Aroma lipid peroxidation

The most important precursors for lipid oxidation are unsaturated fats and fatty acids like oleic (18 1), linoleic (18 2), linolenic (18 3) and arachidonic acid (20 4). The more unsaturated ones are more prone to oxidation. Lipid peroxidation and the subsequent reactions generate a variety of volatile compounds, many of which are odour-active, especially the aldehydes. That is why lipid oxidation is also a major mechanism for thermal aroma generation and contributes in a great measure to the flavour of fat-containing food. Lipid oxidation also takes place under storage conditions and excessive peroxidation is responsible for negative aroma changes of food like rancidity, warmed-over flavour, cardboard odour and metallic off-notes. 

Heme (Fe +) and hemin (Fe +) proteins are widely distributed in food. Lipid peroxidation in animal tissue is accelerated by hemoglobin, myoglobin and cytochrome C. These reactions are often responsible for rancidity or aroma defects occurring during storage of fish, poultry and cooked meat. In plant food the most important heme(in) proteins are peroxidase and catalase. Cytochrome P450 is a particularly powerful catalyst for lipid peroxidation, although it is not yet clear to what extent the compound affects food shelf life in situ . 

Table 3.32. Sensory properties of aroma components resulting from lipid peroxidation...

The content of free but)tric and caprylic acid as well as (Z)-3-hexenal rises when cream is whipped (Table 10.39). Pasteurization results in the formation of 2-acetyl-2-thiazoline in whipped cream and the content of (E,Z)-2,6-nonadienal is greatly increased. A model corresponding to Table 10.39 (without No. 12, 14, 17 and 20) approaches the aroma of whipped pasteurized cream and reproduces especially the "creamy" note. Maillard reaction products are also characteristic of the aroma of milk powder. The development of aroma defects during the storage of whole milk powder is due to products of lipid peroxidation, e. g., (Z)- and (E)-2-nonenal. 

Curing prevents WOE. Myoglobin is stabilized by nitrite, therefore, no additional non-heme iron is formed during cooking (Table 12.18). In addition, the MbNO formed has an antioxidative effect (cf. 12.3.2.2.4). Lipid peroxidation does not occur and new aroma substances are formed that are characteristic of cured meat. 

Important aroma substances of raw and cooked mutton are listed in Table 12.25. A special feature is the two branched fatty acids, which are already present in the raw meat and produce the mutton odor. (E)-2-Nonenal and the other odor substances from lipid peroxidation are also present in not inconsiderable concentrations in the raw meat. Only HD3F is formed during cooking. 

If cooked meat is stored for a short time, e. g., 48 h at ca. 4 °C, an aroma defect develops, which becomes unpleasantly noticeable especially after heating and is characterized by the terms metallic, green, musty and pungent. This aroma defect, also called warmed over flavor (WOF), is caused by lipid peroxidation (cf. 12.6.2.1). The indicator of this aroma defect is hexanal, which increases as shown in Table 12.27. 

To simplify the analytical procedure, individual aldehydes (e. g., hexanal, 2,4-decadienal), which are formed in larger amounts during lipid peroxidation, have been proposed as indicators. In most of the cases, however, it was not tested whether the indicator increases proportionally to the off-flavor substances which cause the aroma defect. 

The aroma is influenced by the recipe but also by the fermentation, e. g., the Strecker aldehydes increase and those from lipid peroxidation decrease if the dough matures at lower temperatures (Table 15.60). An extension of the kneading process... 

Preparation and storage of products from both oilseeds is often inhibited by rancidity and bitter aroma defects caused mostly by volatile aroma active carbonyl compounds, e. g., (Z)-3-hexenal, (Z)-l,5-octadien-3-one and 3-methyl-2,4-nonan-dione. The rancidity-causing compounds are formed through peroxidation of linolenic acid, accelerated by the enzyme lipoxygenase and/or by hem(in) proteins (cf. 3.T.2.2). Furan fatty acids are the precursors in the case of the dione (cf. 14.3.2.2.5). Lipid peroxidation is also involved in the formation of another very potent odorant, 2-pentylpyridine, which produces grassy aroma defects in soybean products. Defatted soybean protein isolates contained 60-510 pg/kg of this compound, which with an odor threshold... 

Termination reactions may lead to the formation of both high and low molecular weight products of the peroxidation reactions. Depending on the lipid, some of the low molecular mass compounds may be important flavours (or aromas) of foods (section 5.2.3). For example, short to medium chain aldehydes formed from unsaturated fatty acids may give rise to rancidity and bitter flavours on the one hand or more pleasant attributes such as those associated with fresh green leaves, oranges or cucumbers on... 

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