Some people open a bottle to check its smell when choosing shampoo. What they smell may come not only from the contents but also from the container. In materials recycled from used plastics in particular, odors originating in the original material or recycling process can present a challenge to acceptance.
In 2024, Francesca Di Cicco and colleagues used shampoo bottles made from recycled plastic to investigate how odor, color, and information that recycled material had been used related to product evaluations. Their aim was to understand where sensory features might become barriers to wider adoption of environmentally conscious packaging.
Fifty-eight people evaluate bottles containing unscented shampoo
The study recruited 62 people and analyzed 58 after excluding those who did not meet conditions such as attention checks or did not complete the experiment. Participants were aged 18–44; 57 identified as female and one as non-binary. They were recruited through a university, and all regularly used shampoo.
The bottles contained 0%, 10%, 50%, or 100% plastic recycled from post-consumer products. Two recycled materials with different odors and colors were used: one cream-colored and one gray. Four levels of each produced eight bottles in total; the 0% bottles were white. They contained unscented shampoo and had no labels or logos. The experiment did not compare everyday fragranced shampoos.
Participants read a short description asking them to imagine buying a new shampoo in a store. One randomly assigned group was told the packaging was made of plastic; the other was told it was made of recycled plastic. A researcher opened the cap near the participant’s nose and gently squeezed the container to release air for the participant to smell.
Participants first evaluated the eight bottles blindfolded, then repeated the evaluations without a blindfold after a break. They rated five attributes on 100-point scales: odor pleasantness, familiarity, intensity, product acceptability, and willingness to buy. Willingness to buy was a reported purchase intention, not an actual purchase or sale.
Bottles with more recycled material tended to receive lower ratings
Overall, higher proportions of recycled material tended to be associated with lower odor pleasantness and familiarity, product acceptability, and willingness to buy. However, differences did not occur between every pair of proportions.
For example, among the cream-colored bottles, the 0% and 10% bottles were rated more acceptable than the 100% bottles, and willingness to buy was higher for 0% than for each proportion containing recycled material. Ratings also tended to be lower for gray bottles containing recycled material, although which comparisons differed depended on whether participants received the information and whether they could see the bottles.
Interestingly, odor intensity alone could not explain the results. For the cream-colored bottles, no significant differences in intensity were found between recycled-material proportions. Nevertheless, pleasantness and purchase intention differed. For gray bottles, proportion also affected intensity, with 50% and 100% rated stronger than 10% under some conditions.
Analyses combining odor and product ratings also found, for both materials, that greater pleasantness was associated with greater acceptability and willingness to buy. Familiarity was related to some ratings as well. The authors argue that deodorizing recycled materials requires attention not just to reducing intensity but to how the odor is perceived and whether it fits expectations for the product. This analysis alone does not establish that pleasantness causes purchasing.
What did color and recycled-material information do?
Removing the blindfold lowered ratings further under some conditions. For 50% gray bottles accompanied by recycled-material information, acceptability and willingness to buy were lower when visible than when unseen. For 50% cream-colored bottles with the same information, odor pleasantness declined.
However, blindfolded evaluations always came first, and evaluations with vision always came later. Nor did the experiment independently change only the color by repainting the bottles. The result therefore cannot be treated as a general rule that gray reduces willingness to buy, or as a pure effect of color alone. The authors also state that a dedicated experiment is needed to establish the mechanisms combining color and odor.
Meanwhile, the information that the bottles were made of recycled plastic did not significantly improve ratings by itself. Nor was the prediction supported that people with greater environmental concern or motivation to protect the environment would respond more strongly to that information.
This does not mean environmental information is always unhelpful. In this study, it consisted of just one sentence in a scenario description. Even if participants remembered it, how much they used it in their judgments is unknown. Many participants also had high environmental concern and motivation, making adequate comparisons with people scoring low difficult.
Not just removing odors, but explaining what they mean
The authors discuss the possibility that a departure from the smell expected of shampoo may raise concerns about content quality or the odor transferring to the user. Indeed, open-ended responses included concerns about the bottle’s odor transferring to the person or the contents. However, the study did not measure chemical migration or human safety, and those concerns do not demonstrate an actual hazard.
The authors further suggest that the distinctive colors and odors of recycled materials need not be treated solely as defects to remove: they could be understood as characteristics communicating resource circularity. Specific explanations of how recycling processes relate to odor and what environmental benefits they offer might change perceptions. This is a proposal for future testing, not an improvement demonstrated by the study.
For that reason, the authors do not simply recommend changing colors to improve impressions. Coloring requires resources and affects handling when materials are sorted and recycled again. The study’s practical point was the need to consider visual and odor evaluations together with designs that make packaging easier to circulate through reuse and recycling.
The study involved two materials and a limited group of participants, so it cannot stand for evaluations of recycled plastics in general. It did not investigate price differences, choices in real stores, or impressions after continued home use. Fragranced shampoo, in particular, combines the contents’ fragrance with the container’s odor and may not produce the same results.
Summary
In this study, odors from recycled-material bottles containing unscented shampoo were associated with product acceptability and willingness to buy. Pleasantness and familiarity mattered alongside intensity, and simply informing participants that recycled material had been used did not demonstrably improve ratings.
Making packaging more circular requires considering how users perceive its characteristics as well as material performance. The authors identified a need to pursue both technologies that reduce distinctive sensory features of recycled materials and ways of communicating why those features arise and what value they represent.
References
Di Cicco, F., Loos, M., Vos, I., Gibson, R., Naidoo, Y., Gifford, S., & Smeets, M. A. M. (2024). The Smell of Sustainability: Understanding Consumers’ Acceptability of Recycled Plastic Packaging with an Off-Odor. Sustainability, 16(12), 4896. DOI: 10.3390/su16124896 · Full original article.
Original article: © 2024 the authors. Licensee MDPI, Basel, Switzerland. CC BY 4.0. The research received funding and test bottles from Unilever, and four of the authors are employees of the company.
