Kuswandi, Bambang, Fitria Damayanti, Jayus Jayus, Aminah Abdullah, and Lee Yook Heng. 2015. “Simple and Low-Cost On-Package Sticker Sensor Based on Litmus Paper for Real-Time Monitoring of Beef Freshness”. Journal of Mathematical and Fundamental Sciences 47 (3):236-51. https://doi.org/10.5614/j.math.fund.sci.2015.47.3.2.