3) As stated in the lab manual, the casein originally has a negative charge at t

Question

3) As stated in the lab manual, the casein originally has a negative charge at the pH of normal milk. Why did adding the acetic acid cause this casein to precipitate (curdle) out of the milk during this experiment? Be sure to explain your answer in context of what we've learned in this class about solubility! (2 pts) Hint: What is the liquid component of the milk mostly made out of? 4) Would you still expect the calcium ions (Ca?") to still be associated with the casein once it precipitates (curdles)? Why or why not? Explain your answer! (2 pts)

Explanation / Answer

ph of milk is 6.6. isolectric point of casein is 4.6. casein present in milk as colloidal suspension. in the form of micelles containing casein, phosphate, calcium, water and enzymes. when acetic acid added in milk it will affect milk in 2 ways :-

1. it increases H+ ion concentration in milk so pH of milk starts decreasing and when reaches around 4.6, as isoelectric point of casein is 4.6. H+ ions neutralizes -ve charge on casein. at that point casein micelle becomes insoluble in milk and gets precipitated.

2. At low pHcalcium hydroxyphosphate present in casein micelle will be dissolved and ionized calcium will form that will penetrate casin micelle and make strong internal calcium bonds. so these casin micelle will aggregate with one another and get precipitated.

question no 4:- yes. after curdling calcium ions still associate with casein. when calcium dissolves at low pH ionized calcium will form that penetrate casein micelle and make bonds with casein. only those calcium ions will be present with precipitated casein. so amount of calcium may be less than milk but still calcium along with casein will be present in curdled milk.

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