A rub and a click to smart-check your fish

Bengaluru: How fresh is your fish? The answers, typically hinging on the seller’s word or complex quality assessment processes that require scientific expertise, could get easier with a new indicator that tracks the quality of fish, right in your kitchen.

A team of researchers in the Department of Chemistry at Christ (Deemed to be University), Bengaluru, has developed a smartphone-assisted colorimetric fish quality indicator that monitors the fresh, spoiling and spoiled states of fish. The indicator was developed using printing paper and red cabbage and works on a single-step rub-coating method. Rohu fish samples, in different storage conditions, were used for the study.

Credit: DH Graphic

Halochromic materials (that change colour with changes in pH) are traditionally used to monitor fish spoilage, but existing methods to develop these halochromic indicators are time-consuming and cannot be used in household settings, the researchers said.

The team comprising Chaithra KP, Prasiddha Nagarajan, and Vinod TP used the simpler rub-coating method to bring out the anthocyanins (halochromic materials present in red cabbage) onto the paper, without extracting the compounds from the vegetable. By rubbing red cabbage on paper, they created a colorimetric interface that displayed a visible colour change when it was used to assess the quality of the fish — from purple (fresh) to blue (spoiling) and then to blue-green (spoiled). These colour changes correspond to the total volatile basic nitrogen (TVB-N) and pH changes.

To analyse these colour changes, the team used the Android smartphone application 'Color Grab'. The app provided a closer reading of the colours, changing in accordance with TVB-N and pH values. These values are reflected as colour changes due to the presence of the anthocyanins.

While red cabbage has been used to track food quality in existing research, the new study, the researchers noted, is different in that it employs the rub-coating process and combines it with a smartphone-based colour analysis. The interface’s performance was also compared with indicators prepared using anthocyanins extracted from red cabbage.

Vinod, Associate Professor in the Department of Chemistry, said that while the extraction method — using organic solvents — delivered “slightly better” results in terms of uniformity, the difference was not significant. “This is a proof of concept — the study pitches a fish spoilage indicator that can be put together with materials available at home and does not need any expertise, either in chemistry or food sciences,” Vinod told DH.

The research findings were published in the peer-reviewed journal, Colloids and Surfaces A: Physicochemical and Engineering Aspects.

How the indicator works

In the experiments, the cabbage-rubbed paper was placed 2 cm near the fish samples.

The TVB-N in a gaseous state — set off by the spoilage in fish — came in contact with the paper and the freshness levels were indicated in the colour changes, in about 30 seconds.

“The smartphone app was needed to quantify these changes because the appearance of colours can be subjective through the naked eye. With an RGB analysis, you can mark the intensity of these colours in numbers,” Vinod said.

The researchers pitch the indicator as an alternative to best-before labels. The findings, they said, could help minimise wastage and fabricate quality indicators for other perishable foods.