Biotechnology and Bioprocess Engineering 2024; 29(1): 177-183  
Colorimetric detection of milk spoilage at low temperatures: a novel PDA/ZnO@NC membrane for pH‑based freshness monitoring
Zhaowei Li1 · Huisoo Jang2 · Sungjin Park3 · Sun Min Kim1,4,5 · Tae‑Joon Jeon4,5,6
1 Department of Mechanical Engineering, Inha University, Incheon 22212, Korea
2 Industrial Science and Technology Research Institute, Inha University, Incheon 22212, Korea
3 Department of Mechanical and System Design Engineering, Hongik University, Seoul 04066, Korea
4 Department of Biological Sciences and Bioengineering, Inha University, Incheon 22212, Korea
5 Biohybrid Systems Research Center (BSRC), Inha University, Incheon 22212, Korea
6 Department of Biological Engineering, Inha University, Incheon 22212, Korea
Correspondence to: Sun Min Kim
sunmk@inha.ac.kr
Tae‑Joon Jeon
tjjeon@inha.ac.kr

Zhaowei Li and Huisoo Jang have contributed equally to this work.
Received: October 11, 2023; Revised: November 27, 2023; Accepted: December 8, 2023; Published online: February 13, 2024.
© The Korean Society for Biotechnology and Bioengineering. All rights reserved.

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Food waste is a huge problem worldwide, and spoilage during storage is a major contributor. Food freshness monitoring systems can help reduce food waste by providing consumers with information to more accurately predict the shelf life of their food. In the case of milk, pH can be used as a freshness indicator. As milk spoils, microbial activity converts lactose to lactic acid, which produces hydrogen ions (H+) and consequently lowers the pH of the milk. In this paper, we propose a paper sensor that can detect the pH of milk in a refrigerated state (− 4 ℃) by color change. Polydiacetylene (PDA)/zinc oxide (ZnO) nanocomposite, which changes its color from blue to red as the pH decreases, was used as the color changing material. Specifically, the nanocomposite was applied to a nitrocellulose (NC) membrane, which is approved as a food packaging material for monitoring freshness during transport or on the shelf. The freshness monitoring function of the sensor was verified by applying artificially spoiled milk samples to the PDA/ZnO@NC membrane. In particular, it was demonstrated that the PDA/ZnO@NC membrane was shown to detect spoiled milk samples with pH 4.5 in 10 min at low temperature. Furthermore, the color change of the sensor upon detection of spoilage remained stable even after long-term storage. In conclusion, the PDA/ZnO@NC membrane presented in this study can be widely applied to the freshness monitoring of various food products stored at low temperatures whose spoilage is evaluated by pH.
Keywords: Polydiacetylene · Spoiled milk detection · Freshness indicator · Food waste

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