Non-invasive measurement of bacteria levels in packaged ready meals

We all know that pathogenic microbes can cause product spoilage and food poisoning when eaten. The trick for ready meal producers is to minimise microbial contamination in their products and to ensure storage and transport environments do not foster the proliferation of microbes while simultaneously maximising ready meal shelf life.

Manufacturers also need to know about rates of microbial growth so that when they assign their ‘best before’ dates they can ensure only good quality product reaches their customers. To avoid the risk that any particular ready meal will go bad and cause illness, it is frequently given an unnecessarily short shelf life. This, of course, impacts production and retail schedules.

As microorganism growth is driven by many factors, until now it has been far from easy to accurately estimate the amount of bacteria within food containers at any given time. A better understanding of the growth process of microorganisms could reduce food waste and prevent people from being sickened by food poisoning — or both.

Now a group of researchers from Zhejiang Normal University in China and Umeå University in Sweden report a fast, accurate and non-invasive technique for monitoring bacterial growth.

“Microorganism growth is always associated with the production of carbon dioxide (CO₂),” said Jie Shao, associate professor at the Institute of Information Optics, Zhejiang Normal University, Jinhua, China. “By assessing the level of CO₂ within a given closed compartment — bottle or bag — it’s possible to assess the microbial growth.”

The researchers reported the results* in Applied Optics, a journal of The Optical Society (OSA).

Several detection techniques are currently capable of rapid and accurate measurements of gas compositions. Those based on optical spectrometry are most appealing because they’re non-invasive, boast high sensitivity, provide instant responses and are potentially useful for assessment of bacterial growth.

“A technique referred to as ‘tunable diode laser absorption spectroscopy’ (TDLAS) is particularly suitable because it combines all of these properties with an ease of use and low cost,” Shao said.

So the group decided to develop an easy-to-use instrument based on TDLAS to assess bacterial growth of various types of samples under a variety of conditions.

TDLAS is by far the most common laser-based absorption technique for quantitative assessments of species within a gas phase. It can be used to measure the concentration of specific gaseous species — carbon monoxide, CO₂, water or methane, to name a handful — within gaseous mixtures by using absorption spectrometry based on tunable diode lasers.

“One major advantage TDLAS offers is its ability to achieve very low detection limits, on the order of parts per billion,” Shao said. “Apart from concentration, it’s also possible to determine other properties of the gas under observation — temperature, pressure, velocity and mass flux.”

The group’s basic set-up simply involves a tunable diode laser as the light source, beam-shaping optics, a sample to be investigated, receiving optics and one or more detectors.

“The emission wavelength of the laser is tuned over a characteristic absorption line transition — of the species within the gas being assessed,” Shao explained. “This causes a reduction of the measured signal intensity, which we can use to determine the gas concentration.”

When the wavelength is rapidly tuned across the transition in a specific manner, it can be combined with a modulation technique called ‘wavelength modulation (WM), which gives the TDLAS technique an enhanced sensitivity. It’s referred to as WM-TDLAS.

By applying the technique to transparent containers of organic substances such as food items or medical samples, bacterial growth can be quickly evaluated. “Although we anticipated that the WM-TDLAS technique would be suitable for assessing bacterial growth, we didn’t expect this level of accuracy,” Shao noted.

In contrast with conventional and more invasive techniques that require contact with the tested items, the WM-TDLAS method is truly non-invasive, making it ideal for monitoring the status of food and medical supplies, or as a tool to determine under which environmental conditions bacterial growth is expected to be severe. “It can provide real-time analysis,” Shao said.

Next, the researchers plan to enhance the technique to “allow for assessments of microbial growth in a large variety of samples — expanding beyond food items and medical supplies”, Shao added.

*Paper: Jie Shao, Jindong Xiang, Ove Axner, and Chaofu Ying, ‘Wavelength-modulated tunable diode-laser absorption spectrometry for real-time monitoring of microbial growth’, Appl. Opt. 55, 2339-2345 (2016).

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