Fabrication of natural colorimetric indicators for monitoring freshness of ready-to-cook idli batter

Natural colorimetric indicators have been fabricated using annatto and pomegranate extract powder and compared with chemical colorimetric indicator fabricated using bromophenol blue (BPB) to monitor the freshness of ready-to-cook (RTC) idli batter. The fabricated colorimetric indicator labels work on increasing headspace carbon dioxide (CO₂) concentration and decreasing pH of stored batter, where indicators showed visible color change from lightest to darkest for spoiled samples. The headspace CO₂ concentration of RTC idli batter increased from 0.15 ± 0.05 to 18.07 ± 0.21% (v/v) and pH decreased from 6.7 ± 0.06 to 4.5 ± 0.06 at optimum storage condition (35 ± 1°C) up to 24 h. The percent titratable acidity (% TA) increased from 0.44 ± 0.02 to 0.90 ± 0.02% up to 24 h. The viable lactic acid bacteria count (LABC) and total bacterial count (TBC) increased while yeast and mold count (YMC) was decreased with increasing fermentation time up to 24 h for batter stored at optimum condition. The color change of indicators was quantitatively measured with captured red, green, and blue (RGB) images. Pixel intensity for natural and chemical colorimetric indicators were decreased with increasing fermentation time. The leaching of chemical dye was observed on stored batter, and natural colorimetric indicators are nontoxic and successfully used as “on package sticker” for monitoring freshness of RTC idli batter.