PTR-TOF-MS analysis for influence of milk base supplementation on texture and headspace concentration of endogenous volatile compounds in yogurt

In the present study, the effects of milk fat (0.3% and 3.5% w/w), solids non-fat (8.4% and 13% w/w), and modified tapioca starch (0%, 0.5%, 1.0%, 1.5%, and 2.0% w/w) concentrations on the textural and physicochemical properties as well as the concentration of several endogenous flavor compounds in the headspace of set and stirred yogurts were investigated. The novel proton transfer reaction time-of-flight mass spectrometry technique was implemented for the non-invasive determination of the amounts of volatile organic compounds in the samples headspace. Milk fat and skim milk powder supplementation of the milk samples increased significantly the firmness and adhesiveness of yogurts (p<0.001) and improved the stability of the formed gels by increasing their water holding capacity and reducing the amounts of expulsed whey (3.94 and 5.1 g for the milk fat and SNF-fortified samples). Acetaldehyde was significantly (p<0.001) higher in the low fat-unfortified systems (6.15±0.48 and 5.6±0.60 ppmv, respectively). A similar trend was also reported in the case of 2-propanone (0.91±0.11 and 1.13± 0.07 ppmv), diacetyl (334±37 and 350±34 ppbv), 2,3-pentanedione (54±6 and 55±6 ppbv), and 2-butanone (56± 7 and 68±5 ppbv) for the same systems. In contrast, the concentration of flavor compounds in the headspace with hydroxyl groups (ethanol and acetoin) increased (p<0.001) by solid non-fat fortification of milk base (350±32 and 206±7 ppbv, respectively, for the systems fortified with skim milk powder). Modified tapioca starch addition improved the textural properties and gel stability of yogurts whereas affected only the ethanol concentration (222±16 and 322±55 for the control and 2.0% w/w containing systems, respectively). Our data suggested that the reinforcement of textural and structural properties combined with the protein binding affinity of the flavor compounds seemed to be responsible for the aforementioned observations. In the case of stirred yogurts, the gel breakdown did not provoke significant changes in the headspace concentration of the most compounds, with the exception of ethanol, acetoin, and 2,3-pentanedione being significantly (p<0.05) higher in the stirred yogurts (267±29, 153±11, and 38±1 ppbv, respectively) than set style ones (232±19, 134±9, and 45±3 ppbv, respectively)