Dye-incorporated Chitosan-based CO2 Indicator to Monitor Food Freshness

Introduction: Quality and freshness indicators that are placed inside sealed food packages give consumers an easier way to find microbial spoilage of food. As the microbial growth and subsequent spoilage leads to production and accumulation of carbon dioxide (CO₂), the partial pressure of CO₂ in headspace of food package can be considered as a quality indicator of food.

Purpose: The aim of this study is to develop a chitosan-based CO₂ indicator which exhibits prominennt visual color change depending on different CO₂ levels. With the increase of CO₂ levels in the headspace of the package, chitosan suspension develops transparency due to its dissolution at lowered pH.

Methods: For fabricating a CO₂ indicator, 0.3% brilliant blue R-250 dye was incorporated into the chitosan dissolved in 0.1 M HCl aqueous solution. The pH of the chitosan-dye mixture was adjusted to 7 and stirred for 2 h. The CO₂ indicator was stored in an incubator filled with CO₂ gas to investigate the time-dependent visual changes. Transparency level and color of the CO₂ indicator solution were measured every 20 minutes.

Results: At the start of incubation, the indicator solution was opaque because chitosan is insoluble at neutral condition (pH 7). Upon increase of CO₂ levels, the pH of indicator solution decreased due to carbonic acid formation; thereby, the solution became transparent and color changed to dark blue. The signal response of the CO₂ indicator was time dependent; pH of the CO₂ indicator was decreased below pH 6 under 100% CO₂ incubation. In addition, under this condition, transparency and blueness of the CO₂ indicator increased over time.

Significance: The visual chitosan-based CO₂ indicator can indicate partial pressure changes of CO₂ and has a potential to be applied to examine optimal ripeness of fermented vegetables such as kimchi and microbial spoilage.