Microstructure of Dairy Products

Mamdouh El-Bakry, Antonio Sanchez, Bhavbhuti M. Mehta, Mamdouh El-Bakry, Antonio Sanchez, Bhavbhuti M. Mehta

• Publisher: Wiley-Blackwell
• Year: 2018
• Print ISBN:
  9781118964224
• eBook ISBN:
  9781118964200
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ciencia de los alimentos

Provides the most recent developments in microscopy techniques and types of analysis used to study the microstructure of dairy products 

This comprehensive and timely text focuses on the microstructure analyses of dairy products as well as on detailed microstructural aspects of them. Featuring contributions from a global team of experts, it offers great insight into the understanding of different phenomena that relate to the functional and biochemical changes during processing and subsequent storage. 

Structured into two parts, Microstructure of Dairy Products begins with an overview of microscopy techniques and software used for microstructural analyses. It discusses, in detail, different types of the following techniques, such as: light microscopy (including bright field, polarized, and confocal scanning laser microscopy) and electron microscopy (mainly scanning and transmission electron microscopy). The description of these techniques also includes the staining procedures and sample preparation methods developed. Emerging microscopy techniques are also covered, reflecting the latest advances in this field. Part 2 of the book focuses on the microstructure of various dairy foods, dividing each into sections related to the microstructure of milk, cheeses, yogurts, powders, and fat products, ice cream and frozen dairy desserts, dairy powders and selected traditional Indian dairy products. In addition, there is a review of the localization of microorganism within the microstructure of various dairy products. The last chapter discusses the challenges and future trends of the microstructure of dairy products.

• Presents complete coverage of the latest developments in dairy product microscopy techniques
• Details the use of microscopy techniques in structural analysis
• An essential purchase for companies, researchers, and other professionals in the dairy sector 

Microstructure of Dairy Products is an excellent resource for food scientists, technologists, and chemists—and physicists, rheologists, and microscopists—who deal in dairy products.

1
Microscopy Techniques for Dairy Products – An Introduction

Mark A.E. Auty

Food Chemistry and Technology Department, Teagasc Food Research Centre, Teagasc, Moorepark, Co. Cork, Ireland

1.1 Introduction

The textural properties of a particular dairy product are strongly influenced by the three‐dimensional arrangement of its structural elements and their interactions (Heertje, 1993). To fully understand the behavior of dairy products, it is therefore not enough to know the chemical composition and bulk physical properties but how they interact and affect the spatial arrangement or organization of the food constituents at the nano‐ and micro‐length scales. Food microstructure studies therefore provide a link between physico‐chemical properties, process behavior and organoleptic qualities of a particular dairy product (Figure 1.1). Linking microscopy with rheological and sensory techniques in particular is necessary for a fuller understanding of food behavior, requiring a multivariate approach to experimental design. This chapter gives a brief overview of the different types of microscopy used to study dairy foods with a focus on confocal microscopy as this is arguably the most useful single technique of benefit to both researchers and food industry technologists.

Figure 1.1 Diagram showing inter‐relationships between microstructure and functionality of dairy products.

1.1.1 Brief History and Background

In the seventeenth century, Antonie van Leeuwenhoek, using a high magnification hand lens, first viewed fat droplets in milk (Leeuwenhoek, 1674). Despite this early start, microscopy of dairy products, and food in general, remained unexplored, with little published literature until well after the development of electron microscopy techniques in the 1940s. As food manufacturers began using microscopes in the 1950s and 1960s, it became apparent that the structural arrangement of food components strongly influenced food processing and quality. Most of the early food‐related electron microscopy work was performed on dairy products, mainly yoghurt and cheese (for reviews see Brooker, 1979; Kalab, 1979a, b, c, 1981, 1993; Holcomb, 1991; Schmidt and Bucheim, 1992). Despite the enormous influence of light microscopy on medical research at the end of the last century and the improvement in optic materials and design, conventional light microscopy of food products remained largely neglected although Lewis (1978) and Flint (1994) published selected methods for light microscopic examination of a range of food ingredients and products, including milk powders and dairy spreads. Contrast between the component of interest and surrounding food material may be achieved by optical techniques, chemical staining, or a combination of both (Flint, 1994). Despite this, optical microscopy of dairy products remained largely neglected until the development of commercial confocal microscopes in the 1990s. In the past 20 years, there has been considerable research interest in food microstructure as a key to understanding structure‐function relationships. A wide range of microscopy techniques is now available for the study of food microstructure, with more being developed (for a review see Morris and Groves, 2013). These techniques are frequently employed to study dairy products such as cheese (El‐Bakry and Sheehan, 2014). The food researcher now has a large toolbox of techniques, the choice of which depends on the particular application. However, a correlative approach employing various microscopy techniques is required to provide a fuller understanding of complex multiphase nano‐ and microstructures (Aguilera and Stanley, 1990; Lewis, 1993). This approach has led to the development of hybrid microscopes such as the RISE (WITech, Ulm, Germany) system which combines a scanning electron microscope with Raman confocal, focused ion beam and even atomic force microscopes, permitting examination of the same sample area by different microscopy techniques.

