Why do carrots curl? Research reveals mechanisms behind aging of root vegetables

Chopped carrot pieces are one of everyone’s most loved foods and a snacking staple: a mainstay of school lunchboxes, picnics and party platters all year round.

Researchers from the University of Bath have discovered the secret science of preparing these popular root vegetables and quantified the processes that cause them to shrivel up if left unused for too long.

Mechanical engineering student Nguyen Vo-Bui conducted the research as part of his final year studies, under the limited circumstances of COVID-19 lockdowns in 2021. The research paper, “Modeling cut carrot curling longitudinally induced by the vascular cylinder-cortex interference pressure”, is published in Royal Society Open Science.

Without access to laboratories, Nguyen aimed to identify the geometric and environmental factors that have the most influence on the longevity of carrots. Working in his kitchen, he characterized, analytically modeled and verified the aging of over 100 Lancashire Nantes carrot halves, cut lengthwise, using finite element (FE) models normally used in engineering structures.

The research team concluded that residual stress and dehydration were the two key factors behind curling behavior. The starchy outer layer of the carrot (the cortex) is stiffer than the soft central vein (also known as the vascular cylinder). When cut lengthwise, the two core halves curl because the stress difference becomes unbalanced. Dehydration causes further loss of stiffness, thus enhancing the curling effect.

Their recommendations to manufacturers include handling carrots in cold, humid, airtight, and humidity-controlled environments to protect their natural properties and increase their edible lifespan.

They say the study provides a methodology for predicting the deformation of cut root vegetables, adding that the procedure is likely to be applicable to other plant structures. The study gives food producers a new mathematical tool that could be applied to the design of food packaging and handling processes, potentially reducing food waste.

Carrot, one of the most important crops in the world in terms of market value, is known for its high production efficiency, but despite this, wastage is high. About 25-30% of these problems occur before processing and packaging, due to deformation, mechanical damage or infected sections. Freshly cut, minimally processed carrots are a convenient ready-to-use ingredient that allows use of carrots that might otherwise be thrown away, reducing food waste.

Dr Elise Pegg, a senior lecturer in the Department of Mechanical Engineering at Bath, is one of the authors of the research paper and supervised the study. She said: “We mathematically represented the curvature of a cut core sample over time and showed the factors that contribute to the curvature.

“Our motivation was to look for ways to improve the sustainability of carrot processing and make it as sustainable as possible. We produced a methodology that a food producer could use to change their processes, reduce food waste and make easier packaging and transportation. Understanding the bending behavior of such systems can help us design and manufacture products with greater durability.

“A question like this would normally be studied from a biological perspective, but we did this work using purely mechanical principles. I’m very happy for Nguyen: it’s a measure of his ingenuity and dedication to produce such interesting research in a difficult situation.”.

Over the course of a week, the curvature of the core halves increased, with the average radius of curvature of each core increasing from 1.61 m to 1.1 m. A 1.32-fold reduction in stiffness was also observed, correlating with core drying; on average, their weight decreased by 22%.

Nguyen added: “It was interesting research: applying mechanical principles to vegetables was surprising and fun.

“One of the big challenges was to design an experiment that could be carried out in a confined environment, without access to normal laboratories and equipment. Now being able to publish this work in an academic journal and potentially be used by industry food industry is truly rewarding.

“This project inspired me to continue my studies at the University of Bath and I am now studying residual stresses in porous ferroelectric ceramics for my PhD.”

In addition to having to use a suitcase to collect the 30 kg of carrots required by the experiment from a farmers’ market, another challenge was finding ways to use them afterwards. Carrot cake, Indian carrot dessert Gajar Ka Halwa, carrot pesto and many other dishes kept Nguyen and his roommates fed for several days.