Teor de ácido lático como indicador de qualidade do leite: validação de métodos e monitoramento.

Abstract

The potential use of lactic acid content in natura and UHT milk as a indicator of quality and its relation to titratable acidity were investigated. The performance of official methods to detect and determine milk adulteration by the addition of neutralizers was also evaluated.The performance of the rosolic acid and phenolphthalein qualitative methods was verified by evaluating the rates of false positive results, selectivity, sensitivity, reliability, loss of reliability region, limit of detection, agreement and concordance. The quantitative methods of ash alkalinity, high-performance liquid chromatography with ultraviolet detection and photodiode array (HPLC-UV/DAD) to determine organic acids, and reflectometry for lactic acid analysis were evaluated through the selectivity, linearity, detection and quantification limits, veracity, repeatability and intermediate precision. The chromatographic and reflectometric methods were applied to samples of fresh and UHT milk collected in the south, southeast and northeast regions of Brazil. The official methods presented poor performance. It was only possible to detect the presence of acidity neutralizers in milk at high levels (0.050 g/100 mL of sodium hydroxide; 0.300 g/100 mL of sodium bicarbonate and greater than 0.080 g/100 mL of sodium carbonate), for which the titratable acidity had already exceeded the lower limits regulated by the legislation. Lack of selectivity in the presence of UHT milk stabilizers was also observed in the ash alkalinity method. The chromatographic and the reflectometric methods for quantification of lactic acid were considered adequate for the purpose of use and presented satisfactory correlation, being the reflectometric method more advantageous in relation to the sensitivity, variability of results, practicality and less time of analysis. The content of lactic acid (71.8 ± 43.1 mg/L), citric acid (2,503.8 ± 245.5 mg/L), orotic acid (83.9 ± 7.9 mg/L) and titratable acidity (0.16 ± 0.01 g/100 mL) were determined in 54 samples of UHT milk purchased from commercial establishments in Curitiba, Belo Horizonte and Recife. Greater variability was observed for lactic acid and a lack of correlation between the levels of this acid and the titratable acidity. Different ranges of lactic acid values were observed for in natura (8.1 ± 6.1 mg/L) and UHT (103.3 ± 63.2 mg/L) milk of the same region, whereas the titratable acidity was similar (0.16 ± 0.02 g/100 mL). Lactic acid content increased 45-fold during controlled acidification of the milk (37 ± 1 °C/9 hours), unlike the contents of the citric and orotic acids that remained constant. The importance of lactic acid evaluation in milk monitoring, its advantage in relation to titratable acidity and the need to establish reference values for effective monitoring were evidenced.