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Storage modulus (G′), loss modulus (G″), strain, tan (δ) and complex viscosity (η*) of 20 semi-hard cheeses were measured by dynamic oscillatory analysis after 2, 30 and 60 days of ripening. On the same cheeses and at the same ages, tryptophan and riboflavin fluorescence spectra were recorded. The aim was to predict the rheology parameters of ripened cheeses from spectra recorded on these cheeses at a young stage. Using partial least square, tryptophan fluorescence spectra recorded at 20 °C on 2-days-old cheeses predicted G′, G″, strain, tan (δ) and η* measured at 80 °C on the 60-days-old cheeses with correlation coefficients (R) of 0.98, 0.97, 0.98, 0.98 and 0.97, respectively. Riboflavin fluorescence spectra gave slightly lower correlation coefficients of 0.88, 0.88, 0.92, 0.87 and 0.88, respectively. Dependent only on visible light, the riboflavin fluorescence spectra potentially provide viable and economic prediction of the rheology of ripe cheese.

This study examines the use of near infrared (NIR) spectroscopy for the determination of chemical parameters of Emmental cheeses from different geographic origins of Europe. A total of 91 Emmental cheeses produced during the winter in Austria (n=4), Finland (n=6), Germany (n=13), France (n=30) and Switzerland (n=38) were investigated. Fat, total nitrogen (TN), water-soluble nitrogen (WSN), non-protein nitrogen (NPN), sodium chloride (NaCl) and pH were analysed by reference methods. The diffuse reflection of different cheeses was measured by a Buchi NIRLab N-200 spectrometer using a rotating measuring cell between 1000 and 2500 nm. The best results for fat content (R2=0.92; ratio of standard deviation to root mean square error of prediction (RPD)=3.61) and NaCl (R2=0.39; RPD=1.28) were obtained after maximum normalisation and smoothing, whereas the best results for TN (R2=0.82; RPD=2.34), WSN (R2=0.77; RPD=2.08), NPN (R2=0.80; RPD=2.17) and pH (R2=0.43; RPD=1.32) were obtained when the spectra were subjected to the first derivation between maximum normalisation and smoothing. It can be concluded that NIR spectroscopy can be used for the determination of fat, TN, WSN and NPN of European Emmental cheeses with varying degrees of accuracy. However, the NIR failed to determine NaCl and pH parameters.

Correspondance auteur: Dufour E. E-mail: dufour@enitac.fr; International audience; The potential of mid-infrared and intrinsic fluorescence spectroscopies was investigated for determining the geographic origin of different hard cheeses. Mid-infrared (1700-1500 cm(-1) region), fluorescence emission spectra, following excitation at 250 and 290 nm, and fluorescence excitation spectra following emission at 410 nm were recorded directly on cheese samples. Twelve experimental hard cheeses were made using identical and controlled cheese-making conditions with milks originating from three different regions in Jura (France). The results of factorial discriminant analysis (FDA) performed on the fluorescence and mid-infrared spectra of the experimental cheeses showed a good discrimination of the cheeses. In a second step, 25 Gruyere Protected Denomination of Origin (PDO) cheeses manufactured at different altitudes in Switzerland, i.e., 11 Gruyere PDO cheeses from the lowlands (< 800 m), eight Gruyere PDO cheeses from the highlands (1100-1500 m) and six L'Etivaz PDO cheeses (1500-1850 m) were analysed. Only 80% of correct classification was obtained by applying FDA to the mid-infrared spectra of the investigated cheeses. A better classification (100%) was achieved based on the fluorescence spectra. Front-face fluorescence spectroscopy appeared to be rapid, low cost and efficient for the geographic determination of Gruyere cheeses

The potential of near-infrared (NIR), mid-infrared (MIR) and front-face fluorescence spectroscopies combined with chemometric methods was investigated for discriminating Emmental cheeses from different European geographic origins. A total of 91 Emmental cheeses produced during winter in Austria ( n = 4 ), Finland ( n = 6 ), Germany ( n = 13 ), France ( n = 30 ) and Switzerland ( n = 38 ) were investigated. Cheese samples were classified by factorial discriminant analysis. From NIR, a good classification of cheeses was observed for 89% and 86.8% of the calibration and validation spectral data sets, respectively. The MIR results were comparable to those observed with NIR. Within the 3000–2800 cm−1 region, 84.1% and 85.7% of the cheeses were correctly classified for the calibration and validation data sets, respectively. The classification obtained with the tryptophan fluorescence spectra was considerably lower (only 67.6% and 41.7% for classification and validation spectra, respectively). But tryptophan fluorescence spectra allowed a good discrimination of Emmental cheeses made from raw milk or from thermised milk. Common components and specific weights analysis were performed on the physico-chemical, infrared and tryptophan fluorescence data sets. These results showed that spectroscopic techniques may provide useful fingerprints and allow the identification of Emmental cheeses according to the geographic origin and production conditions.

