Application of laser backscattering imaging for the physico-chemical characterisation of antimicrobial silica particles functionalised with plant essential oils

https://doi.org/10.1016/j.jfoodeng.2020.109990Get rights and content

Abstract

The capacity of orthogonal imaging applied to laser-backscattering for characterising antimicrobial particles based on immobilised essential oils was tested. Different particles were synthesised using various particle and oils. Samples were characterised physico-chemically and by an imaging technique. The technique recorded the generated patterns because of the laser-particles interaction during the sedimentation process. The series of images were transformed into an orthogonal image. Data extraction varied depending on the fragmentation degree of image length. After the multivariate analysis, the physico-chemical results showed variability due to particle size. That variability diminished the effect of oils for large sizes. The imaging data collected these properties, which could be used to recognise both particle size and oil type. Thus the prediction of the properties was successful. The position in the physico-chemical space of variance was also predicted. Hence this technique could complement a low-cost method to evaluate the properties of functionalised particles with oils.

Introduction

Emerging problems that negatively impact some synthetic antimicrobials and consumer health have been evidenced. Facts like abuse of antimicrobial substances due to inadequate traditional food-conservation methods increase the antibiotic resistant strains of bacteria and fungi, which all render the development of new strategies to prevent food spoilage and contamination necessary (Pisoschi et al., 2018). One of the main approaches in this area is to use naturally-occurring antimicrobial chemicals. Research into modifications in natural compounds has focused on changes in their properties. Some examples are solubility, dispersion across food matrices, avoiding volatility, etc. The main aim is to amplify the antimicrobial effect and to reduce the impact on products’ organoleptic properties (Weiss et al., 2009). Accordingly, immobilisation of compounds on solid matrices has a high potential as regards the above-mentioned aim. This process provides incremented antimicrobial capacity from a small amount of compound compared to its free version. One of the main groups of these compounds is plant essential oils. Our research group has developed and tested these compound types by immobilising them onto particles of different materials, sizes and morphologies (silica) with successful results for both solid and liquid foods (Ribes et al., 2019; Ribes et al., 2017; Ruiz-Rico et al., 2017). The result is many possible particle types whose physico-chemical properties (zeta-potential, particle size distribution, surface area, uniformity, etc.) may vary significantly depending on the selected combinations. We observed that these properties could be determining factors for the effectiveness of a pre-designed antimicrobial. This effectiveness is associated with behaviour in relation to the food matrix. Therefore, controlling the properties of these compounds is a crucial aspect for their industrial and biotechnological applications (Dickinson, 2012). However, different equips and devices, and a relatively long time, are needed to acquire a complete data pool (particle size properties, zeta potential, thermos-gravimetric analysis, etc.).

So numerous methods can characterise properties related to particle size and distribution due to repercussions on other properties. Some of the most frequently used methods are the liquid sedimentation method, microscopy and laser diffraction scattering. The liquid sedimentation method has been widely used as the standard method (ISO 13317-4:2014). It is based on a direct mass measurement to give the mass distribution of the equivalent spherical particle diameter. Microscopic methods are relatively simple, but need a long measuring time when particle size distribution is wide. The particles tracking analysis (PTA) has reported good results by image analysis procedures to determine not only size properties, but also the behaviour of particles during sedimentation (Śliwa et al., 2015). Laser-based measuring techniques can be classified into three groups according to their measuring principle: local filter technology, Fraunhofer diffraction, and laser-backscattering (Emmerich et al., 2019). Laser-backscattering is the most employed technique for inline applications. One example is dynamic light scattering (DLS) (Yin, 2012). It measures a stable flow of dispersed particles to acquire information with a light detector. It requires accurate calibration from a direct method (Bell et al., 2012). Laser-backscattering methods have also been applied to model and characterise food properties, and to process both solid and fluid food matrices. These approaches are based on a simple device, which also includes image analysis procedures. In this case, the laser's interactions with samples are captured as diffraction patterns in digital images. The image capturing regime depends on the type of required information. Static patterns from different samples can be captured in single images to study the static properties of a given matrix. When the characterisation of a sample depends on the evolution of properties over time, many imaging captures should be carried out in a dynamic regime.

The patterns from these digital images are processed and transformed into numerical data. This information can be used to predict simultaneously food features and process parameters for non-destructive physico-chemical monitoring. Some examples of static characterisations with which we work are the prediction of rheological properties from vegetable-based creams (Verdú et al., 2018) and the physico-chemical properties of biscuits with different fibre contents (Verdú, Barat and Grau, 2019a). The dynamic study of patterns allowed us to monitor the texture of milk during fermentation for yogurt production (Verdú, Barat and Grau, 2019b). In the present work, orthogonal imaging was applied to simplify the collection of laser patterns information with time by a dynamic analysis approach. This image mode means capturing information from the image sequence given by a dynamic process in a single image. Hence this approach reduces the imaging transform need and can improve previous applications to capture dynamic information over time.

This work focused on studying the application of orthogonal imaging for the physico-chemical characterisation of antimicrobial silica particles functionalised with plant essential oils.

Section snippets

Experiment procedure

The experiment focused on characterising antimicrobial particles in terms of their physico-chemical properties by orthogonal images applied to the laser-backscattering technique. In order to obtain wide variability in the properties of antimicrobial particles, they were manufactured by modifying some factors: particle size distribution, F1; type of immobilised essential oil, F2 (Fig. 1). F1 had three categories (a, b and c) of the used commercial silica particles, whose particle size

Physico-chemical characterisation

A PCA was carried out to simultaneously study the physico-chemical properties of particles from the analysed variables (Fig. 3A). This analysis provided a geometrical space of variance from which the relations between both physico-chemical properties and categories could be studied in an extensive lot of particles. This space of variance could represent a space formed by a given lot of particles used in industry or a laboratory from which its properties could be interpreted after model training.

Conclusions

Orthogonal imaging applied to the laser-backscattering technique could be used to characterise the physico-chemical properties of different categories of functionalised silica particles with plant essential oils. Differences in particle size generated most of the observed physico-chemical variance, and diminished the effect of the different essential oils when size increased. Imaging technique information could be used to characterise particles in size and essential oil type terms. A common

CRediT authorship contribution statement

Samuel Verdú: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Supervision, Visualization. Maria Ruiz-Rico: Methodology, Formal analysis, Supervision, Writing - original draft. Alberto J. Pérez: Software, Data curation, Writing - original draft. José M. Barat: Project administration, Funding acquisition, Supervision. Raúl Grau: Conceptualization, Writing - original draft, Writing - review & editing, Supervision, Project administration, Funding

Acknowledgement

The authors gratefully acknowledge the financial support from the University Polytechnic of Valencia for Programme “Ayudas para la Contratación de Doctores para el Acceso al Sistema Español de Ciencia, Tecnología e Innovación, en Estructuras de Investigación de la UPV (PAID-10-17)”, “Generalitat Valenciana” for their postdoctoral fellowship (APOSTD/2019/118) and the Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and FEDER-EU (Project RTI2018-101599-B-C21

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