Laser tool for detecting bacterial growth in packaged food

The researchers are developing a new, low-cost, fast and accurate laser tool that can detect bacteria growth inside packaged food without having to open the package and take a sample.

The researchers – from Zhejiang Normal University in China and Umeå University in Sweden – describe the device and how they tested it on two types of bacteria in the journal Applied Optics.

The reason packaged food has “sell-by” and “use-by” dates is to avoid the risk that it may go bad – due to growth of bacteria and other microorganisms – and cause illness.

Such precaution means food often has an unnecessarily short shelf-life. A better understanding of the growth process of microorganisms – and its detection – could help reduce food waste and perhaps also the number of people who get food poisoning, note the researchers.

Similarly in medicine, a tool that could quickly and non-invasively detect bacterial growth could save time and reduce waste of precious medical resources.

For example, it is important to be able to measure the quality of blood samples quickly and accurately. If they are contaminated, they may have to be discarded and repeated. Also, bacterial growth in medical blood supplies – while rare – means the blood has to be thrown away. And if it is not detected, then there is the risk of infecting patients and possibly death.

A rapid screening tool means a larger percentage of blood could be directly tested for possible bacterial contamination.

Optical spectrometry is sensitive and gives instant results

However, microorganisms are complex beings – their growth is driven by many factors. This makes it difficult to estimate how much bacteria might be present inside sealed packages of food or blood.

For their study, the team focused on the fact that bacteria let off gas – for example carbon dioxide – as they multiply. First author Jie Shao, assistant professor at the Institute of Information Optics at Zhejiang, says:

“By assessing the level of [carbon dioxide] within a given closed compartment – bottle or bag – it’s possible to assess the microbial growth.”

The technology that shows much promise for accurate measurement of gas composition is optical spectrometry. It is highly sensitive, provides instant results and can be used non-invasively, such as through glass or the see-through films and plastics used to package food.

For their study, the researchers focused on one particular optical technology called “tunable diode laser absorption,” or TDLAS. They decided to investigate it because it combines all technical requirements of a measurement tool “with an ease of use and low cost,” explains Prof. Shao.

TDLAS can measure concentrations of various gases – including carbon monoxide, carbon dioxide, water and methane – within a mixture. It uses tunable diode lasers to measure their presence via absorption spectrometry – a technique that can detect compounds from the specific and unique way their elements absorb different wavelengths of light.

The TDLAS device the team is developing comprises a tunable laser diode as the light source, beam-shaping optics, a place to carry the sample, plus receiving optics and one or more detectors.

Detects gases at parts per billion

The device works on the principle that when you shine a light on a sample, the different gases it contains will each absorb a particular wavelength.

By getting the tunable diodes to emit different wavelengths, when a gas in the sample absorbs a particular wavelength, the device determines the amount of gas present from the reduction in the measured signal intensity (the signal-to-noise-ratio).

Combining the rapid tuning technique with a method called “wavelength modulation” or WM, makes TDLAS even more sensitive. This technique is called “WM-TDLAS.”

Prof. Shao says one of the features that makes the technology particularly attractive is that it can detect gases at very low concentrations – in the order of parts per billion.

“Apart from concentration,” adds Shao, “it’s also possible to determine other properties of the gas under observation – temperature, pressure, velocity and mass flux.”

In their paper, the team describes how they tested the WM-TDLAS approach on two types of bacteria, Staphylococcus aureus and Candida albicans, and found it can produce “high signal-to-noise-ratio data from bacteria grown in confined spaces and exposed to limited amounts of nutrients.”

Prof. Shao says the method can provide real-time analysis, and notes:

“Although we anticipated that the WM-TDLAS technique would be suitable for assessing bacterial growth, we didn’t expect this level of accuracy.”

The team now plans to develop the technique for detecting microbial growth in a range of other applications beyond food and medical supplies.

Meanwhile, Medical News Today recently learned how laser technology in another medical field has enabled surgeons to open the blood-brain barrier to enable chemotherapy to zap glioblastoma, the most common and aggressive brain cancer.