Chemical Reaction between Orange Juice and Milk

ChemicalReaction between Orange Juice and Milk

ChemicalReaction between Orange Juice and Milk

Anexperiment to determine the chemical reaction that occurs when milkis mixed with orange juice is performed by adding drops of orangejuice to half a glass of fresh milk. The mixture is stirred gentlywith a glass rod. An observation is made for each drop of juice thatis added and stirred. It reaches a point at which milk startscurdling. The rate of curdling or the overall changes occur at ahigher rate after the first instance of curdling. Apart fromobserving the physical changes, the pH of the two components (milkand orange juice) and the final product is critical because it helpsin understanding the reasons behind the reactions.

Reactionbetween the orange juice and milk

Orangejuice + milk = Curdle

ThepH range of milk (6.5-6.7) indicates that it is slightly acidic(Stemwedel, 2008). When tested with a litmus paper, milk gives aresponse that ranges from slightly acidic to neutral. Other sourcesindicate that milk contains lactic acid that acts as a proton orhydrogen donor. However, research has shown that milk can beclassified as an alkaline base, but an acid forming type of food(Tosh, 2015). This means that milk should test alkaline and formacidic products in the human body. The orange juice, on the otherhand, has a low pH of about 3.5, which indicates that it is an acid.Orange juice, similar to other juice extracts obtained from citrusfruits has a high content of citric acid that accounts for their lowpH content.

Milkcontains a mixture of compounds that include water, proteins, andfat. However, the reaction between milk and the orange juice occursbecause of one type of protein that is referred to as casein(Christensen, 2014). Some groupings of casein protein float in themilk since they are not bonded to any other component. Thesegroupings usually have negative charges that make them repel othercompounds, including other groups of casein. This keeps caseingroupings dispersed in the milk. Lowering the pH of milk neutralizesthe negative charges on casein, which reduces their repellentproperty. Consequently, the dispersed casein groupings startclumping, forming large clumps and eventually the milk becomescurdled. The rate of curdling is directly proportional to amount oforange juice that is added to the milk. In addition, the rate ofreaction between the citric acid found on the orange juice and caseinthat is found in milk is affected by levels of temperatures. The rateof reaction is very slow at room or cold temperatures while curdlingbecomes rapid in hot milk (Christensen, 2014).

Addingorange juice to the milk lowers the pH progressively, until theisoelectric point is reached. The isoelectric point of a reactionbetween milk and orange juice is pH 4.6 (Christensen, 2014). This isthe point at which molecules of the reacting compounds carry no netcharge. The milk fails to curdle with the first few juice dropsbecause the isoelectric point had not been reached. At isoelectricpoint, the molecules become zwitterions, carrying negative as well aspositive charges that results in a zero net charge. At this point themilk starts coagulating.

Inconclusion, the chemical reaction between the orange juice and milkcan be attributed to the citric acid found in the orange juice andcasein protein that is found in milk. Curdling occurs when the citricacid neutralizes casein at the isoelectric point.


Christensen,E. (2014). Food science: Why lemon makes milk curdle. ApartmentTherapy.Retrieved June 12, 2015, from

Stemwedel,D. (2008). Friday spog blogging: Experimental results (milk + lemonjuice). ScienceBlogs LLC.Retrieved June 12, 2015, from

Tosh,P. (2015). Alkaline it or why your body’s pH matters. ChrisBeat Cancer.Retrieved June 12, 2015, from