistance to oxidation. Ferric ions are less digestible than iron in haem complexes. Sodium and potassium chlorides present in food are very slightly dissociated, and sodium and potassium ions react with free fatty acids forming soaps (Blumenthal and Stockler, 1986). Soaps are surface active agents, increasing foaming and thus accelerating oxidation. Volatile mineral components, such as selenium or mercury derivatives, are partially lost at high frying temperatures. Many foods contain antinutritional or even toxic substances, which are often partially decomposed or evaporated during frying.

12.3.3 Changes in sensory characteristics

Frying imparts a distinctive flavour to fried products; some flavours are common to all fried foods and some are additional and, specific for particular products, e.g. french fries (Wagner and Grosch, 1998).

The colour of the fried product often differs substantially from that of the original food material. The most important reactions are nonenzymic browning reactions between reducing sugars and free amino acids, called Maillard reactions. Colourless premelanoidins are a group of intermediary products with very low nutritional value. They are rapidly polymerised into macromolecular deep brown melanoidins, which are completely unavailable for human nutrition. To obtain light-coloured potato chips, it is necessary to adjust the concentration of reducing sugars to low values (Califano and Calvelo, 1987). A side reaction is the Strecker degradation of free amino acids under attack of dicarbonylic sugar degradation products. Heterocyclic products, such as pyrazines or furans, have typical fried or roasted flavour notes (Chun and Ho, 1997). Similar browning reactions are caused by interactions of frying oil oxidation products (mostly hydroperoxides or unsaturated aldehydes) with the free amine group of bound lysine (Pokorny, 1981). Lysine thus becomes unavailable for human nutrition.

Oxidised frying oil also contributes in significant degree to the flavour of fried food (Chang et al, 1978). Moderate amounts are necessary to produce the typical fried flavour but greater amounts are objectionable. For this reason the frying oil should not be too fresh for high quality fried foods. The flavour improves by repeated use for frying, but if use is too long then products are of lower quality. The producer's aim is to maintain frying oil for the longest time possible at the stage of optimum performance (Blumenthal and Stier, 1991).

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