Valorization of Sour Milk to Form Bioplastics: Friend or Foe?

A demonstration was developed to introduce students to waste valorization in order to form bioplastics. Waste valorization is the process of reusing, recycling, or composting, from waste, useful products or sources of energy. In this demonstration, waste valorization is introduced by converting sour milk into a bioplastic via the addition of lemon juice upon heating. Utilizing lemon juice to perform the acidification offers a greener procedure than the traditional formaldehyde (used commercially to make galalith) and enhances the transferability in remote locations such as the Amazon Rainforest in comparison to vinegar. Students can establish connections to relevant United Nations Sustainable Development Goals (UN SDGs) by adopting a systems thinking approach. However, through this, it is noteworthy that this process is also used (particularly in the Indian subcontinent) to make paneer, a farmer cheese. While this also enables students to make a link to additional UN SDGs pertaining to "zero hunger", there is an ethical discussion to be had as to whether such a process that is utilized to feed malnourished citizens should be used to make a decorative bioplastic. As such, despite this demonstration being transferrable, instructors may consider carefully whether to utilize this resource, and, if so, to use this as an opportunity to teach the importance of ethics in science.

Introduction

Given the highly significant global issues surrounding plastic pollution owing to the fact that plastics are oil-derived and hence take thousands of years to biodegrade, there is significant demand for the production of green and sustainable alternatives. Such plastic substitution would contribute toward addressing the United Nations Sustainable Development Goals (UN SDGs),[1] which seek to address global challenges such as those relating to poverty, inequity, climate and environmental degradation, prosperity, and peace and justice. Bioplastics are plastics that are derived from renewable biomass sources and offer an alternative to their oil-derived counterparts. As such, for the next generation of scientists, engineers, and policymakers to address such global challenges, allowing us to transition toward a more sustainable society, education in the production of bioplastics is paramount. This subject matter is also aligned with the increased emphasis from the American Chemical Society (ACS) to enhance teaching in polymers and materials chemistry.[2,3]

There are some examples in the literature of bioplastics being incorporated into curricula to include at the modular level where students have the opportunity to complete a series of core foundational experiments in preparation for a student-directed project in this area.[4] Another example from Hudson and colleagues outlines an activity to valorize waste lobster shells (in the form of chitin) to produce plastic objects.[5] Indeed, educational resources which simultaneously offer opportunities to teach waste valorization allow students to see how to achieve a whole systems thinking “closed-loop” manufacturing of products with all input materials fully utilized. In doing so, instructors can align such resources with the UN SDGs, specifically Goal 12, which aims to reduce the environmental impact of wastes and improve utilization and management within the context of circular economy principles.[6−10]

In 1893, Auguste Trillat discovered the means to insolubilize casein, one of two proteins (together with whey) from milk, by immersion in formaldehyde.[11] In doing so, a galalith bioplastic can be formed through facilitating cross-links between neighboring casein molecules, which has found application in the production of buttons, pen barrels, knitting needles, buckles, and more. However, despite being cheap, galalith could not be molded, leading to its demise by commercial end users in the 1940s in the UK (though production continued in Brazil until the 1960s). Despite this, to this day, galalith is used as part of outreach demonstrations to show the formation of a bioplastic. However, this is far from a “green bioplastic” given that, according to the Global Harmonized System of Classification and Labeling of Chemicals (GHS), formaldehyde is suspected of causing genetic defects (H341); it may cause cancer (H350) and causes damage to organs (eyes) (H370). As such, this is highly unsuitable for educational use at any level.

Green Alternative to Galalith for Demonstrations

A reported alternative for making bioplastics from milk is to substitute formaldehyde with an acid such as vinegar.[12] Given that the isoelectric point of casein is 4.6,[13] and the pH of milk is 6.6, upon acidification of milk with vinegar, this instigates coagulation of the casein and precipitation of a curd, which can be extracted. Upon drying, a bioplastic can be formed.

Following the ACS Global innovation imperative in Belém, Pará, Brazil, in 2016 to develop green chemistry experiments for remote locations,[14] a white paper was produced with recommendations to develop low-cost and transferrable green chemistry experiments for implementation in Brazil and developing countries.[15] This served as motivation to collaboratively develop a bioplastic demonstration, using reagents that are readily available in communities within the Amazon Rainforest and surrounding areas while simultaneously incorporating opportunities to show the power of waste valorization, aligning with the UN SDGs. To this end, sour skimmed milk (5–10 days past expiration date) was used as the waste feedstock (given this has a higher casein content than semiskimmed and full fat milk). Furthermore, as lemons are grown in the Amazon Rainforest, vinegar was substituted with lemon juice to complete the acidification. All reagents are totally benign with no known hazards, and the demonstration was conducted in the UK ahead of a (different) bioplastic-themed laboratory experiment as part of a summer transitional course in chemistry for 30 students studying natural sciences according to the following steps.

Using a measuring cylinder, measure 50 mL of sour skimmed milk, and then, pour the milk into a 100 mL beaker.

Using a stirrer hot plate, heat the milk to 60 °C. Use a thermometer to check the temperature while heating the milk, continuously stirring it with a glass rod. If you wish to add color to the bioplastic, add a small volume (∼5 mL) of food coloring now.

While the milk is heating, measure 3 mL of lemon juice into a 5 mL measuring cylinder.

Once the milk has reached 60 °C, remove the beaker from the hot plate and place it on a desk.

Add the lemon juice and stir the milk and lemon juice mixture using a glass rod for about 5 s.

Using a spatula, scrape out the solid from the beaker onto a paper towel. Use a paper towel to dab the white solid to help dry the surface. Do this until as much water has been removed from the solid as possible.