Many of the common techniques used to study food microstructure have been adapted from specimen preparation procedures for biological tissue. However, there are particular problems associated with the preparation of food products for microscopic examination that the researcher should be aware of. Many foods have high levels of moisture, fat or sugar and preserving the original microstructure of such materials may be difficult, particularly for electron microscopic studies that may require low moisture, conductive specimens. Dried ingredients, such as spray dried powders, crystalline sugars, starches etc. with a moderately small particle size (<100 µm) may be examined in their natural state and require little sample preparation. Highly refractile or opaque solid and semi‐solid food materials however, need to be rendered thin enough to transmit light and generally this is achieved either by compression or sectioning. Soft materials may be compressed or smeared across a microscope slide. Solid food materials and cellular tissues may be chemically fixed, dehydrated then embedded in paraffin wax or plastic resin prior to sectioning by microtomy. Alternatively, frozen sections, approximately 5–20 µm thick, may be cut in a cryostat. Sectioned material can then be observed using any of the optical or chemical contrast techniques described below. Powdered ingredients, for example spray‐dried milk powder particles, should be mounted in a clear, immiscible liquid such as sunflower oil that should be viscous enough to restrict Brownian movement of the particles.

The main microscopy techniques used to study dairy products are listed in Table 1.1.

Table 1.1 Main microscopy techniques used in food microscopy.

Technique	Radiation type	Incident radiation wavelength	Approximate resolution	Application
Stereo‐microscopy	Photons	400–750 nm	5 µm	Overview of microstructure
Light microscopy	Photons	400–750 nm	200 nm	Ingredient localization
Fluorescence microscopy	Photons	350–400 nm (u.v.) 400–750 nm (visible)	200 nm	Ingredient localization
Confocal microscopy	Photons	350–400 nm (u.v.) 400–750 nm (visible)	200 nm	Ingredient localization, 3D information
SEM	Electrons	0.001–0.01 nm	4 nm	Large depth of field‐simulated 3D view
TEM	Electrons	0.01–0.01 nm	1 nm	Fine structural detail, macromolecular interactions
AFM	N/A (physical cantilever)	N/A		Surface topology, nano‐mechanical behavior

1.2 Conventional Optical Microscopy Techniques

1.2.1 Conventional Light Microscopy – Optical Contrast

1.2.1.1 Bright Field

Bright field illumination employs an axial cone of light from the condenser, which is transmitted through the specimen and is commonly based on Koehler illumination. This technique is useful if there is inherent contrast in the specimen, for example in highly colored food products, otherwise stains or dyes may be...

Table of contents

• Cover
• Table of Contents
• List of Contributors
• Preface
• Chapter 1: Microscopy Techniques for Dairy Products – An Introduction
• Chapter 2: Light Microscopy and CSLM Techniques, Principles and Applications
• Chapter 3: Electron Mıcroscopy Technıques
• Chapter 4: Emerging Techniques for Microstructural Analysis
• Chapter 5: Quantitative Image Analysis in Microscopy
• Chapter 6: Microstructure of Milk
• Chapter 7: Microstructure of Cheese Products
• Chapter 8: Microstructural Aspects of Yogurt and Fermented Milk
• Chapter 9: Microstructure of Milk Fat and its Products
• Chapter 10: Microstructure of Ice Cream and Frozen Dairy Desserts
• Chapter 11: Whey Wastes and Powders
• Chapter 12: Microstructure of Selected Traditional Indian Dairy Products
• Chapter 13: Using Microscopy for Microorganism Localization within Dairy Products
• Chapter 14: Microstructure of Dairy Products: Challenges and Future Trends
• Index
• End User License Agreement