Oxidized flavor stability of milk is highly associated with consumer acceptance. This study characterized oxidized flavor stability as affected by milk concentration. The selected volatiles in raw milk (RM), heated milk (HM), concentrated milk, and distillate were investigated using solid-phase microextraction-gas chromatography-mass spectrometry. Concentrated milk and distillate showed higher levels of heptanal (14.52-17.24μg/kg), nonanal (5.08-8.64μg/kg), 2-heptanone (25.22-73.25μg/kg), and 2-nonanone (10.83-17.28μg/kg) compared with RM and HM. Based on the odor activity values of different volatiles, oxidized volatiles may contribute to fatty, greasy, soapy, and dyestuff oxidized odors in concentrated milk. Oxidized volatiles of nonanal, 2-heptanone, and 2-nonanone were found to be higher in concentrated milk at 50°C than at 40°C. In addition, 18 and 24 volatiles were found in unconcentrated milk and distillate, respectively. Straight aldehydes and methyl ketones showed much higher levels in distillate than in RM and HM, which also had higher levels compared with their corresponding concentrated milk samples. These results indicated that oxidized volatiles were mainly evaporated into distillate as flavor loss. They also suggested that lipid oxidation could continue drastically even under the concentration processing at a low temperature and reduced pressure.

The quality of milk powder can decrease during storage. In this study, the characteristics of 12 kinds of commercial milk powder from China were investigated. Changes in various indicators were tracked to comprehensively estimate the oxidation stability of different commercial milk powders. The components of the commercial milk powder were different. The percentages of milk fat, protein, and carbohydrates ranged from 9.8 to 28.5 g/100 g, 15.0 to 24.0 g/100 g, and 32.0 to 67.5 g/100 g, respectively. The water activities ranged from 0.2394 to 0.5286. The diameters of the milk fat globules in different commercial milk powder ranged from 13.99 to 41.09 nm. At the same time, the peroxide value of the control sample was low (≤0.14 mEq/kg). After 3 mo of storage, the peroxide values of some of the commercial milk powder increased significantly. The changes in the thiobarbituric acid values during storage did not follow a common trend. The contents of free fat in the different control samples were 0.21 to 1.67 g/100 g, and these values did not increase during storage. After 3 mo of storage, the hydroxymethyl furfural values and b color values of the different commercial milk powder reached their highest levels. The concentrations of typical oxidized flavor compounds in different commercial milk powder increased greatly with prolonged storage time. The level of hexanal was the highest, and the contents in all the samples ranged from 28.56 to 4,071.28 μg/kg after 6 mo of storage and from 5.91 to 6,281.37 μg/kg after 12 mo of storage. Free radicals were found in some of the stored milk powder, and these were shown as single peaks or multiple peaks. The ratios of the peak areas and masses reached 12.42 × 10 6 to 14.26 × 10 8 . However, the presence of free radicals in the commercial milk powder was not consistent. The water activities and diameters of the fat globules in the commercial milk powder were highly correlated with their oxidation stabilities during storage.

Interactions between stabilizer and milk protein are believed to influence the stabilizing behavior of the milk system. We investigated changes in fluorescence intensity induced by interactions of soybean soluble polysaccharide (SSPS) and milk protein (Mp) during acidification. The fluorescence intensity (If) of Mp increased as pH decreased from 6.8 to 5.2. Compared with Mp alone, If of SSPS-Mp mixtures increased as the pH decreased from 6.8 to 5.2. We found that the If of the SSPS-Mp mixture decreased in a pH range from 5.2 to 3.6, which indicated a change in the polarity microenvironment around the Trp residues. We also found that the maximum emission wavelength (λmax) shifted from 337 to 330nm as pH decreased from 6.8 to 3.6, in further support of SSPS interacting with the polar portion of Mp during acidification. Furthermore, an excited monomeric molecule (pyrene exciplex) was found as a ground-state pyrene formed and a broad band was shown at about 450nm. The intensity ratio of the first peak to the third peak (I1:I3) of Mp increased slightly, and the ratio of intensity of pyrene exciplex to monomer (Ie:Im) decreased because pyrene molecules were located in a less hydrophobic microenvironment during acidification. However, the ratio of I1:I3 decreased clearly at pH below 5.6 and the ratio of Ie:Im showed the opposite trend in the SSPS-Mp mixture. Changes in intrinsic and exogenous fluorescence intensity confirmed that interactions of SSPS and Mp could change the polarity of the microenvironment and that SSPS probably interacted with the polar portion of Mp. These results could give insight into the behavior of stabilizers in acid milk products.