Latest advances in imaging techniques for characterizing soft, multiphasic food materials

Highlights

• The microstructure of food determines its physicochemical and nutritional properties.

• Food systems are complex, colloidal multiphasic materials containing biomolecules.

• Suitable characterization techniques are required to investigate microstructure.

• Application of several imaging techniques to food systems were critically reviewed.

Abstract

Over the last two decades, the development and production of innovative, customer-tailored food products with enhanced health benefits have seen major advances. However, the manufacture of edible materials with tuned physical and organoleptic properties requires a good knowledge of food microstructure and its relationship to the macroscopic properties of the final food product. Food products are complex materials, often consisting of multiple phases. Furthermore, each phase usually contains a variety of biological macromolecules, such as carbohydrates, proteins and lipids, as well as water droplets and gas bubbles. Micronutrients, such as vitamins and minerals, might also play an important role in determining and engineering food microstructure. Considering this complexity, highly advanced physio-chemical techniques are required for characterizing the microstructure of food systems prior to, during and after processing. Fast, in situ techniques are also essential for industrial applications. Due to the wide variety of instruments and methods, the scope of this paper is focused only on the latest advances of selected food characterization techniques, with emphasis on soft, multi-phasic food materials.

Introduction

In the last few decades, demands arising from both governmental institutions and consumer associations set specific goals for food science and technology, seeking to improve the nutritional quality of food products. Obesity, lactose intolerance and celiac diseases are examples of diet-related conditions that require addressing [3,12,28,100]. Moreover, vegan, vegetarian, and religion-based diets [41] represent a significant share of consumers' preferences. Broadly speaking, there is an increasing trend in developing “clean label” products [8] and introducing functional foods (e.g., antioxidants, probiotics) to the market [77,139]. Lastly, enhancement of food shelf-life is necessary to reduce food waste and to improve products' quality during storage.

Food manufacturers and researchers are tackling these challenges through 1) New Product Development (NPD), 2) Existing Product Development (EPD) and 3) monitoring quality attributes online during manufacturing, using robust, rapid and reliable technologies. Food scientists manipulate micron-sized structures, such as biological macromolecules and colloidal assemblies, because these structures critically participate in transport properties, physical and rheological behaviour, as well as textural and sensorial traits in edible systems [4]. Generally, the vast majority of food materials can be described as soft condensed matter, where basic building blocks self-organize into larger, more complex structures with intricate phase diagrams [156].

Specific food structures and properties can be generated at the microscale in a variety of ways: health sensitive ingredients in foods can be replaced with alternative substances that mimic organoleptic and textural properties of the original ingredients [116]; digestion and absorption profiles of the main food biopolymers can be modified with the aid of single or multiple emulsions, through encapsulation or gel formation [109]. Additionally, in the last decade, nanotechnology has been applied to manipulate and design food structures. In fact, the effectiveness of nutrients and antioxidants delivery can be improved by organizing the assembly of food components on the nanoscale [70].

Despite significant work in food science research, further efforts are required to understand the relationship between processing, microstructure and macroscopic properties, such as sensorial attributes (taste and texture) and nutritional aspects (calorific density and delivery profiles). Investigating food microstructure is, however, a challenging task: foods are complex multicomponent and multiphase systems, and the microstructural elements are difficult to distinguish in both their natural and processed states [4], not to mention their reciprocal interaction in the final product. Simulative technologies represent an additional tool for researchers to investigate the physiochemical properties of single molecules or larger assemblies. With the increasing accessibility of computational tool packages, simulations are becoming a common addition to research [13,46,47]. The behaviour of proteins, carbohydrates, lipids and colloidal particles can be predicted by computational efforts and used to complement results from experimental techniques [45]. Complex, multi-phasic systems such as chocolate during the conching process have been modelled using molecular dynamics [58]. The authors successfully simulated the interactions between lecithin molecules and sucrose crystals in a non-aqueous matrix (cocoa butter). In addition, computational power can be invested in high-throughput, image-analysis algorithms to study dynamic processes with microscopic techniques image [52,144].

Typically, the spatial distribution of phases and localization of particles in multiphasic materials is determined via specific imaging techniques that exploit a broad range of electromagnetic radiation, from X-Rays through visible to infrared waves. Spatial resolution, together with sample preparation, are key factors in determining which technique is most suitable for a specific material.

Some of the latest advances in characterization techniques applied to food microstructure over the past five years are presented here. A particular emphasis is put on the analysis of soft, multiphasic systems. The advantages and limitations of each technique are discussed in this article, together with consideration of their use in tackling specific challenges in the food industry, including nutraceutical delivery, shelf-life improvement, calorific reduction and the incorporation of “clean label” ingredients. Table 1 summarizes the techniques considered in this review paper, along with the main advantages and limitations of each technique.