The effect of heat treatment and storage time in the formation of cholesterol oxides and alteration of the fatty acids composition in eggs. / Efeito do processamento termico e tempo de estocagem na formação de oxidos de colesterol e na alteração da composição de acidos graxos em ovos.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

A new method was determined for the simultaneous determination of cholesterol and cholesterol oxides in eggs, using high performance liquid chromatography (HPLC) with ultra-violet (UV) and refractive index (RI) detectors. External standardisation was used for quantification and the cholesterol and cholesterol oxides were confirmed by liquid chromatography interfaced to atmospheric pressure chemical ionisation coupled to mass spectrometry (APCI-MS). The best conditions for sample saponification and extraction of the non-saponifiable matter were defined using complete factorial designs with central points. Under the chromatographic conditions used, cholesterol was separated from the following oxides: 19-hydroxycholesterol (19-OH), 20a-hydroxycholesterol (20a-OH), 22(R)-hydroxycholesterol (22(R)- OH), 24(S)-hydroxycholesterol (24(S)-OH), 22(S)-hydroxycholesterol (22(S)- OH), 25-OH, 7-keto, 7b-OH, 7a-OH, 5,6a-epoxy, 5,6b-epoxy and triol. The proposed method was shown to be higly sensitive and precise, with recovery of cholesterol from 92 to 102% and of cholesterol oxides from 93 to 96%. The DL found for the cholesterol oxides varied from 0.002 to 0.079 mg/g and for cholesterol it was 0.026 mg/g. The QL values encountered were from 0.002 to 0.042 mg/g for the cholesterol oxides and 0.088 mg/g for cholesterol. The cholesterol content determined for the certified reference material was 19.0 ± 0.2 mg/g of egg yolk, identical to the value declared on the certificate. A comparative study was carried out between the method of lipid extraction according to Folch et al. (1957) followed by methyl ester preparation according to Joseph &Ackman (1992) by saponification and methylation with boron trifluoride, and the method of direct methylation according to Wang et al. (2000). The method of lipid extraction followed by esterification presented more precise and exact results and was thus validated using the certified powdered egg reference material (SRM 8415). The cholesterol, cholesterol oxide, total lipid and fatty acid contents were determined in samples of egg powder, eggs enriched with omega 3 and normal eggs. Two commercial brands of egg powder were stored at 25 ± 2ºC in the dark and analysed at zero time and subsequently every month during the period of their shelf lives (6 and 12 months, respectively). Two batches of eggs enriched with omega 3 and normal eggs were stored in the dark at 25 ± 2ºC and at 5 ± 1ºC in the refrigerator, for 45 and 21 days respectively. The samples were analysed at zero time and at 3 further periods during storage. At each time, for each type of storage, 36 eggs were analysed, divided into 3 parts. One of the parts (12 eggs) was analysed raw and the other two were submitted to two types of heat treatment, boiling and frying. The cholesterol content did not decrease in any of the samples during storage. In both the eggs enriched with omega 3 and the normal eggs, the lowest cholesterol contents were observed in the fried eggs. Five oxides were identified in the powdered eggs: 7-keto, 7b-OH, 7a- OH, 5,6a-epoxy and 5,6b-epoxy, and their contents increased during storage. Three oxides were identified in the eggs enriched with omega 3 and the normal eggs: 7-keto, 7b-OH and 7a-OH, their concentrations being higher in the fried eggs. During storage the behaviour of the oxides varied according to the type of egg. In the eggs enriched with omega 3, the concentrations of the oxides 7-keto and 7a-OH increased during storage. On the other hand, the 7b-OH content decreased up to time 1 in the cooked and raw samples but increased during storage in the fried samples. In normal eggs, the 7-keto content increased from zero to time 1 and then reduced up to the end of storage in both the raw and heat-treated samples. For 7b-OH, the content increased from zero to time 2 and then remained constant. For 7a-OH, the increase was from zero to time 1, subsequently decreasing. In both the normal eggs and those enriched with omega 3 the different storage conditions only influenced the oxide 7-keto, which was greater in eggs stored at 25ºC. In egg powder, the total lipid content was 35 g/100g and remained constant during the storage period. In the eggs enriched with omega 3 and the normal eggs, the total lipid contents decreased in the fried eggs. The principal fatty acids determined in all the samples were: C14:0, C16:0, C18:0, C16:1 n7, C18:1 n9, C18:2 n6, C20:4 n6 and C18:3 n3. In addition, C22:6 n3 and C18:1 n9 trans were principal components in the eggs enriched with omega 3 and the egg powder respectively. In all samples the unsaturated fatty acid contents decreased during storage, evidence that lipid oxidation had occurred with the production of free radicals which contributed to the increase in cholesterol oxides. In the eggs enriched with omega 3 and the normal eggs, the heat treatments, specifically frying, reduced the unsaturated fatty acid concentrations.

ASSUNTO(S)

fatty acids eggs colesterol acidos graxos cholesterol cholesterol oxides ovos oxidos de colesterol

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