The simple and everyday practice of curd formation involves concepts taught in biology (microbial fermentation), chemistry (conversion of milk lactose to lactic acid) and physics (coagulation of milk proteins with acid).
Can we use this process to offer a hands-on introduction to what it means to think like a scientist?
Doing science may be a powerful way to understand the scientific method, and what it means to be a scientist. Applications of the scientiﬁc process extend beyond laboratories to our everyday life.
One application of this process is in understanding curd formation (see Box 1).
Box 1. A quick look at the basics:
- The term ‘curd’ in this article (and in India) refers to the fermented product obtained by the addition of a small amount of curd (or buttermilk) to milk. This product is called yoghurt in some other parts of the world.
- Curd is formed due to several species of bacteria, like Lactobacillus sp., Lactococcus sp., and Streptococcus sp., that convert the lactose in milk to lactic acid (which makes curd sour). These bacteria are collectively called lactic acid bacteria (or, LAB).
Setting curd is an age-old practice, common to many parts of the world, that has undoubtedly evolved through repeated observation, prediction and enquiry.
This everyday practice offers plenty of learning opportunities to curious young students who may have just begun to explore the world of biology.
For e.g., have you ever wondered why we always need a small amount of ‘old’ curd to get a batch of ‘fresh’ curd? Or, if it were possible to get curd simply by leaving milk undisturbed for long enough? Or, why the temperature of milk or the season of the year seem to affect the rate of curd formation?
These, and many other such questions around curd formation can be answered through simple, cost-effective experiments. A small space, the motivation to observe, and the willingness to play around with milk and curd (and clean up later) may be all that is needed!
Apart from introducing students to the scientific method, these experiments touch upon a wide range of topics in school biology (see Box 2).
Box 2. Links to the school science curriculum:
The experiments around curd formation described here are linked to concepts of:
- Fermentation, food preservation, and microbiology from the chapter on ‘Microorganisms: Friend and Foe’ in the NCERT Grade VIII curriculum;
- Physical changes, reversible and irreversible changes from the chapter on ‘Changes Around Us’ in the NCERT Grade VI curriculum; and,
- pH, acids and bases from the NCERT Grade VII curriculum.
How do people make curd?
This activity is ideally introduced a day before the other activities. Share the corresponding activity (see Activity Sheet: How do people make curd?) with students as homework. Introduce them to the idea of a ‘science reporter’ and ask what they expect to find.
Once students have completed this activity, encourage them to reflect on how they would go about testing the things they’ve learnt from it. This discussion could be used to introduce the concept of a hypothesis.
In addition to the process of curd formation, this activity can also be used to discuss science – society relationships. For e.g., is science limited to labs? Are people with no professional training in science also capable of engaging with the scientific method in their everyday lives?
How is curd different from milk?
Discuss the difference between milk and curd with your students. Once they have arrived at some consensus, ask them if they could describe a method to distinguish between the two liquids without tasting them (tasting anything in the laboratory should be discouraged).
Encourage them to work in groups of 3 – 4 to try the corresponding activity (see Activity Sheet: How is curd different from milk?)
Students are likely to report that curd is not as runny as milk, has a sour smell, and a lower pH. A milk smudge is much smoother than that of curd (which appears clumpy) and milk tends to concentrate at the centre of the smudge.
This activity may also be useful in provoking a discussion on the ‘many senses’ and ‘extended tools’ used for observation in science.
Turning milk into curd
Ask your students to describe how milk is turned into curd. Introduce the popular practice of adding a small amount of old curd to milk to get new curd — how many of them have observed this? What does the old curd contain that turns milk into a new batch of curd? Does the old curd change the chemistry of milk, or its biology?
Share the corresponding activity sheet (see Activity Sheet: Curd formation) with students.
Encourage them to work in groups of 3 – 4 to design an experiment to determine the exact set of conditions required for curd formation.
