Citation: Lipi ´ nska, E.; Pobiega, K.; Piwowarek, K.; Bla ˙ zejak, S. Research on the Use of Thermal Imaging as a Method for Detecting Fungal Growth in Apples. Horticulturae 2022, 8, 972. https://doi.org/10.3390/ horticulturae8100972 Academic Editors: Jianwei Qin and Elazar Fallik Received: 3 September 2022 Accepted: 18 October 2022 Published: 20 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). horticulturae Communication Research on the Use of Thermal Imaging as a Method for Detecting Fungal Growth in Apples Edyta Lipi ´ nska *, Katarzyna Pobiega * , Kamil Piwowarek and Stanislaw Bla ˙ zejak Department of Food Biotechnology and Microbiology, Institute of Food Sciences, Warsaw University of Life Sciences—SGGW, Nowoursynowska 159cStr., 02-776 Warsaw, Poland * Correspondence: edyta_lipinska@sggw.edu.pl (E.L.);katarzyna_pobiega@sggw.edu.pl (K.P.) Abstract: This study assesses whether thermal imaging can be utilized for detecting and monitoring the level of plant product contamination with apples used as an example. The growth of Penicillium expansum, Botrytis cinerea, and Rhizopus stolonifer on apples contaminated with these fungi was investigated by measuring temperature changes using a thermovision camera. The results showed a significant relationship between the temperature of apples and the growth of microorganisms, as well as that the temperature changes occurred in two stages (temperature of the contaminated apples increased on the first day but then decreased). Significant differences were found between the temperature of the apples showing microbial growth and the noncontaminated control sample, which indicates that the thermal imaging technique has a potential application in microbial quality control. Automation of the production process and attempts on the intensification of production capacity have resulted in the need to improve individual stages of product quality control. Thermovision- based methods have a high potential in this field, as they represent an innovative and noninvasive alternative to conventional microbiological diagnostic methods. Keywords: thermal imaging; microbiological quality; apples 1. Introduction Apples are an excellent example of both a raw material and a product that can be consumed in either natural or processed form (juices, pomace, dried fruit). Fresh apples are susceptible to contamination by microorganisms such as yeasts (Saccharomyces, Pichia, Kloeckera, Candida, etc.), molds (Penicillium, Aspergillus, Rhizopus, Botrytis, Fusarium, Mucor), and coliform bacteria [1]. Filamentous fungi are cosmopolitan and oligotrophic organisms. They can easily modify their metabolism according to the prevailing physical conditions and the type of nutrients present in the substrate. These organisms are also capable of secreting various enzymes and thus can transform macromolecular compounds into simpler absorbable forms [2]. Molds are aerobic organisms that grow on the surfaces of products, forming abundant air mycelium. However, mechanical removal of a visible mycelium does not guar- antee the complete elimination of the fungus, as its other portion, including deep mycelium and metabolites (e.g., mycotoxins), remains in the substrate. Thus, these microorganisms occur both on the surface and inside of the fruit [3]. Fungal species, such as Penicillium expansum which causes a common apple disease called soft rot, can cause molding of the seed core in apparently healthy apples with no surface changes. The most common apple diseases of fungal origin are caused by gray mold (Botrytis cinerea). Spores of this mold can easily penetrate a fruit through any skin damage and removed stalks. They are also capable of growing at a wide range of temperatures, and thus cause extensive damage to raw materials [4]. Mold diseases pose a significant threat to plant producers, due to the rapid development of infections and huge economic losses, as well as affect consumers’ health. Therefore, it is necessary to ensure early detection of changes in raw materials [5]. Horticulturae 2022, 8, 972. https://doi.org/10.3390/horticulturae8100972 https://www.mdpi.com/journal/horticulturae