Quantitative Determination of Thermally Derived Off-Flavor Compounds in Milk Using Solid-Phase Microextraction and Gas Chromatography

Abstract

Many volatile compounds generated during the thermal processing of milk have been associated with cooked, stale, and sulfurous notes in milk and are considered as off-flavor by most consumers. A headspace solid-phase microextraction (HS-SPME)/gas chromatographic technique for the quantitative analysis of thermally derived off-flavor compounds was developed in this study. The extraction temperature, time, and sample amount were optimized using a randomized 2(3) central composite rotatable design with 2 central replicates and 2 replicates in each factorial point along with response surface methodology. Calibration curves were constructed in milk using the standard addition technique, and then used to quantify 20 off-flavor compounds in raw, pasteurized, and UHT milk samples with various fat contents. The concentrations of these volatiles in raw and pasteurized milk samples were not significantly different. However, dimethyl sulfide, 2-hexanone, 2-heptanone, 2-nonanone, 2-undecanone, 2-methylpropanal, 3-methylbutanal, heptanal, and decanal were found at higher concentrations in UHT milk as compared with raw and pasteurized milk samples. In addition, the concentration of methyl ketones was greater in UHT milk with higher fat content. The calculated odor activity values suggested that 2,3-butanedione, 2-heptanone, 2-nonanone, 2-methylpropanal, 3-methylbutanal, nonanal, decanal, and dimethyl sulfide could be important contributors to the off-flavor of UHT milk. The HS-SPME technique developed in this study is accurate and relatively simple, and can be used for the quantification of thermally derived off-flavor compounds in milk.

Multi-channel open tubular traps for headspace sampling, gas chromatographic fraction collection and olfactory assessment of milk volatiles

“…Higher concentrations were found of, amongst others, the sulphur compound dimethyl sulphide, the ketones: 2-hexanone, 2-heptanone, 2-nonanone and the aldehydes: 2-methylpropanal, 3-methylbutanal and decanal in UHT milk when compared to concentrations measured in raw and pasteurised milk [3]. Calculated odour activity values (OAV) for dimethyl sulphide, 2,3-butanedione, 2-heptanone, 2-nonanone, 2-methylpropanal, 3-methylbutanal, nonanal and decanal show that these compounds may well be important contributors to the off-flavour of UHT milk [3]. The formation of 2,3-butanedione is however not only heat-induced but can also point to microbial activity in milk [3].…”

“…Powerful odour active volatiles detected in heated milk include 2-heptanone, 2-nonanone and nonanal, with 2-heptanone and 2-nonanone identified as the most intense volatile flavour compounds of UHT milk [12]. Higher concentrations were found of, amongst others, the sulphur compound dimethyl sulphide, the ketones: 2-hexanone, 2-heptanone, 2-nonanone and the aldehydes: 2-methylpropanal, 3-methylbutanal and decanal in UHT milk when compared to concentrations measured in raw and pasteurised milk [3]. Calculated odour activity values (OAV) for dimethyl sulphide, 2,3-butanedione, 2-heptanone, 2-nonanone, 2-methylpropanal, 3-methylbutanal, nonanal and decanal show that these compounds may well be important contributors to the off-flavour of UHT milk [3].…”

“…The thicker film column was less sensitive to any residual moisture which improved FID signal stability and also retention time repeatability of the early eluting compounds. Table 2 shows the results of the linear regression analysis of the calibration (n=8) based on the method of standard addition [3]. 2,3-Butanedione was excluded from the quantification results because the purity of the purchased reference standard was compromised.…”

“…Calculated odour activity values (OAV) for dimethyl sulphide, 2,3-butanedione, 2-heptanone, 2-nonanone, 2-methylpropanal, 3-methylbutanal, nonanal and decanal show that these compounds may well be important contributors to the off-flavour of UHT milk [3]. The formation of 2,3-butanedione is however not only heat-induced but can also point to microbial activity in milk [3].…”

“…The ultra high temperature (UHT) treatment of milk imparts an undesirable cooked (cabbage, sulphurous, stale, heated, sterile milk) flavour to packaged long life milk [1][2][3][4]. Analytical methods that have been used to study the aroma of dairy products are gas chromatography mass spectrometry (GC-MS) and gas chromatography olfactometry (GCO) [5][6][7][8].…”