This activity is likely to take at least 5 – 6 hours for completion. If possible, it would be best to start the experiment at the beginning of the day, with incubations extending to the last period.
The cultures need to be observed at regular intervals — the 3 – 4 students in each group can take turns to do this. Once students have completed this activity, discuss what they’ve learnt from it. Each group can be invited to share their design and results, and the class as a whole can be encouraged to identify similarities and points of difference.
Students are likely to report that even if it thickens, milk left to itself does not form curd. The thickening of milk, while similar in appearance to curd, is a result of spoilage by bacteria from the milk, air, the utensil in which the milk is stored, and/or handling.
By adding ‘old’ curd to milk, we give the lactic acid bacteria in the old curd a chance to multiply, produce lactic acid, and inhibit the growth of food spoiling bacteria. This activity may also be useful in introducing a discussion on ‘standardization’ and its relevance to science.
What if you don’t have ‘old curd’ to begin with?
There is a popular belief that if you don’t have old curd, you could add some lemon juice or a chilly to milk to turn it into curd. Ask students to respond to this question with a show of hands: “How many of you believe this could work?”
Share the corresponding activity sheet (see Activity Sheet: Starter for a new batch of curd) with students, and encourage them to test this hypothesis in groups of 3 – 4.
Once students have completed this experiment, ask for another show of hands to the question posed before. Encourage students to reflect on what convinced them to change their minds.
Students are likely to report that fresh curd is formed only by the addition of some old curd (source of lactic acid bacteria). Adding lemon juice or chillies to milk causes the precipitation of milk casein — resulting in the formation of paneer or cottage cheese, but not curd.
Hence, what happens with these ‘starters’ is a chemical change — the precipitation of casein by acid. In contrast, the use of curd as a starter results in a biological change — the microbial fermentation of milk to curd. This activity may also be useful in introducing a discussion on ‘controls’ in scientific experiments (see Box 3).
Box 3. Experimental controls:
Controls help prove that an intervention (e.g., the addition of a small volume of curd to milk) causes the effect being studied (e.g., results in the formation of a new batch of curd).
In this experiment, beaker 2 or the beaker where a small amount of curd is added to milk, is the ‘positive control’ because we have determined this as the optimum condition in our previous experiment, and we are sure that it will form curd.
In contrast, beaker 1 or the beaker where nothing is added to the milk, is the ‘negative control’.
We know that the milk in this container will not turn into curd.
An introduction to experimental controls can also lead to a discussion on questions, like: “How important are these controls? How accurate is our inference from an experiment that does not involve controls?”
Does the temperature of milk matter?
Curd formation requires certain environmental and physical conditions. Most students may be familiar with the idea that curd formation is faster when old curd is added to warm milk, and incubated in a warm place.
Some students may suggest that the milkneeds to be ‘hot’ rather than ‘warm’. The teacher can then ask students to form a hypothesis, for e.g.,: “Can we say that the higher the temperature, the faster the process of curd formation?”
Share the corresponding activity sheet (see Activity Sheet: Does the temperature of milk matter?) with students, and encourage them to test this hypothesis in groups of 3 – 4.
Once students record their observations, ask students if they’d like to change their hypothesis. A discussion can then be conducted on:
“What is the temperature range in which curd formation was fastest? Why?”
The LABs which ferment milk to curd also inhabit the human gut. They grow best at our body temperature (~ 37ºC). Therefore, using milk warmed to this temperature helps provide these bacteria with conditions that are optimal for their growth and multiplication.
On the other hand, using milk that is very hot (> 45 °C) will kill these bacteria — reducing the possibility of getting curd. Milk that is cold impedes bacterial growth, slowing the rate of curd formation considerably.
Similarly, incubating milk with old curd in a warm place, like a hot water bath or incubator set at 37ºC will speed up the process of curd formation by providing an environment in which LAB’s thrive.
Thus, this experiment helps demonstrate the significance of temperature in biological processes.
