Influence of the heating temperature on the fat composition of milk fat with emphasis oncis-/trans-isomerization

Milk fats from barn and pasture feeding (WMF and SMF) were analysed for changes in the content of fatty acids, triglycerides and cholesterol after 15-minute heating at 200, 225, 250, 275 and 300°C, respectively. The lower temperatures of 200 and 225°C are realistic in cooking and frying processes, whereas temperatures of 250 to 300°C were chosen to show the effects of overheating the fat. The main focus was put on identification and quantitation of individual isomers of C18 : 1 (trans δ4, δ5 to δ16, cis δ9 to δ15) as well as of C18 : 2 including the conjugated linoleic acid (CLA) isomer cis δ9, trans δ11. The trans isomers among these fatty acids are of great importance as regards nutritional physiology. Furthermore, determinations of the content of α-tocopherol and of free fatty acids as well as of the iodine and peroxide value were performed. Primarily, with rising temperature there was an increase of the elaidic acid (trans δ9) content from 0.3% to 0.5% (WMF) and from 0.2% to 0.7% (SMF) between the base fat and 300°C, whereas in this range the vaccenic acid (trans δ11) content decreased from 1.1% to 0.9% (WMF) and from 3.8% to 3.4% (SMF). The other trans-C18 : 1 isomers as well as the cis isomers δ12 to δ15 exhibited no general trend at these temperatures. The content of oleic acid strongly decreased with the WMF (max. 2.90%), whereas there was no comparable decrease with the SMF (max. 0.48%). This effect possibly was attributable to the content of α-tocopherol being higher by relatively 71% in the SMF. At the higher temperatures particularly strong decreases (base fat → 300°C) were found for cis δ9, cis δ12, trans δ11, cis δ15, cis δ9, trans δ11 (CLA) and linoleic acid (WMF: 1.55 → 0.67%, 0.17 → 0.09%, 0.50 → 0.03%, 0.42 → 0.06%; SMF: 1.05 → 0.67%, 0.54 → 0.38%, 1.71 → 0.35%, 0.61 → 0.17%). All together, absolute losses of fatty acids amounted to max. 4.6% and 6.5% (SMF and WMF, resp.). As losses of linoleic and linolenic acids were not compensated by formation of isomerization products, formation of oxidation products possibly having a high health-threatening potential must have taken place to a greater extent. On the other hand, a great part of these oxidation products are volatile and their amount in the remaining fat was not recorded by the analytical method applied. Anyway, isomerization and oxidation are low at 200°C, but these undesirable effects strongly increase with higher temperatures.