The invention of paper is closely connected with the development of human civilisation. A large percentage of the world's valuable historical heritage is recorded on paper carriers and stored in conditions that create favourable conditions for the growth and proliferation of microorganisms. The most important microorganisms threatening cultural heritage are microscopic filamentous fungi. Filamentous fungi found in archives, libraries and other memory institutions are frequently from the genera Penicillium, Aspergillus, Cladosporium, Chaetomium and Trichoderma (Pinheiro, Sequeira, and Macedo 2019).
Paper, as an organic substrate with hygroscopic character, provides filamentous fungi with nutrients that serve as a source of energy. Fungi can cause severe damage to paper-based heritage objects, associated with various chemical, physical or aesthetic changes. Physical processes of fungi-induced degradation occur mainly under the influence of hyphal growth or by forming fruiting bodies on the contaminated object. The formation and spreading of mycelium over the object's surface changes the visual properties, ultimately affecting the aesthetic value (Savković et al. 2022). Certain filamentous fungi are characterised by producing of paper-staining pigments or weak organic acids that can induce discolouration (Dunca and Ardelean Elena 2014a). The production of organic acids lowers the pH of the paper, which can lead to acid hydrolysis (Híreš 2015a). Another consequence of the degradative activity of fungi might be so-called foxing – the formation of isolated reddish-brown spots (Szulc et al. 2018). Thanks to their enzymatic activity, structural alternations associated with the degradation of the main component of paper – cellulose fibres – occur, leading to the loss of the characteristic structure of the paper and changes in its properties. Paper progressively loses its strength, becomes brittle, and even object destruction might occur (Híreš 2015b; Dunca and Ardelean Elena 2014b). Besides, filamentous fungi can participate in degrading historical glues, oil binders or other essential added components (e.g. adhesives, antioxidants, etc.) (Dunca and Ardelean Elena 2014b; Sterflinger and Piñar 2013).
Before any intervention by the conservator, the cultural heritage objects should be diagnosed to evaluate their current state, including assessment of biological damage. Knowing a complete set of information about the microbial communities contaminating artefacts is necessary. Proper detection, characterisation, and identification of microflora using a suitably chosen combination of methods are crucial for appropriately assessing potential threats to cultural heritage artefacts (Savković et al. 2019). Unfortunately, the numbers of standard approaches used for detecting and identifying microorganisms are time-consuming, destructive and not very simple. Spectroscopic techniques are a promising, non-destructive, fast and simple alternative.
Fischer et al. (Fischer et al. 2006) and Shapaval et al. (Shapaval et al. 2012) present methods that can serve as a basis for differentiation and strain characterisation of various species of filamentous fungi based on Fourier Transform Infrared Spectroscopy (FTIR). Lecellier et al. (Lecellier et al. 2014) showed that FTIR spectroscopy is promising for routine fungal identification. A study by Salman et al. (Salman et al. 2010) shows differences between examined fungi genera using FTIR ATR spectroscopy. Surface Enhanced Raman Spectroscopy (SERS) was employed to discriminate fungi found in biofilms formed on earthen architecture walls in the region known as Paraíba Valley (or São Paulo Historical Valley) (Fazio et al. 2018).
UV-Vis-NIR FORS (Fibre Optic Reflectance Spectroscopy) is a helpful tool as the technique has several advantages: (1) it requires short times of analysis (a good quality spectrum can be obtained within a few seconds); (2) easy to use; (3) non-destructive and non-invasive technique — no physical or chemical alteration to the analysed area; (4) availability of not very expensive instrumentation (Aceto et al. 2014); (5) instruments do not set any limitation to the size or shape of analysed objects; (6) portability of instrumentation (Analytical Methods Committee AMCTB No 75 2016). UV-Vis-NIR spectroscopy has been increasingly used in cultural heritage protection in recent years. Its variant with optical fibres is becoming one of the most widely used tools for analysing heritage objects and materials (Analytical Methods Committee AMCTB No 75 2016), as it enables the acquisition of spectral data directly in collection-keeping institutions. The method has already been used for the identification and characterisation of pigments (Garofano et al. 2016), the study of degradation mechanisms, and differentiation between different textiles (wool, silk, cotton, and bast) (Analytical Methods Committee AMCTB No 75 2016), characterisation of discolouration in historical plastic objects (Angelin, Eva Mariasole; Cucci, Costanza; Picollo 2020), identification of iron-gall inks (Gál et al. 2013), identification of corrosion products on bronze artefacts (Liu et al. 2021), and many others. Nevertheless, it has rarely been applied in studying the microbial contamination of historical artefacts.
This research aimed to investigate the potential of portable UV-Vis-NIR fibre optics reflectance spectroscopy as a tool for the non-destructive study of filamentous fungi contaminating paper and heritage paper documents.