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Catching food freshness in real time: An undergraduate biology experiment

Rohini Karandikar

Food spoilage is covered in undergraduate biology courses with limited scope for practical experience. This article describes a simple module that not only helps students explore the topic experimentally but also develop a deeper understanding of various scientific concepts and gain experience in design thinking. 

Rohini food title image
Image by Freepik.

Food spoilage causes food wastage, foodborne illnesses, and huge economic losses worldwide. Food spoilage is also a topic covered in undergraduate biology curricula. However, it is mostly taught theoretically and has limited scope for practical experience, largely due to safety issues associated with spoiled food. Food spoils due to many environmental factors, such as humidity, temperature, and food-spoiling bacteria, among others. While preservatives prolong their shelf-life, there is a growing tendency to buy preservative-free foods. Hence smart packaging that provides a visual indication of food freshness is in vogue.

The undergraduate students at the National Initiative on Undergraduate Science (NIUS) of Homi Bhabha Centre for Science Education (HBCSE), Mumbai worked on a module to experimentally design a simple food-freshness tracking system for milk-based foods to track spoilage in real-time. This article describes the system and what students learned in the process of making it. The module was conceptualized and facilitated by Anuttama Kulkarni, an INSPIRE Faculty Fellow at HBCSE, who is also the author of the original research article that describes it.

Kulkarni came up with the idea for the module while searching for experiments for a 4‑day camp at HBCSE, aimed at developing conceptual understanding and experimental skills among undergraduate students. During the search, she found articles about food-freshness indicators designed for meat-based foods. Kulkarni gave students a similar task to design food-freshness indicators for dairy foods. Students worked on a prescribed protocol that was designed and tested by Kulkarni. However, some aspects of the experiment were designed by students. The students worked in a BSL‑2 laboratory and were adequately trained on biosafety guidelines as per the American Society for Microbiology (ASM).

Students designed the food-freshness indicators using low-cost resources such as Dylon colour catchers (DCC), which are laundry sheets that can catch dyes released by coloured clothes while washing. DCC sheets are easily available on e‑commerce platforms. Students were asked to cut out discs from the DCC sheets and impregnate them with pH indicators– dyes that change colour with increasing acidity or alkalinity. Here, students were challenged with the task of selecting appropriate pH indicator dyes based on the availability of the dyes in the laboratory. Since the dyes change colour close to their pKa, understanding the concept of pKa was crucial for the experimental design.

Figure 1: Characteristics of the four indicator dyes and a representative experimental setup at pH 7 using water as the sample. (Image from Kulkarni’s article, republished here with her permission under the license CC BY-NC-ND 4.0 )

Students tested three milk-based foods – two types of yogurts (commercially available as well as home-set) and kheer (a sweet dish prepared using milk, sugar and wheat) – and water, as a control. They poured 5 ml of the samples into 10 ml plastic containers. The students then placed the DCC discs on the sticky side of cellophane tapes. Next, they used the cellophane tapes to seal each food container so that the discs directly faced the respective food samples (Figure 1 and Figure 2).

Figure 2. Procedure to set up the indicators. Plastic containers containing the samples (water in this image) are covered using cellophane tape. DCC discs impregnated with dyes are stuck to the sticky surface of the cellophane tape before covering the containers so that the discs face the sample directly. Care is taken not to touch the tape where the discs are to be stuck as this might interfere with the experiment. (Image from Kulkarni’s article, republished here with her permission under the license CC BY-NC-ND 4.0 )

At this point, students wondered how food spoilage would change the colour of the discs. This followed a discussion where Kulkarni used fermentation as an example, where CO2 is released and reacts with water to form carbonic acid, which releases acidic vapours. In a spoiling milk product, bacteria break down nutrients such as lactose into acids. Some of these acids, being volatile, release vapours. Students then discussed the highly absorbent nature of the DCC discs which could absorb the vapors. As the acidic vapours come in contact with the pH indicator dyes in the discs, the dyes change colour. This discussion enabled a better understanding of the concept of vapour release and absorbance by the discs in their experiment.

Students monitored the colour changes in the DCC discs at room temperature at 0, 6, 24 and 48 h of starting the observation. These time intervals were chosen at the time of standardizing the experiment. Importantly, these were the time points when the colour changes in the discs were distinctly visible. At the same time, students drew a sample of food from each setup to check the microbial count using a bacterial spread plate assay. This assay enabled them to quantify the bacterial load in the samples. Thus, the extent of food spoilage was correlated with bacterial growth, providing additional evidence of food spoilage due to microbial activity.

In the end, students were asked to select the best indicator dye from the four dyes tested. Students also had to answer questions on experimental design such as Comment on the significance of including water in the assay”, and critical-thinking questions like Why do you think each pH-indicator is responding differently?” in their observation sheet.

This experiment offers a context to investigate ideas of experimental designs, conceptual understanding, and design thinking of undergraduate biology students. Through a single experiment, students could apply the concepts of pH, pKa, and bacterial growth kinetics, among others. Kulkarni says, The relevance or the relatable nature of the food samples we studied caught students’ attention. Also, the idea that acidity can be monitored just by absorbing the vapors, without really touching the food sample— came as a surprise to students.” So, besides creating something new, students learnt something new by doing’!

Speaking about the application of such modules, Subhojit Sen, Assistant professor, [Ramalingaswami fellow, UM-DAE Center for Excellence in Basic Sciences (CEBS), Mumbai] highlights the experiential learning opportunity and the relevance to everyday life this module brings. Sen adds, Having such tools at hand will bring evidence-based temper back to the user, rather than having to simply believe the labels of expiry dates.”

The novelty of the experimental module was more than just a learning experience. Kulkarni remarked, the experiment sparked students’ curiosity in application-oriented research, while giving them an interdisciplinary learning experience. Such an experience of applying varied concepts in real-life, using low-cost resources is rare.”