Water- and fat-soluble vitamins in milk after different thermal treatments and steam injection.

To prepare a good cappuccino, it is preferable to use fresh whole milk as its high-fat content contributes to the flavour and its protein content is useful for a good whipping of the milk contributing to its creaminess. The foam is produced by the nozzle of the coffee machine, which is necessary to obtain a compact cream, but it is also important that the temperature of the milk must not exceed 65-70°C, to avoid an unpleasant aroma and the lack of foam formation. Furthermore, the nutritional quality of the milk should remain unchanged after the steam injection treatment. In the present work, we focused the attention on the water- and fat-soluble vitamins content in pasteurized bovine milk samples (HQ-milk) treated at different temperatures (40-80°C) and steam injected pressure using a nozzle of Simonelli Group coffee machine. The treatment time was 20, 30, 40, 50, and 60 seconds. Water- and fat-soluble vitamins content on the treated HQ-milk has been determined by RP-HPLC using a C18 column (HiQSil C18 HS, 5 μm, 4.6 mm i.d, 250 mm, Kya Tech Corporation), connected to an HPLC Agilent 1260 Infinity LC System (Agilent Technologies), according to two different protocols from Albalá-Hurtado and co-workers [1, 2]. Quantification was achieved by a calibration curve obtained relating the concentrations (M) of each vitamin standard loaded in the column to the peak area corresponding to each concentration. The results showed that if the only temperature is applied, vitamin B2, nicotinic acid, and nicotinamide were not affected by the increasing temperature because the exposure time from 20 to 60 seconds is too short to cause a decrement in the vitamins content. However, when the HQ-milk is treated with increasing temperature together with steam injection, an interesting result regarding the nicotinic acid concentration was obtained. In fact, the concentration of nicotinic acid increases by about 6.5 folds starting from 7.5 M at 25°C to reach a mean value of 45 M at a temperature range of 50-70°C. It is also interesting to note that the increase of nicotinic acid concentration is time-dependent: if the HQ-milk is treated for 20 seconds with the steam injection (time normally used to heat the milk during the preparation of the cappuccino), nicotinic acid reaches its highest concentration (54 M) at 70°C, but if the milk is treated for 60 seconds, the highest concentration (43 M) is reached at 50°C. Nicotinic acid (Niacin) is used to prevent and treat niacin deficiency that may result from alcohol abuse, malabsorption syndrome, Hartnup disease, poor diet. Furthermore, nicotinic acid is used to treat dyslipidemias since reduces triglycerides and LDL cholesterol and raises HDL cholesterol levels [3]. In our experiments, we observed that the increase in nicotinic acid dependent on the steam injected and the treatment time is accompanied by a simultaneous decrease in quinolinic acid. The latter is generated within the kynurenine pathway resulting from L-tryptophan catabolism and may be present in the milk since it is related to the composition of the cows’ diet [4]. Quinolinic acid may be converted to nicotinic acid by a decarboxylation reaction induced by the temperature and the steam injected pressure [5]. However, this hypothesis needs to be further confirmed. On the contrary, nicotinamide and B6 decrease when temperature and steam injection increased, whereas vitamin B2 resulted not affected by the treatment. Among fat-soluble vitamins, the results showed that vitamin E and vitamin D3 did not undergo variations after temperature and steam injection treatment while vitamin A was slightly affected. In conclusion, these results indicate that the nutritional quality of HQ-milk, from the point of view of the concentration of vitamins, seems not influenced by the thermal treatment and by the injected steam. It was observed, after this treatment, an increase in milk nicotinic acid content, very interesting considering the beneficial effects on human health exerted by this vitamin.