Eficácia da espectroscopia por ultrassom e do congelamento na avaliação da composição química do leite

Abstract

Objective to evaluate the ultrasonic spectroscopy method and the effect of freezing the milk fat and protein analyses of cow's milk. 100 samples of cow's milk were analyzed immediately after collecting in expansion tank of a dairy herd (fresh), frozen at-20oC for 30 days and again examined the levels of fat and protein. All the analyses of fat and protein were made in triplicate.The analyses of fat, known as official method, using the butirômetro method of Gerber. The analyses of crude protein, called official method, using the micro-Kjedahl method. Ultrasound spectroscopy analysis was made with the equipment calibrated at the factory to cow's milk (Master Classic Complete, Electronic Products By Akzo Was, São Leopoldo, RS, Brazil). The effects of method (official versus ultrasound spectroscopy) and freezing of milk were analyzed as a completely randomized design, with the effect of the repeated method in time (in natura or frozen for 30 days). Additionally, simple linear regression was adjusted between the values obtained with the Ultrasound method and official methods. Interaction effect was observed (P < 0.01) between the analytical method (official versus ultrasound spectroscopy) and freezing the milk about the levels of fat and protein. Ultrasound spectroscopy in 0.51 percentage points overestimated (P < 0.01) the fat content in fresh milk and on 0.36 percentage points (P < 0.01) in the milk to freezing. However, there was high correlation coefficient (r = 0.82) between fat obtained by ultrasound spectroscopy with official method specifically in fresh milk, indicating potential use of ultrasound spectroscopy through development of calibration equations.The freezing of the milk by 30 days did not affect (P = 0.15) the fat content of milk obtained by the official method (Gerber butirômetro), but reduced the fat content (P < 0.01) obtained with ultrasound spectroscopy. Ultrasound spectroscopy underestimated in 0.29 percentage points (P < 0.01) the protein content in fresh milk and 0.87 percentage points in the milk to freezing. The freezing of the milk by 30 days increased (P < 0.01) the protein content of milk obtained by the official method (Kjeldahl method), but reduced protein content (P < 0.01) obtained with ultrasound spectroscopy. The fat and protein

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content of milk obtained by the method of ultrasound spectroscopy are divergent from those obtained by the official methods, independent of the sample be in natura or subjected to freezing. However, the method of ultrasound spectroscopy can be used to estimate the levels of fat since fit the calibration equations with a comprehensive database. It is not recommended to analysis of the levels of protein and fat from milk subjected to freezing.