Citric acids reaction in milk (Milk Curdling)

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Cindy1234
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Citric acids reaction in milk (Milk Curdling)

Post by Cindy1234 »

Dear Expert,

I am preparing for my science fair project and my topic is “What is the effect of citric acids in milk?” I have read Science Buddies websites and collected a lot of information. But I still want to ask if different citric acids and acid level will affect the curdling process in the milk?

In addition, I tried to search for reference books but cannot find one that include exactly what I want. I think something related to cheese chemistry is relevant. Do you have any recommendations for books that I should read to find more useful information?

Thank You!

Ashley
JacquelineK
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Re: Citric acids reaction in milk (Milk Curdling)

Post by JacquelineK »

Hi there!
I would agree that looking into cheese chemistry would be the best option to answer your question. One of the main uses of milk and acid is to make different types of cheese. The links below are two different experiments on different acids used in the cheese making process and the effects in regards to yield, quality, etc. of the cheese. Hope this gives you an idea of where you should be looking to help with your question. Good luck!
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3931878/
https://www.scientificamerican.com/arti ... -activity/
AshleyT
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Re: Citric acids reaction in milk (Milk Curdling)

Post by AshleyT »

Dear Expert,

I performed a preliminary experiment by adding lemon juice (squeezed from lemon) to 3 types of milk (1. whole milk; 2. low-fat milk; 3. skimmed milk) and waited for 30 mins. Then I dripped all the liquid (whey) out from the curdled milk. I have the following questions which I still not quite sure:

1. How long should I wait for the milk to curdle? For my preliminary experiment, I waited for 30 mins. But then I saw the whey keep coming out from the curdled milk. Will the whey liquid stop separating from the milk curd? How long should I do this test?

2. I would like to test which milk will curd the most. Originally, my hypothesis is that the higher the fat content in milk (i.e whole milk), the milk will curd the most. I tried to WEIGHT the milk curd (after separating with the whey) but seems the weight of all 3 kinds of milk curd are similar. How should I measure the milk curd to get a result for me to explain?

3. Is there any correlation between the ratio of lemon juice added to milk portion that affect the curdling process of milk? What is the relation? Faster? or Bigger milk curd?
I cannot find related research on this.

Thank you for your help in advance.
Ashley
JacquelineK
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Re: Citric acids reaction in milk (Milk Curdling)

Post by JacquelineK »

Hi there!
Congrats on getting this far in your experiment. The questions you have asked are good ones indeed. You thinking like a true scientist!

1) Milk is primarily made up of fat, protein and lactose. Whey and Casein make up most of the proteins. Because Casein is poorly soluble in water, its proteins build spherical structures called micelles that allow them to stay in suspension as if they were soluble. By either adding more acid, or letting the mixture sit for a longer period of time, etc. the endpoint, at which all of the curd forming protein has come out of the suspension, is same. A good way to test your question would be to let the mixture sit for different times and see if there is difference between the amount of whey vs. curds.

2) What an excellent hypothesis! I believe what you are essentially weighing is both the curd and residual solution/liquid. Try drying out the curds, therefore you are left with just the dry ingredients.

3) The coagulation of milk is like any other enzymatic reaction. There are four main factors that affect the rate of enzymatic reactions including temperature, pH, enzyme concentration, and substrate concentration. So, adding more of these factors to an extent could increase the rate at which curds will form.
I hope this information answers some of your questions. Cheers!
AshleyT
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Joined: Sun Jan 23, 2022 6:30 am
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Re: Citric acids reaction in milk (Milk Curdling)

Post by AshleyT »

Dear Jacqueline,

Thanks for your information.

I did 2 more follow up experiments using 1. Low-fat milk (4.3g protein for each 100 ml) 2. Skimmed milk (3.3g protein) and 3. Fresh milk (3.0g protein). Since protein is the element that makes milk curd, so I think that the weight of curd will be highest for low-fat milk, then skimmed milk, then fresh milk. However, my results showed that fresh milk’s curd weight is higher than skimmed. Did I make something wrong here? Or there is other factor that make milk curd?

Another question is: I added 10ml lemon juice to 40ml of milk. I tried to convert these liquid to grams (by formula). I tried to add to the he weight of whey and curd together but the sum is smaller than the weight of the original liquid. That means some weight is lost during the process of milk curdling. Am I correct? How can I search for a reason for this?

Thank you!
Ashley
JacquelineK
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Joined: Tue Aug 16, 2022 11:55 am
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Re: Citric acids reaction in milk (Milk Curdling)

Post by JacquelineK »

Hi there,

1. Based on the information you have gathered, it looks like there is another component of milk that may factor into curd yield. That component is the fat %. Fat does not directly participate in curd formation, but is interlocked within the complex colloid that is formed during the acidification from the casein. So, based on your results, not only does protein contribute to curd yield, but the % of fat influences it as well. To further test this theory, and get a definite answer to your question, you could use milk with the same % fat, but differing amounts of protein.

2. Yes, you are correct! In chemistry, it is very common to lose material. Some evaporates, some gets stuck behind, or stuck to the tools.
In your experiment, whether your weighed the dried curd, or the wet curd, the result could be a similar scenario. Water is lost via evaporation, which could account for the difference in weight. A good place to start researching is reading different scientific articles on "the acidification of milk," for any other lingering questions.

Hope this answers some questions. Good job with your experiment! The questions you are asking are that of a seasoned scientist! Cheers.
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