Kuswandi, B., F. Damayanti, J. Jayus, A. Abdullah, and L. Y. Heng. “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, vol. 47, no. 3, Dec. 2015, pp. 236-51, doi:10.5614/j.math.fund.sci.2015.47.3.2.