An improved cryoscopic procedure for lactose determination in milk

Summary 

An improved, simplified cryoscopic method for lactose determination in mil is present. Some advantages of this method over the original one are: a) improved accuracy since cryoscopic measurements are performed on the same cryoscope tube; b) only two tubes are need determination; c) better precision since variations in results due to diferences within cryoscopic tubes are also eliminated; d) dilution of milk samples allows higher degree of hydrolysis; e) neutralization (pH 6,70) of samples permits the determination of the actual lactose content even in acidified milk (and whey) samples. The cryo-enzymic procedure furnishes results which are 0,06% lower than the cloramine-T standard method.

Key-words: Lactose; Milk; Cryoscopy; Hydrolysis.    

Resumo 

Apresenta-se um método crioscópico melhorado e simplificado para a determinação de lactose no leite. Algumas vantagens do método sobre o original são: a) melhor exatidão, porque as determinações se realizam num mesmo tubo crioscópico; b) somente dois tubos são necessários para a determinação; c) as variações entre resultados devido à diferença entre tubos são eliminadas, obtendo-se melhor precisão; d) a diluição das amostras permite maior grau de hidrólise; e) a neutralização (pH 6,7) das amostras permite calcular o teor real de lactose no leite acidificado e amostras de soro. O método fornece resultados inferiores em 0,06% em relação ao método de cloramina-T.

Palavras-chave: Lactose; Leite; Crioscopia; Hidrólise.

Introduction 

Milk cryoscopy has been used by quality control laboratories to monitor adder water to milk through freezing point depression (FDP) determinations. The introduction of thermisor cryoscopes in these laboratories have enormously facilited the determination of the FDP of a large number of samples. Recent application of cryoscopy in Dairy products analyses comprises measurement of lactose hydrolysis (RAMET et al., 1979; BAER et al., 1980; ZALAZAR et al., 1981) of lactose content in aqueous solutions and in whey (ZARB; HOURIGAN, 1979; YEH CHEN et al., 1981) along with sucrose in ice cream mixes (FRANK; CHRISTEN, 1984). Furthermore, it enables salt determinatios in butter (WOLFSCHOON et al., 1983a, 1984), in young Prato Cheese (WOLFSCHOON et al., 1983b) and cheese-brines (WOLFSCHOON et al., 1983c; 1985).   

Although high performance liquid chromatography (PIRISINO, 1983; STEFEN, 1975) has been developed for accurate lactose, glucose and galactose determinations, attention has Always been focused on simple procedures for lactose estimation.

The Zarb and Hourigan (1979) cryoscopic method is based on the fact that When lactose is hydrolyzed by β-galactosidase, it is splitted into galactose and glicose; this implies an increase of the molality since both monosacharides are readily soluble in the aqueous phase of milk. As a direct consequence of this, the freezing point of mil kis further depressed. The difference in freezing point before and after cleavage is proportional to the lactose contente. Zarb and Hourigan (1979) however, did not evaluate their method against a standard procedure for lactose determination in milk; therefore experiments were carried out at the Dairy Institute Cândido Tostes to simplify and to improve the accuracy of the cryoscopic method (ZARB; HOURIGAN, 1979) for lactose determination and to test the applicability of it for lactose content estimation in milk. This paper reports on the results obtained.

References  

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[82] WOLFSCHOON-POMBO, A. F. An improved cryoscopic procedure for lactose determination in milk. Arq. Bras. Med. Vet. Zootec., v. 39, n. 2, p. 2023-212, 1987.