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International audience; Front-face fluorescence spectroscopy technique coupled with chemometric tools was used for predicting the freshness state of a Lebanese dairy product called Qishta, stored up to 20 days. Acid, peroxide and thiobarbituric acid reactive substances (TBARS) values reached no more than 0.93 mg NaOH g−1 fat, 6.22 meq O2 Kg−1 fat, and 0.0313 mg malonaldehyde (MA) kg−1 sample, respectively, throughout the investigated storage time. In parallel, fluorescence emission spectra of tryptophan and riboflavin, and fluorescence excitation spectra of vitamin A were recorded and showed the highest fluorescence intensity for the Qishta samples aged of 20 days and the lowest intensity for the fresh ones. The primary and secondary indicators of lipid oxidation showed that Qishta can be stored for 20 days without any alteration despite the increase in the TBARS after 16 days of storage. Principal component analysis (PCA) applied on riboflavin emission spectra allowed better discrimination between Qishta samples with a clear distinction of those aged 20 days while some overlapping was noticed between samples aged below 16 days. A high correlation (R2 = 0.923) was observed between the peroxide value and the intensity of the riboflavin fluorescence recorded at 460 nm.

Qishta is a Lebanese dairy product obtained by heating whole milk for 2 to 3 hours in an open shallow vessel. During the process, milk is added to readjust its level and compensate the amount of evaporated water. The process consists of harvesting the coagulum formed at the milk surface. Our findings showed that these aggregates result from the denaturation of milk proteins and their interactions with the fat globules. This work has demonstrated that Qishta has almost the same amount of proteins and fat estimated at 12% and therefore the product has a composition similar to that of Ricotta cheese made from whole milk. The characterisation of Qishta using mass spectrometry has allowed to determine and quantify the proteins present in Qishta. These analyses have demonstrated also the presence of “Cross links” between proteins which are involved in the Qishta coagulum formation. Lanthionine and lysinoalanine were identified as neoformed amino acids during the heat treatment. These amino acids are involved in the cross links formed during the heat treatment of milk. These amino acids will establish covalent bonds between the proteins of Qishta coagulum. The confocal laser scanning microscopy analyses of Qishta samples previously treated with Fast Green and Nile red dyes have allowed to highlight the protein-fat interactions considered as the source of coagulum formation.Our findings showed the significant effect of the fat amount present in the initial milk on the process and the yield of Qishta. Increasing the amount of milk fat leads to an increase in the Qishta yield. A milk with 3.2% of fat has given the maximum yield. However, the use of skim milk in the production did not nor lead to any coagulum formation.The effect of ageing during Qishta storage for 20 days at 4 ℃ on the fat oxidation and therefore on the Qishta shelf life has been realized. The results of the primary and secondary indicators of oxidation analyses have showed the fat present in Qishta was not oxidized during the study period.In addition, the emission spectra of tryptophan, riboflavin and the excitation spectra of vitamin A were directly recorded using frontal fluorescence on Qishta stored at 4 ° C for 20 days.The analysis of these spectra by multidimensional statistical tools such as principal component analysis and discriminant factor analysis has allowed to significantly differentiate Qishta samples. Indeed, these analyses have allowed to significantly distinguish between the samples and subsequently predict their shelf life of the product.; La Qishta est un produit laitier Libanais obtenu par traitement thermique de 2 à 3 heures du lait entier dans un plateau peu profond. Durant le procédé, le lait est ajouté ponctuellement afin de réajuster le niveau du lait et de compenser la quantité d’eau perdue suite à l’évaporation. Le procédé consiste à récupérer le coagulum qui se forme à la surface du lait. Les agrégats formés proviennent de la dénaturation des protéines et de leurs interactions avec les globules gras. Ce travail a permis d’établir que la Qishta est composée de 12% en protéine et 12% en lipide, composition similaire à celle de la Ricotta. La caractérisation de la Qishta par spectrométrie de masse a permis d’identifier et de quantifier les protéines présentes dans le produit. Ces analyses ont permis aussi de démontrer la présence de « Cross links » entre les protéines. Les résultats montrent la formation de lysinoalanine et de de la lanthionnine lors du traitement thermique. Ces acides aminés vont établir des liaisons covalentes entre les protéines du coagulum de la Qishta. L’analyse par microscopie confocale de la Qishta préalablement traitée par le Nile Red et le Fast Green a permis de mettre en évidence les interactions lipoprotéiques à la base de la formation du coagulum.Les études menées ont permis aussi de démontrer l’effet significatif de la quantité de la matière grasse sur la production et le rendement de la Qishta. L’augmentation de la concentration de la matière grasse dans le lait conduit à une augmentation du rendement. Le rendement maximal est obtenu pour un lait entier à 3,2% en matière grasse. Cependant, l’utilisation du lait écrémé pendant le procès n’aboutît pas à la formation de la Qishta.L’effet du vieillissement lors du stockage de la Qishta pendant 20 jours à 4 ℃ sur l’oxydation de la matière grasse et par la suite sur sa durée de vie a été réalisé. Les résultats des analyses des indicateurs primaires et secondaires d’oxydation montrent que la matière lipidique n’a pas été oxydée pendant la durée étudiée. Par ailleurs, les spectres d’émission du tryptophane, de la riboflavine et les spectres d’excitation de la vitamine A ont été enregistrés directement au moyen de la fluorescence frontale sur de la Qishta stockée à 4°C pendant 20 jours. L’analyse de ces spectres par des outils statistiques multidimensionnelles telles que l’analyse en composante principale et l’analyse factorielle discriminante ont permis de différencier d’une manière significative les échantillons de la Qishta. En effet, ces analyses ont permis de distinguer d’une manière significative entre les échantillons et par la suite de prédire leur durée de vie.

This study aims at exploring the chemical composition of a traditional Lebanese dairy product known as Qishta, describing the process of how to prepare it and understanding the mechanisms leading to its formation. The process of making Qishta can be divided into two phases: a hot phase during which milk is heated in a stainless-steel large shallow vessel, and a cold phase consisting of draining, cooling and packaging. According to milk temperature, two reaction zones were identified: zone A with an average temperature of 100 &deg, C, and zone B with an average temperature of 60 &deg, C. The results showed that Qishta had a moisture, fat, protein, lactose and ash content of 68%, 11.7%, 12.1%, 5.4% and 1.6%, respectively. Our findings showed that Qishta is a lipoprotein product having an equal amount of fat and proteins (&asymp, 12%), this composition is almost similar to that of Ricotta cheese made from whole milk. In addition, our results assert that the interactions between caseins and whey proteins lead to gel formation. Milk initial fat percentage had a significant effect on Qishta production. The highest yields were obtained when the initial fat percentage was 3.6% (182.5 g of Qishta).