Determination of the thermal properties of leaves by non-invasive contact‑free laser probing

The thermal properties of materials provide valuable data for quality monitoring and the rational design of process steps where heating is required. Here we report a rapid, simple and reliable technique that determines the most important thermal properties of leaves, i.e. the specific heat capacity (cp) and thermal conductivity (λ). Such data are useful when leaves are heated during processing, e.g. for the precipitation of host cell proteins during the extraction of high-value products such as recombinant proteins produced by molecular farming. The cp of tobacco (Nicotiana tabacum) and Nicotiana benthamiana leaves was determined by infrared measurement of the temperature increase caused by a near‑infrared laser pulse of defined length and intensity. We used the sample temperature profiles to calculate λ based on exponential fits of the temperature decline, taking convective heat transfer and thermal radiation into account. We found that the average cp was 3661 ± 323 J kg−1 K−1 (n = 19) for tobacco and 2253 ± 285 J kg−1 K−1 (n = 25) for N. benthamiana, whereas the average λ was 0.49 ± 0.13 (n = 19) for tobacco and 0.41 ± 0.20 (n = 25) J m−1 s−1 K−1 for N. benthamiana. These values are similar to those established for other plant species by photothermal imaging and other methods. The cp and λ values of leaves can be determined easily using our non-invasive method, which is therefore suitable for the in-line or at-line monitoring of plants, e.g. during the highly regulated production of biopharmaceutical proteins.