The Frying Process

How proline is converted to NPYR has not yet been fully elucidated and could conceivably occur by either of two pathways (29, ). One pathway involves the initial N-nitrosation of proline, followed by decarboxylation, while in the other, proline is first decarboxylated to pyrrolidine followed by N-nitrosation to NPYR. Since the conversion of N-nitrosoproline (NPRO) to NPYR occurs at a much lower temperature than the transformation of proline to pyrrolidine, the pathway involving intermediacy of NPRO is thus the more likely route ( ). It has been reported that preformed NPRO in raw bacon is not the primary precursor of NPYR in cooked bacon (29,33-5), as shown by the fact that ascorbyl paImitate, when added to bacon, inhibits the formation of NPYR (33). However, this by no means rules out the intermediacy of NPRO which could be formed at the higher temperatures attained during the frying process (29,36). 

The mechanism of NPYR formation has been studied by Coleman (37) and Bharucha et al. ( ). Coleman (37) reported that the requirement for a high temperature, the inhibitory effects of water and antioxidants, and the catalytic effect of a lipid hydroperoxide are consistent with the involvement of a free radical in the formation of NPYR. Similarly, Bharucha et al. (29) suggested that, since both NPYR and NDMA increase substantially towards the end of the frying process, N-nitros-amine formation during frying of bacon occurs essentially, if not entirely, in the fat phase after the bulk of the water is removed and therefore by a radical rather than an ionic mechanism. These authors speculated that, during the frying of... 

Amino acid analysis confirmed that Maillard browning reactions occurred during the frying process, since basic amino acids such as lysine and arginine decreased significantly, whereas all others decreased only slightly (Table I). 

Oil acts as a heat transfer medium during the frying process which rapidly heats, cooks, and sometimes puffs, dehydrates, or forms a crust around the product. The mechanics of deep fat frying are shown in Fig. 34.32.173 The entering material carries oxygen that temporarily aerates the hot oil, and water that forms steam, with both rapidly swept out... 

In the frying process, food, such as vegetables, meat, or seafood, is brought in direct contact with hot oil. The food surface becomes golden yellow to dark brown in color and develops a pleasant fried food flavor. 

Fried food flavor develops as the frying process continues... 

Free radicals are formed whenever oil containing unsaturated fatty acids is heated in the presence of a metal initiator, (25) such as iron, nickel, or copper. Free radicals are formed in the oil during frying. The metal initiator in the frying process can come from several sources as given below ... 

Polymerization of oil occurs under heat with or without the presence of oxygen. Heat can cleave the oil molecule or fatty acid. These cleaved compounds can then react with each other, forming large molecules. These polymers are referred to as thermal polymers. In the frying process, excessive fryer heat and excessive fryer down time can produce high levels of thermal polymers. Thermal polymers can be detected in the fresh product by expert panelists because they generally impart a bitter aftertaste to the fried food. 

The reactions in the oil in the frying process are very complex. Figure 3, published by Carl W. Fritsch, illustrates the various pathways for oil reaction in the fryer (10). 

The dehydration in frying is very rapid. It can vary from 10 seconds up to a few minutes, depending on the type of product and the frying process. 

Table 2.1 lists some of the pore parameters obtained from a 3D image analysis of a fried product. Miri et al. (2006) studied the effect of frying time and temperature on crust formation and pore characteristics of French fries using the x-ray micro-CT technique, and conclnded that the method allowed not only a fundamental understanding of the frying process, but also a process design that would result in specific microstructnres. 

It is difficult to give an accurate definition of the perfect frying medium because there are so many applications and so many factors that affect the frying process. Some general guidelines and characteristics of a good frying oil are ... 

The volatiles in the frying oil increase in the beginning, but then decrease as frying continues. The most important volatiles for the quality of frying oil are saturated aldehydes Cg-Cg, enals (e.g., 2-decenal), dienals (e.g., 2,4-heptadienal), and hydrocarbons (hexene, hexane, heptane, octane, nonane, and decane). The presence of volatile oxidation products formed during the frying process was discussed by Perkins (1996) and Nawar (1998). 

Christie, W.W. and Dohson, G. 2000. Formation of cychc fatty acids during the frying process, Eur. J. Lipid Sci. TechnoL, 102, 515-520. 

Morton, l.D. 1998. Geography and history of the frying process, Grasas Aceites, 49, 247-249. 

Deep-frying operations also affect the flavor of fried foods directly through absorption as a film on the surface. The frying process is a complicated thermal-chemical process that produces fried foods with desirable color, flavor, and texture. Food placed in hot fat is heated quickly to a point where water is vaporized, and the resulting steam canses a boiling action in the oil. This boiling action increases aeration in the oil, which results in an increased oxidation of the oil with the formation of the primary oxidation products—hydroperoxides. These hydroperoxides are extranely... 

Bubble end point—If the frying process is extended to a considerably long time, the rate of moisture removal diminishes due to effective decrease of moisture content, and no more bubbles escape from the surface of the product. This final stage is referred to as the bubble end point. 

During the frying process the lipid material is exposed to high temperature in the presence of air and moisture. Under these conditions a wide variety of reactions takes place and the main groups of alteration compounds generated are summarized in Table 59.5 [11,25,7,16]. 

Some products, e.g., doughnuts, must be turned over a little more than half way through the frying process in order to get a symmetrical shape and a uniform crust color. For these products conveyors that automatically turn the dough pieces at the proper time are used. 

Kawas, M.L. and Moreira, R.M. 2001. Characterization of product quality attributes of tortilla chips during the frying process, J. Food Eng., 47 97-107. 

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