Here, the teacher may also ask: “What would happen if curd was added in unequal amounts to all beakers?” The answer is — we would not be able to determine if curd formation was an effect of the amount of curd added or the temperature.
This also emphasises the need to vary only one parameter at a time (here, the temperature).
This is exactly how the scientific method works, by varying only one variable at a time, while keeping all the others constant.
Some parting thoughts
Each of these experiments helps highlight the role of various factors in curd formation. In doing so, they raise some interesting questions. For e.g., milk gets spoilt if not consumed within a day. But if turned into curd, it stays fresh for longer.
So, isn’t fermentation also a process of food preservation? Students begin to understand that fresh curd is a live source of bacteria, and turning milk into curd is akin to a process of breeding microbial ‘pets’.
Thus, performing these experiments will help students understand what these little creatures like or dislike.
Although the article provides experimental designs, it may be best to encourage students to come up with their own experimental designs, whenever possible (see Box 4).
Box 4. Experiments on sources of milk and effect of other additives: Students could be encouraged to design and conduct experiments to check how curd formation is affected by using:
- Milk from different animals, like cow, goat, buffalo, sheep, camel, and horse etc.
- Milk subjected to different types of processing, like fresh, boiled, pasteurised, or milk stored in tetra-packs.
- Milk additives, like chocolate powder, nutrition supplements, etc.
It is likely that students might take some time designing their own experiments, but doing this will help them develop a hands-on understanding of the scientific method.
Not only will this help students feel a sense of ownership towards their experiments, but it will also help them discover several ways of using simple resources, like milk and curd, as ‘live educational tools’.
- Practical applications of science provide a powerful lens to introduce students to the scientiﬁc method.
- Curd formation is one application of the scientific process in our day – to – day lives.
- Different aspects of the scientific method, such as hypothesis making, standardisation and controls, can be introduced through activities around curd setting.
- These activities can also be used to discuss science – society relationships.
The author would like to thank Dr. Ritesh Khunyakari, Associate Professor, Hyderabad campus, Tata Institute of Social Sciences, for his valuable inputs during the development of this learning unit as well as the current article.
She would also like to thank all the students who participated in ﬁeld- trials, and teachers and members of the Vigyan Pratibha team who provided valuable inputs and feedback on the development of this learning unit.
- This article is based on a learning unit — ‘The journey from milk to curd’ developed as a part of the Vigyan Pratibha programme of HBCSE.
Vigyan Pratibha is a talent nurture programme in science and maths for Grade VIII‑X students from diverse backgrounds of Kendriya Vidyalayas, Jawahar Navodaya Vidyalayas and Atomic Energy Central Schools.
The programme is aimed towards developing a deeper understanding of concepts and critical thinking skills by providing students with tasks which go beyond textbook-based classroom teaching.
Teachers’ Notes. (URL: https://vp.hbcse.tifr.res.in/w…) and Students’ Worksheets (URL: https://vp.hbcse.tifr.res.in/w…) for this learning unit are available online. Teachers need to register for access to Teachers’ Notes.
- The image used at the beginning of the article is by Ji-Elle, Wikimedia Commons. License: CC-BY-SA. https://commons.wikimedia.org/wiki/File:Curd-Sri_Lanka.jpg
- ‘Lactic Acid Bacteria and their Fermentation Products’. Montiville, T.J., Matthews K.R. and Kniel K.E. (2017). Food Microbiology: an Introduction, ASM Press.
- ‘Lactic Acid Bacteria’. World of Microbiology and Immunology. Retrieved May 11, 2019 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/lactic-acid-bacteria
- ‘A Milk Curdling Activity’. Lohner S. (2017). Scientiﬁc American.
About the author
Rohini Karandikar is a post-doctoral fellow in science education at the Homi Bhabha Centre for Science Education, TIFR, Mumbai.
She is currently working on Vigyan Pratibha, and School Science Research and Development —Participatory Action Research projects.
She can be contacted at firstname.lastname@example.org