Frying 295 entration of these products(known as polar products)exceeds 25-27%o, the frying oil has to be replaced by fresh oil. At still higher levels of polar prod ucts, 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 alde hydes, 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-0-o-C bonds. The content of polymers is a good indicator of the degree of frying oil degra- dation. When their content reaches 10%o, 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 cor pounds)or HPSEC (for polymers): this is best done in combination with column chromatography(Sanchez- Muniz et al, 1993). Among other methods, the spec- trophotometry, determination of permittivity(dielectric constant), specific gravity or different colour tests can be used(Xu, 2000) o Frying oil can be used for a longer time if it is purified from insoluble parti s and polar substances by using a suitable adsorbent, such as magnesium sil icate(Perkins and Lamboni, 1998). Commercial products for this pupose are available(Gertz et al, 2000). Their combination with antioxidants is recom- mended (Kochhar, 2000). Membrane processes have been proposed for purifica tion 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 veget able 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, 1. e. the palm olein fraction( Che Man and Hussin, 1998), modified soybean, sunfower, 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 marketcentration 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 prod￾ucts, 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 alde￾hydes, 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 degra￾dation. 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 com￾pounds) or HPSEC (for polymers); this is best done in combination with column chromatography (Sánchez-Muniz et al, 1993). Among other methods, the spec￾trophotometry, 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 parti￾cles and polar substances by using a suitable adsorbent, such as magnesium sil￾icate (Perkins and Lamboni, 1998). Commercial products for this pupose are available (Gertz et al, 2000). Their combination with antioxidants is recom￾mended (Kochhar, 2000). Membrane processes have been proposed for purifica￾tion 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 veget￾able 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. Frying 295