Henrik Stapelfeldt

Light and heat sensitivity of red cabbage extract in soft drink model systems

Anthocyanins of red cabbage (Brassica oleracea L.) display colour over a larger range of pH-values than the majority of anthocyanins from other natural sources, being pink at pH 3, violet at pH 5 and blue at pH 7. Using reversed phase high-performance liquid chromatography with diode array detection, cyanidin was found to be the only anthocyanidin of red cabbage extract and, tentatively, identified as cyanidin-3,5-diglucoside and cyanidin-3-sophoroside-5-glucoside, the latter found in various acylated forms. The apparent quantum yield for photobleaching has been determined for red cabbage extract at 25°C in air-saturated McIlvaine buffer, using monochromatic light at each of the irradiation wavelengths, 313, 366, and 436 nm, in continuous photolysis experiments, in order to provide an objective measure for the sensitivity of this food colorant to ultraviolet and visible light. The quantum yield was found to depend on both pH and irradiation wavelength, ranging from 0.2×10−4 mol einstein−1 for 436 nm light at pH 7.0 to 3.7×10−4 mol einstein−1 for 366 nm light at pH 5.0. The thermal stability at pH 3.0 in McIlvaine buffer of four different anthocyanin extracts was compared and, for the temperature range investigated (25–80°C) the degree of stability was red cabbage>blackcurrant>grape skin>elderberry. The thermal stability of the same anthocyanin extracts was also compared for a non-carbonated soft drink medium of pH 3.0 yielding the same order of stability but with rates of degradation approximately twice as high as in buffer, which may indicate a detrimental effect of sugar and ascorbic acid. Due to the high thermal stability of red cabbage extract in solution, photobleaching will be the primary destabilising factor for red cabbage anthocyanin-coloured products in display.

Effect of heat treatment, water activity and storage temperature on the oxidative stability of whole milk powder

Abstract The oxidative status of high-heat, medium-heat and low-heat whole milk powder was investigated at moderately accelerated storage conditions, with exposure to atmospheric air at 25 or 45 °C and at three water activities (0.11, 0.23 and 0.33 at 25 °C, and 0.11, 0.17 and 0.31 at 45 °C) for 2 months using: (i) electron spin resonance spectrometry for measurement of the level of free radicals, (ii) determination of thiobarbituric acid reactive substances (TBARS) as a measurement of secondary lipid oxidation products, (iii) size-exclusion HPLC for measurement of fluorescent protein polymerization products, and (iv) sensory evaluation. Lipid oxidation was affected greatly by storage temperature, with a maximum level of free radicals being detected after 47 days at 45 °C, and with the highest level in low-heat powder, irrespective of water activity. The sensory quality dropped to an unacceptable level for low-heat powder within 33 days of storage, as confirmed by measurement of TBARS, and the increasing TBARS value was parallelled by a decrease of ‘free’ thiol groups to an unmeasurable level in low-heat powder, in contrast to medium- and high-heat powders, in which the initial level of free thiol groups was only reduced by one-third after 63 days of storage. In contrast to common beliefs, initial powder quality was retained best at water activities between 0.11 and 0.23, where powders showed no significant differences in sensory quality, irrespective of preheat treatment. No difference in sensory score was found for storage at different water activities at 25 °C, whereas storage at water activity 0.31 at 45 °C markedly affected all powders. This effect was ascribed to a combination of increased autoxidation rate and increased Maillard reaction rate giving rise to protein polymerization, in effect decreasing the powder solubility, as often seen during storage under tropical conditions.

Effect of high hydrostatic pressure on the enzymic hydrolysis of β-lactoglobulin B by trypsin, thermolysin and pepsin

Hydrolysis of beta-lactoglobulin B (beta-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 delta V# = -63 ml mol-1 at pH 2.5, 30 degrees C and gamma/2 = 0.16. Digestion of beta-lg by trypsin and thermolysin is likewise enhanced by pressure, and the pressure effect has been traced to pressure denaturation of beta-lg B, which by high-pressure fluorescence spectroscopy has been shown to have a large negative volume of reaction, delta V(o) = -98 ml mol-1, at pH 6.7, 30 degrees C and gamma/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.

