MONITORING THE MILK ACIDIFICATION BY THE CONDUCTOMETRIC METHOD
Rodica Caprita, A. Caprita
Abstract
Milk has conductive properties because it contains charged compounds, especially mineral salts. The determination of the electric conductivity of individual milk components allowed the identification of individual contributions to overall conductivity. The addition of NaCl to milk caused an increase in conductivity that was only 77.78% of that in aqueous solutions, due to the equilibrium between soluble and colloidal salts, and other soluble substances, which interfere with the salts and decrease its conducting power. Milk acidification and the decrease of pH are due to the lactose fermentation when lactic acid is formed. The phosphoric acid, the citric acid, and their acid salts have a buffering role in milk, so that the incipient milk acidification is at first buffered. Lactic acid added to fresh milk decreased the pH and increased the electrical conductivity of milk. The increase in electric conductivity for addition of lactic acid to raw milk is from 5.2 mS/cm to 5.7 mS/cm, and for addition of lactic acid to deionized water is from 0 mS/cm to 0.56 mS/cm. We observed that when the same quantity of lactic acid is added to milk the curve for electrical conductivity increase is parallel to that of aqueous solutions, but with lower absolute values. The lower incrementally increase in conductivity is due to the uptake of protons by the buffers present in milk. HPO42- changes into H2PO4- as pH decreases, which has a lower molar conductivity. The variation of the electrical conductivity incrementally with variation of lactic acid concentration makes possible the use of this biophysical parameter for monitoring milk acidification.
Key words: pH, electric conductivity, lactose, milk, lactic acid
MONITORIZAREA ACIDIFIERII LAPTELUI PRIN METODA CONDUCTOMETRICA
Rodica Caprita, A. Caprita
Rezumat
Laptele are proprietati conductive deoarece contine compusi încarcati electric, în special saruri minerale. Prin determinarea conductivitatii electrice a componentelor din lapte am identificat contributia fiecarui component la conductivitatea globala a laptelui. La adaugarea de NaCl în lapte cresterea conductivitatii electrice a fost doar cu 77,78% din cea observata la solutia apoasa, datorita atât echilibrului care exista între sarurile solubile si cele coloidale, cât si datorita prezentei altor substante solubile care interfera cu sarurile si astfel le reduce puterea conductiva. Acidifierea laptelui si scaderea pH-ului se datoreaza fermentatiei lactice cu formarea acidului lactic. Acidul fosforic, acidul citric si sarurile lor acide din lapte au rol de tamponare, astfel încât acidifierea incipienta a laptelui este tamponata. Acidul lactic adaugat la laptele proaspat scade valoarea pH-ului si creste conductivitatea electrica. Conductivitatea electrica a crescut la adaugarea acidului lactic în laptele integral de la 5,2 mS/cm la 5,7 mS/cm, iar la adaugarea acidului lactic în apa deionizata de la 0 mS/cm la 0,56 mS/cm. Am observat ca pentru aceeasi cantitate de acid lactic adaugata în lapte, curba de variatie este paralela cu cea a solutiei apoase, dar cu valori absolute mai mici. Scaderea conductivitatii este o consecinta a preluarii protonilor de catre sistemele tampon prezente în lapte. Odata cu scaderea pH-ului, HPO42- se transforma în H2PO4-, care are o conductivitate molara mai scazuta. Variatia incrementala a conductivitatii electrice cu variatia concentratiei în acid lactic face posibila folosirea acestui parametru biofizic în monitorizarea acidifierii laptelui.
