Thermal indicators in milk, which had been subjected to one of the six industrial processes of thermization, pasteurization, direct and indirect UHT-sterilization, pre-sterilization and in-bottle sterilization, were studied. The following three indices of heat damage were analyzed by high performance liquid chromatography (HPLC), hydroxymethylfurfural (HMF), lactulose and acid-soluble ␤-lactoglobulin(␤-LG). Average amounts found were 1710 mg/L of ␤-LG and 2.49 mol/L of HMF in pasteurized milk. In UHT milk, the amounts for direct and indirect processes were 389 and 322 mg/L of ␤-LG, 12.0 and 250 mg/L of lactulose and 5.6 and 8.7 mol/L of HMF. In sterilized milk the amounts were 1120 mg/L of lactulose and 22 mol/L of HMF, without any detectable presence of undenatured whey proteins. On the basis of the time/temperature profiles, a sterilization factor, expressed as seconds, was defined for each thermal treatment. By applying discriminant analysis each industrial process could be classified independently at the 95% confidence level (pasteurization, UHT-treatment and in-bottle sterilization), but direct-UHT treated milk could not be discriminated from indirect-UHT milk, nor thermized milk from raw bulk milk. The simultaneous application of several heat-induced parameters improves the classification of industrial processed milks, and is therefore a useful tool for optimization of the processing conditions.

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New analytical techniques were used to study the kinetic behavior of 5-(hydroxmethyl) furfural (HMF) and blockage of available lysine in milk and model systems. Both determination of HMF by the chromatographic method and estimation of available lysine by the ortho-phthaldialdehyde (OPA) method at sterilization temperatures proved valid. Activation energies for the lysine loss reaction were 91.14, 112.41 and 66.67 kJ·mol−1 in the model systems and the milk respectively in fluorimetric determination with OPA. Activation energy for HMF determination ranged from 118.5 to 93.04 kJ·mol−1

Hydroxymethyfurfural (HMF) content and loss of available lysine were measured as indices of heat damage in various Spanish commercial milks (pasteurized, UHT sterilized and in‐bottle sterilized milks) with similar processing dates. HMF level was determined by the traditional colorimetric procedure and by the reversed‐phase HPLC method. Available lysine was measured by an alternative method with o‐phthaldialdehyde as fluorescent marker. A significative relationship has been found between the HMF content and loss of available lysine and both are suitable for use as heat‐induced indices. When loss of lysine (y) was expressed as percentage and HMF (x) as μm, the function of the correlation line was, y=0.47x−0.089 (r=0.958, P<,0.005).

Homogenized lactose/caseinate model systems and milk with different fat content were heated at 110 to 150 °C for up to 30 min. Free and total hydroxymethylfurfural (HMF) were measured by reverse-phase high-performance liquid chromatography. Multifactor analysis of variance was applied to determine whether milkfat content contributed to free and total HMF formation. There was a negative effect of milkfat on the formation of total HMF in both systems. However, free HMF formation was promoted by higher milkfat content in milk, but not in the lactose-caseinate model. Milkfat affected also the free and total HMF formation in commercial pasteurized milk, ultra heat-treated milk, and bottle-sterilized milk.

Delgado-Andrade C., Rufián-Henares J. A., Morales F. J. (2008): Optimised procedure to analyse Maillard reaction-associated fluorescence in cereal-based products. Czech J. Food Sci., 26: 339-346.Fluorescent Maillard compounds measurement provides more specific information on the extent of the Maillard reaction than other unspecific tools to monitor the reaction, and is suitable, as the first approach, to assess the nutritional quality of foods as related to protein damage. This work presents an optimised laboratory procedure for the measurement of total fluorescent intermediate compounds (FIC) associated with Maillard reaction, described and evaluated in a cereal-based product. Total FIC are evaluated using increased pronase E concentrations and different incubation times for the enzymatic hydrolysis, as well as three different sample clean-up steps after the enzymatic digestion. The effects of basic/acid media are considered for the stability of the fluorescent compounds. The standardised procedure is finally applied to breakfast cereals as a model of cereal-based products, analysing the correlation between total FIC production and fibre and protein contents. It is demonstrated that fluorescent compounds are mainly linked to the protein backbone in ready-to-eat breakfast cereals. Fluorescence measurement is presented as an inexpensive, rapid and accurate procedure to study the extent of Maillard reaction in breakfast cereals.

The amounts of two heating-induced products were studied (lactulose and 5-hydroxymethyl-furfural) in UHT milk samples, directly and indirectly heat-treated, during a period of 90 d storage at five different temperatures. The results indicate an increase in 5-hydroxymethyl-furfural, as well as lactulose, taking place in the range of temperatures between 40–50°C.

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