Light-induced changes in packaged cheeses: a review

Abstract This review presents current knowledge on light-induced effects on packaged cheeses. As research in this area is somewhat limited and involves highly non-standardized light exposure conditions, the review includes, if deemed necessary, effects observed in other dairy products. Most of these effects may be explained by general lipid oxidation mechanisms combined with knowledge on the spectral balance between the singlet oxygen quencher, β -carotene, and the sensitizer, riboflavin. As determined by Lambert-Beers law, β -Carotene absorbs light in a concentration-dependent manner, which would otherwise be absorbed by riboflavin, thereby inducing quality changes. Consequently, these processes may be prevented by total exclusion of light and by storage in an oxygen-free atmosphere. Unfortunately, quality changes are apparent at residual oxygen levels even as low as 0.5% in headspace, which is often considered an acceptable residual oxygen level in industry. For a given product composition, spectral distribution and photon flux of the light source determine the extent of quality changes, since photochemical processes have limited temperature dependence, in contrast to the consecutive lipid autoxidation process. Hence, precautionary measures include changes of light source and targeted prevention of photon flux relative to the cheese by use of creative packaging. In order to optimize packaging and display conditions, a substantial need exists for analytical methods, which reflect the sensory perception of the consumer. Once established, optimization to include marketing and consumer aspects will harbor no major obstacles.

Early prediction of the shelf-life of medium-heat whole milk powders using stepwise multiple regression and principal component analysis

Abstract Fifteen medium-heat whole milk powders manufactured at the same plant under identical processing conditions from 15 different batches of raw milk were subjected to accelerated storage at 50 °C and a water activity of 0.31 in air. With a view to the development of a new prediction method, 22 chemical and technological variables and 13 time-dependent changes over 2 days were measured at the beginning of the experiment. The development of oxidized flavour in the stored powders was subsequently assessed weekly for 6 weeks by a sensory panel. The shelf-life of each powder was determined as the time at which the mean flavour score crossed a threshold value (8 or 6 on a scale from 15 to 0). The relationship between shelf-life and the properties of the fresh powders, together with the time-dependent changes, was investigated by the use of Stepwise multiple regression with maximum R2 improvement, and pseudo-optimal sets of regression equations were found which minimized the PRESS (predicted sum of squares) statistic. The initial time-dependent change of TBA-reactive substances was the variable which correlated best (r = −0.66) with the shelf-life at flavour threshold 8, followed by the yellowness Hunter b-value of the powder (r = 0.54), while the variables which alone correlated best with the shelf-life at flavour threshold 6 were the density of milk powder after 1250 tappings (r = 0.58), TBA-reactive substances (r = −0.56), and the density of milk powder after 100 tappings (r = 0.53). The variables selected by PRESS minimization for the regression equation of threshold 8 (R2 = 0.949) were the α-lactalbumin content, the time-dependent change in β-lactoglobulin, the time-dependent change in TBA-reactive substances, the density after 1250 tappings, and the solubility index. The variables selected for the regression equation of threshold 6 (R2 = 0.980) were the α-lactalbumin content, the time-dependent change in β-lactoglobulin, the density after 1250 tappings, the total fat content, the time-dependent change in the redness Hunter a-value of the powder, and the initial moisture content. Good predictive abilities of the regression equations were indicated by small PRESS values, and the method appears ready for validation in industrial production. Differences between the milk powders were recognized by principal component analysis. One powder deviated clearly, but not extremely, from the other 14 powders on the first and second principal components, mainly because of a large initial amount of free radicals, a rapid generation of further free radicals, and a low initial concentration of fluorescent oxidation products. The first 7 principal components explained only 84% of the total variation, and no other groupings of the milk powders were apparent.

Pressure denaturation and aggregation of β-lactoglobulin studied by intrinsic fluorescence depolarization, Rayleigh scattering, radiationless energy transfer and hydrophobic fluoroprobing

beta-Lactoglobulin (beta-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 beta-lg in neutral aqueous solution with ionic strength 0.16 (NaCl) at 25 degrees C, was delta V degree = -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 beta-lg-1-anilinonaphthalene-8-sulphonic acid (ANS) complex with 1:1 stoichiometry, as determined by Jobs method of continuous variation. The unfolding of beta-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 approximately 50 MPa, prior to irreversible pressure-induced gel formation of beta-lg. Using results from this and previous studies, we propose a three step pressure denaturation model for beta-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 beta-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.

Effect of specific wavelengths on light-induced quality changes in Havarti cheese

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) x 10(-5) mol x einstein(-1) and (9-13) x 10(-5) mol x 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) x 10(-5) mol x 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.

