Ph.D DISSERTATION TITTLE

 CUTTING TIME PREDICTION IN CHEESE MAKING BY BACKSCATTER OF NEAR INFRARED RADIATION

SUMMARY

The cheese industry demands the development of simple, sanitary and non-destructive measuring instruments in the line in order to monitor the process of coagulation in the milk and predict control parameters such as clotting time and, above all, cutting time. The objective determination of cutting time improves the cheese yield and the homogeneity of the product.    

The general objective of this study is to analyse the use of a fibre-optic sensor to determine light scattering of near infrared radiation in order to monitor milk coagulation and objectively select the cutting time. To this end, three experiments were carried out using completely randomised block designs. Each experiment was replicated three times. In each of them different experimental factors and levels were selected with the aim of providing information concerning the influence of the different factors on clotting and, especially, on the algorithms for predicting cutting time.

It was shown that the use of such a sensor allows real-time monitoring of the coagulation of goat’s milk, that the instrument is able to predict precisely and efficiently both the Berridge clotting time and the cutting time, and that it is necessary to include in the prediction algorithms a protein term when the protein concentration varies significantly.

At the same time it was also shown that the sensor is very useful for studying the factors of clotting, that temperature has a non-linear effect on coagulation rate, that the activation energy of casein hydrolysis process is increased with protein concentration, and that the effect of calcium chloride is especially strong during the secondary phase of coagulation, independently of pH. It was seen that there are interactions between various variables, that aggregation commences between the induction period and the inflection point of the reflectance/time curve, and that the optical parameter T_max estimates the gelling point before the Berridge clotting time, both responding in the same way to Foltmann’s equation. This converts the last parameter into a potentially useful tool for studying coagulating enzymes. Additionally, it was shown that goat rennet presents greater technological adaptability to goat’s milk than the enzyme which comes from M. miehei.

Finally, it was shown that the diffuse reflectance ratio after T_max responds both to micelle aggregation processes and to curd firming reactions. These phases co-exist, constituting the non-enzymatic phase of milk coagulation.

Therefore, the light scattering of near infrared radiation during coagulation of milk contains information concerning all the reactions involved in the process, i.e., hydrolysis, aggregation and hardening. This explains its high degree of usefulness for predicting clotting and cutting time, making it a potential tool for process control.    

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