The viscoelastic properties and the matrix structures of three different retailed soft cheeses (M1, M2 and M3), for which the manufacturing process was varied, were studied from the surface to the centre of the cheese using dynamic rheology and front-face fluorescence spectroscopy. The storage modulus ( G ′) and the loss modulus ( G ″) values of the samples increased from the surface to the inner part of the cheeses, while strain and tan δ decreased. Protein tryptophan (excitation: 290 nm; emission: 305–400 nm) and vitamin A (emission: 410 nm; excitation: 250–350 nm) spectra were recorded at 20°C in samples cut from the surface to the centre. For each cheese, the data sets containing fluorescence spectra and rheology data were evaluated using multidimensional statistical methods. In addition, the three cheeses were well discriminated by their spectra by applying factorial discriminant analysis. From the tryptophan fluorescence data sets, 94% and 87.7% good classifications were observed for calibration and validation groups, respectively. A better classification (100% and 96% for principal and test samples) was obtained from the vitamin A spectra. Canonical correlation analysis was performed on the rheology and tryptophan fluorescence spectral data sets, and on the rheology and vitamin A fluorescence spectra data sets. The two groups of variables were found to be highly correlated since the squared canonical coefficients for canonical variates 1, 2, 3, 4 were higher than 0.98. These high correlations indicate that cheese rheology is a reflection of its structure at the molecular level.

Tryptophan fluorescence spectra of eight different soft cheeses were recorded directly on cheese samples using front-face fluorescence spectroscopy. Discriminant ability of the data was investigated by discriminant analysis. A correct classification was observed for 95% and 92% of the calibration and validation samples, respectively. It was concluded that tryptophan fluorescence spectra enable the identity of individual cheeses to be finger-printed. Canonical correlation analysis was applied to soft-cheese sensory profile data and fluorescence spectral collection in order to measure the link between the two groups of variables measured on the same samples. The two groups of variables were found highly correlated since the squared canonical coefficients for canonical variates 1 and 2 were 0.93 and 0.80, respectively. A subset of four cheeses was investigated closely in order to establish a molecular basis of the discrimination. It was shown that molecular level information may be derived from the fluorescence spectra.

Sixteen semi-hard cheeses varying by their protein (20.2–24.1%), fat (23.7–31.1%) and dry matter (50.2–57.9%) contents were manufactured and ripened in controlled conditions. Fluorescence (vitamin A) and mid-infrared (2780–3000 cm−1 region) spectra were collected on the 16 different cheeses at 1, 21, 51 and 81 days of ripening. The data tables containing the mid-infrared and fluorescence spectra of the different cheeses were analyzed by principal component analysis and canonical correlation analysis methods. The obtained similarity maps showed that it was possible to discriminate the cheeses at the four different ripening times from the infrared and fluorescence spectra. In addition, the interest to combine the two spectroscopies was demonstrated since a better discrimination of the four stages was obtained using canonical correlation analysis. The examination of the spectral patterns associated with the principal components and the canonical variates showed that the structure of triglycerides acyl chains in fat globules was modified during cheeses ripening. The shift of the methylene bands in the infrared region and the changes of the shape of vitamin A fluorescence spectra agree with partial crystallization of triglycerides from 21 to 81 days of storage.

The critical concentrations for β -lactoglobulin gelation at pH 7 and 8 in 50% ethanol solutions (v/v) were established. Then, the gelation kinetics at 25°C were followed by dynamical rheological methods and Fourier transform infrared spectroscopy. The gelation process was faster at pH 7 compared to pH 8. After 3 min, the infrared spectra of BLG in 50% ethanol solution indicated that the percentage of α helix was higher at pH 8 than at pH 7. Alcohol-induced aggregation of BLG was characterised by the formation of intermolecular hydrogen bonded β -sheet structures for the two pH values. The final gel structures were also investigated by small-angle neutron scattering. The data obtained on the molecular, mesoscopic and macroscopic levels showed that the kinetics of gelation were pH dependent contrary to the spatial arrangements of the aggregates in the final structures.

Limited proteolysis of β-lactoglobulin and β-casein by pepsin was performed in the presence of varying concentrations of ethanol. β-Lactoglobulin started to be cleaved by pepsin only in ethanol concentrations greater than 20%, when its secondary structure began to change. In 25% ethanol, the rate of hydrolysis of β-lactoglobulin was slow (40% remained intact after 40 h of hydrolysis) and many short and hydrophilic peptides were observed. The rate of hydrolysis of β-lactoglobulin reached its maximum in 30 and 35% ethanol (80% of β-lactoglobulin was hydrolysed after 10 h), and a mixed population of hydrophilic and hydrophobic peptides of different lengths was observed. Large hydrophobic peptides appeared first, then some shorter products. The rate of hydrolysis of β-lactoglobulin decreased at ethanol concentrations equal to or higher than 40%, when only a few long, hydrophobic peptides were produced. As seen by circular dichroism, the addition of ethanol to β-casein induced α-helix formation and reduced the rate of casein hydrolysis without changing the peptide profile. The only exception was the yield of a single peptide (Pro81 − Met93).