Flatten the solid on a desk and use a plastic shape cutter to cut out the desired shape.

Place the shaped bioplastic in a Petri dish and put the Petri dish and plastic into an oven (set to 65 °C) for 4 h.

Remove the bioplastic from the oven, allow to it cool, and distribute it to the class.

A useable product can be made such as a Christmas-tree (or other) decoration if a hole is punched into the bioplastic during step 7 and a piece of string is looped through it and tied in a knot (Figure , inset). This demonstration enables students to identify links with the UN SDGs, specifically goals 4 (quality education), 9 (industry, innovation and infrastructure), 11 (sustainable cities and communities), 12 (responsible consumption and production), 13 (climate action), 14 (life below water), and 15 (life on land). A representative student comment following the demonstration and subsequent laboratory experiment was “Helped me to see why we did the experiment and why society needs to shift away from oil-based plastics.”

Figure 1

Representative data depicting bioplastic mass variation as a function of lemon juice addition together with an inset of representative bioplastics made with holes punched for string to be inserted for use as a decoration.

If desired, the demonstration can be modified to be more interactive[16] by requesting for volunteers to assist the instructor in preparing additional samples where parameters such as the volume of lemon juice added is varied and investigated as a function of the resultant mass of the bioplastic. For example, if formulations are made with 1, 2, 3, 4, and 5 mL additions of lemon juice, the trend depicted in Figure is produced showing an increase in bioplastic mass up to 3 mL followed by a decrease. The increase can be rationalized by acidification decreasing the net charge of caseins and instigating the solubilization of colloidal calcium phosphate from the casein micelles into the solution.[17] In doing so, micelles will be brought closer together to form dense clusters, facilitating coagulation and formation of a bioplastic.[18] However, at high lemon juice addition (4–5 mL), the mass of bioplastic produced decreases because the casein moieties will have an overall positive charge (which is likely due to amine and hydroxyl moieties acting as Lewis bases), leading to electrostatic repulsion between casein molecules and reduced coagulation.[19]

Indeed, student participation may be enhanced further by converting this demonstration into an activity where students physically make the bioplastic themselves with the additional option of investigating varying the volume of lemon juice added as a function of bioplastic mass as described. Approximately 7–8 h should be made available for students to complete this with the flexibility of being able to engage with alternatively taught content or take a break while the samples are in an oven.

Systems Thinking Approach Reveals More

While this demonstration to valorize sour milk using green and readily available reagents and processes nicely allows students to consider the life cycle of materials with specific emphasis on waste valorization, taking a whole systems thinking approach to designing this resource uncovered potential issues and additional learning outcomes for students. Systems thinking uses cognitive frameworks, strategies, and tools to enable visualization of interconnections and relationships among components of systems together with an examination of the dynamic nature of systems and how systems-level phenomena emerge from interactions among the systems parts.[20] Within education, this approach can help students to address complex, interdisciplinary, real-world problems that are aligned with the UN SDGs.[21,22] Indeed, examples have been reported of the use of systems thinking to inform curricular change and resource development within green chemistry and sustainability.[23,24]

Through taking this approach and considering transferability for international adoption of this demonstration, it is noteworthy that chhena are curds originating from the Indian subcontinent, made from cow milk by adding lemon juice and straining.[25] Chhena is then pressed and can be processed to make paneer, an unaged, nonmelting farmer cheese. This demonstrates that paneer, which is created via a very similar process to the demonstration, is used as an important source of food to provide nutrition and alleviate hunger in parts of India and elsewhere. As such, this affords an additional opportunity to demonstrate to students that valorization of sour milk in this way also serves to address goal 2 of the UN SDGs pertaining to ending hunger, achieving food security and improved nutrition together with promoting sustainable agriculture. While this acts as an additional learning point for students, it highlights serious ethical implications of making bioplastics from milk, which has formed the basis of outreach demonstrations[12,26,27] and even processes used by industrial companies.[28] This therefore enables instructors to instigate an ethical discussion as to whether such a process should be implemented for commercial application to prepare bioplastics when many malnourished citizens rely on converting sour milk to form paneer, an important foodstuff.[29] Further details are available in the instructor guide as part of the Supporting Information.

On the other hand, while considering the entire system has opened up new learning opportunities for students, some instructors may be reticent to implement this demonstration of bioplastic formation owing to its application to make paneer. It is therefore recommended that, further to creating learning opportunities for students to use systems thinking, instructors should consider the entire system at large when evaluating whether a resource/demonstration is fit for the purpose. Given the ubiquity of this demonstration (using formaldehyde, vinegar, lemon juice, or another option), this indicates a wider issue in that there are not enough transferrable demonstrations/laboratory experiments on the subject of bioplastics in the literature for instructors to implement and that there should be a focus devoted to this area in the near future.

Conclusions

This demonstration enables the widespread implementation of forming a bioplastic from sour milk, showing students how to utilize waste in a productive fashion to create a useful product, (e.g., in the form of a decoration). Through this, students can adopt a systems thinking approach in the context of the life cycle of the sour milk. Further to showing students a potential route to bioplastics, the demonstration can be made more interactive where parameters such as the volume of lemon juice can be varied with the mass of bioplastic being monitored. This exercise allows students to make a connection to relevant UN SDGs by considering the entire system; further to applications as a bioplastic, students can be challenged to recognize that the process demonstrated is used in the formation of paneer. This enables instructors to discuss the ethical implications of applying this process to make a commodity bioplastic in competition with a valuable foodstuff [and hence satisfying a different UN SDG (goal 2)]. Indeed, some instructors may conclude that, given the latter, this may not be an appropriate demonstration for bioplastic formation and perhaps seek to utilize an alternative.