The effects of exposure of slices of Havarti cheeses to monochromatic light of wavelengths 366 nm, 405 nm, and 436 nm, respectively, were studied by tristimulus colorimetry, solid-phase microextraction gas chromatographic analysis of volatiles, and open-end fluorescence spectroscopy. Having determined the photon fluxes of the three wavelengths by ferrioxalate actinometry, it was possible to quantify the effects of light exposure in an absolute manner. For all analyses, the most severe effects were caused by visible light, leading to colour bleaching, change in hue, riboflavin degradation, and formation of the secondary oxidation products hexanal, 1-pentanol, and 1-hexanol. Apparent quantum yields for formation of hexanal and 1-pentanol were found to be insignificantly different for 405 nm and 436 nm exposures, having values of (3–5).10−5 mol.einstein−1 and (9–13).10−5 mol.einstein−1, respectively. These compounds were not formed when exposed to 366 nm light. In contrast, 1-hexanol was formed when exposing cheese to all three wavelengths, resulting in apparent quantum yields of (2–6).10−5 mol.einstein−1. The results obtained are discussed in relation to the interplay between inherent product colorants, light sources, and transmission characteristics of the packaging materials.

β-Lactoglobulin (β-lg) in aqueous solution under pressure showed a marked depolarization of intrinsic fluorescence assigned to a gradually increased rotational diffusion of tryptophyl moieties in pressure-unfolded states. The corresponding change in anisotropy provided a new and more accurate method for determining denaturation volume which, for β-lg in neutral aqueous solution with ionic strength 0·16 (NaCl) at 25 °C, was ΔV°=−73 (SE 3) ml mol−1, corresponding to half denaturation at 123 MPa. The pressure unfolding led to exposure of hydrophobic regions to the protein–water interface that could be probed by fluorescence intensity of a β-lg–1-anilinonaphthalene-8-sulphonic acid (ANS) complex with 1:1 stoichiometry, as determined by Job's method of continuous variation. The unfolding of β-lg impaired the binding capacity of the inner calyx, with a reduction in binding capacity of 50% at 50 MPa, as shown by decreasing cis- parinaric acid fluorescence, decreasing anisotropy and decreasing radiationless energy transfer from tryptophans to this probe with increasing pressure. The pressure-induced reversible exposure of hydrophobic groups to the protein–water interface may, at least partly, explain the initial aggregation reactions, evident from increased Rayleigh scattering from ∼50 MPa, prior to irreversible pressure-induced gel formation of β-lg. Using results from this and previous studies, we propose a three step pressure denaturation model for β-lg for neutral solution at ambient temperature, including an initial pressure-melted state (up to 50 MPa) with partial collapse of the inner calyx and solvent exposure of the free thiol group, followed by a reversible denaturation with exposure of hydrophobic regions (half denaturation at 123 MPa) and with irreversible denaturation with thiol–disulphide exchange becoming increasingly important at higher pressures. Effects of pressure on β-lg, as measured by fluorescence depolarization, were found for the reversible denaturation steps to be similar to the effects of chemical denaturants but different with respect to shift in ANS emission maxima.

The light stabilities of two commercial colorants for Cheddar cheese, an annatto extract and a β-carotene preparation, have been compared in a model system and tested in industrial scale production. The apparent quantum yields for photodegradation by monochromatic light were determined at 25 °C for solutions of the colorants in air-saturated 10 mm-citric acid–disodium hydrogen phosphate buffer containing 100 mg sodium caseinate/l at pH 5·2 and 5·4. Solutions of commercial β-carotene were more photostable than solutions of annatto, and although the quantum yields for photodegradation for both solutions depended significantly on both pH and irradiation wavelength, exposure to u.v. light (313 and 366 nm) caused more photobleaching than exposure to visible light (436 nm). The differences in colour stability observed in the model system were verified by tristimulus colorimetry during 9 weeks illuminated storage at 5 °C of Cheddar cheese produced from milk to which had been added (per 1000 kg) 67 g β-carotene extract or 133 ml annatto.

Lipid oxidation in milk and dairy products is a chain reaction initiated by formation of free radicals (Richardson & Korycka-Dahl, 1983). Thanks to intensive studies on both model systems and actual food, the autocatalytic process, including the formation of secondary lipid oxidation products from the lipid hydroperoxides formed initially, is fairly well understood. However, actually predicting the rate at which the first free radicals leading to spontaneous oxidation are formed in milk from different cows awaits the development of new analytical methods with higher specificity and sensitivity (Nicholson, 1993; Barrefors et al. 1995). Such methods would also be valuable for predicting the stability and shelf life of dried dairy products, which are determined by oxidative phenomena. Electron spin resonance (ESR) spectrometry has the potential for detecting the early events in lipid oxidation, as it is the only spectrometric method that will directly detect the unpaired electron characteristic of the free radical and it is, moreover, a highly sensitive method (Brudvig, 1995). ESR spectrometry has recently been shown to provide quantitative information on the level of free radicals in milk powder that correlates with the level of secondary oxidation products developed upon reconstitution and that also correlates with subsequent sensory evaluation (Nielsen et al. 1997; Stapelfeldt et al. 1997a, b, c). However, in order to explore further the potential of this method for raw milk, it was considered valuable to measure the tendency of milk to form free radicals in relation to its level of α-tocopherol, the most important lipophilic chain-breaking antioxidant (cf. Kamal-Eldin & Appelqvist, 1996).

Hydrolysis of β-lactoglobulin B (β-lg B) by pepsin, a process slow at ambient conditions, is facilitated at a moderately high hydrostatic pressure such as 300 MPa, corresponding to an apparent volume of activation ΔV# = −63 ml mol−1 at pH 2·5, 30 °C and Γ/2=0·16. Digestion of β-lg by trypsin and thermolysin is likewise enhanced by pressure, and the pressure effect has been traced to pressure denaturation of β-lg B, which by high-pressure fluorescence spectroscopy has been shown to have a large negative volume of reaction, ΔV° = −98 ml mol−1, at pH 6·7, 30 °C and Γ/2 = 0·16. Pressure denaturation is only slowly reversed following release of pressure and the enhanced digestibility is maintained at ambient pressure for several hours.

Each of the secondary lipid oxidation products pentanal, hexanal and heptanal was found to react with β–lactoglobulin (β–lg) in a two-phase model System (aqueous phosphate buffer–1-octanol) yielding fluorescent condensation products (emission maximum, 410 nm; excitation maximum, 350 nm). Protein polymers were detected by size-exclusion HPLC, and the rate of reaction paralleled the formation of fluorescent products, with the reactivity being pentanal > hexanal > heptanal. Simultaneously, the reaction also changed the intrinsic fluorescence of β–lg, and in particular pentanal reduced the intensity of tryptophan fluorescence (emission maximum, 332 nm; excitation maximum, 288 nm) by 30%. These findings are discussed with reference to the effect of peroxidizing lipids on the physical properties of whey proteins and the use of protein fluorescence (induced by the reaction with aldehydes) as marker for the oxidative status of milk and whey protein products.