Warmed-over flavour in cooked sliced beef Chemical analysis in relation to sensory evaluation

ZusammenfassungAuf der Grundlage der Autofluorescenz der extrahierten Lipidfraktion wurde eine Analysemethode zur Feststellung des Umfangs des Aufwärmgeschmacks (WOF) in gekochtem Rindfleisch untersucht. In einem Kühllagerungsversuch mit wiedererwärmtem, gekochtem Rindfleisch wurde das Verfahren zusammen mit der zur Messung der WOF-Entwicklung üblichen Extraktionsbestimmung von Thiobarbitursäure-reaktiven Substanzen (TBARS) verwendet. Die objektiven Messungen wurden mit sensorischen Beurteilungen korreliert, dabei stellte sich heraus, daß sowohl die TBARS-Bestimmung als auch die Fluorescenzmethode geeignet sind, die Entwicklung von WOF in wiedererwärmtem, gekochtem Rindfleisch während der Lagerung zu überwachen. Jedoch wurde festgestellt, daß die TBARS-Bestimmung im Vergleich mit der Fluorescenzanalyse eine genauere Übereinstimmung mit der sensorischen Beurteilung aufwies. Für hohe TBARS-Werte und für hohe Konzentrationen von Fluorescenz-Oxidationsprodukten wird der Eindruck vom WOF-Aroma (Geruch) gesättigt und ist somit ein Beispiel des Weber-Fechner-Gesetzes, welches den sensorischen Eindruck als eine logaritmische Funktion der auslösenden Verbindung(en) beschreibt. Praktisch läßt sich aus diesem Ergebnis schließen, daß eine Verdopplung des TBARS-Wertes einer 30%-Erhöhung des WOF-Grades entspricht.SummaryA technique for analysing the extent of warmed-over flavour (WOF) in cooked beef based on autofluorescence of the extracted lipid fraction has been studied. In a chill storage experiment with reheated, oven-cooked beef, the method has been used together with the commonly used determination of thiobarbituric acid reactive substances (TBARS) by extraction, to measure the development of WOF. The objective measurements were correlated to sensory evaluations showing that the TBARS and the fluorescence methods are each suitable for monitoring the development of WOF in reheated cooked beef during storage. However, determination of TBARS was found to agree more precisely with the sensory evaluation than the fluorescence analysis. The impression of WOF smell becomes saturated for high TBARS values and high concentrations of fluorescent oxidation products, and constitutes an example of Weber-Fechners law describing sensory impression as a logarithmic function of the triggering compound(s). A practical implication of this result is that a doubling of the TBARS value corresponds to a 30% increase in the degree of WOF.

Light sensitivity of elderberry extract. quantum yields for photodegradation in aqueous solution

The quantum yield for photobleaching of a commercial elderberry (Sambucus nigra L.) extract, mainly composed of anthocyanins, has been determined at 25°C for air-saturated 10 mM citrate buffer solutions, using monochromatic light at each of the wavelengths 313 nm, 366 nm, and 436 nm, respectively, in continuous photolysis experiments, in order to provide an objective measure for the sensitivity of this food colorant to ultraviolet and visible light. In the pH region investigated, pH 3.0-3.8, typical of fruit-based products, the quantum yield for photodegradation was found to depend only on the wavelength of irradiation, being 2.1 ±0.2.10-4 mol·einstein-1 at 313nm, 1.6±0.2.10-4mol.einstein-1 at 366 nm, and 2.8±0.6.10-5mol.einstein-1 at 436 nm, whereas solution pH only affects the colour intensity. Results show that light bleaching will be the major destabilising factor for anthocyanin-coloured products in display and that exclusion of uv-light will greatly improve their colour stabilty.

Fluorescence of carotenoids. Effect of oxygenation and cis/trans isomerization

Abstract C 40 carotenoids fall, with respect to fluorescence in homogeneous solution, into two distinct groups depending on the presence of a CO group in the molecule. Excitation spectra agree with absorption spectra for the carbonyl derivatives astaxanthin and canthaxanthin. In contrast, zeaxanthin and isomers of β-carotene have a twentyfold increase in fluorescence quantum yield for excitation around 350 nm compared to excitation near the absorption maximum (at approximatively 430 nm). These differences are interpreted in terms of the role of non-emitting 1 (n, π*) states related to the CO group in facilitating non-radiative deactivation of higher 1 (π, π*) states.