Structure of ice cream

The structure of ice cream has been studied in detail using electron microscopy. Trapped air bubbles are found to be separated by only few micrometer-thick layers of the continuous phase. 

The air cell stabilizing effect of agglomerated fat globules, promoted by emulsifiers and the ice-crystal-growth-controlling effect of hydrocoiloid stabilize the foam structure of ice cream to a great extent. This is evident by melt down analysis (see section 5.2) of ice cream exposed to heat shock. 

Figure 12.2. The structure of ice cream mix and ice cream. (A). Fat globules (F) in mix with crystalline fat within the globule and adsorbed casein micelles (C), as viewed by thin section transmission electron microscopy. (B). Close-up of an air bubble (A) with adsorbed fat, as viewed by low temperature scanning electron microscopy. (C). Air bubble (A) with adsorbed fat cluster (FC) that extends into the unfrozen phase, as viewed by thin section transmission electron microscopy with freeze substitution and low temperature embedding.

Adapa, S., Dingeldein, H., Schmidt, K.A., Herald, T.J. 2000. Rheological properties of ice cream mixes and frozen ice creams containing fat and fat replacers. J. Dairy Sci. 83, 2224-2229. Adleman, R., Hartel, R.W. 2002. Lipid crystallization and its effect on the physical structure of ice cream. In Crystallization Processes in Fats and Lipid Systems (N. Garti, K. Sato, eds.), pp. 381-427, Marcel Dekker, New York. 

Fig. 7. The effect of adsorbed protein on structure of ice-cream mix, ice cream, and melted ice cream. A-B, ice-cream mix with no surfactant and with added surfactant, respectively, as viewed by thin-section transmission electron microscopy. f= fat globule, c = casein micelle, arrow = crystalline fat, bar = 0.5 pm. See Reference 24 for methodology. C-D, ice cream with no surfactant and with added surfactant, respectively, as viewed by low-temperature scanning electron microscopy, a = air bubble, f = fat globule, bar = 4 pm. See Reference 34 for methodology. E-F, ice cream with no surfactant and with added surfactant respectively, as viewed by thin-section transmission electron microscopy with freeze substitution and low-temperature embedding. a = air bubble, f= fat globule, c = casein micelle, fc = fat cluster, bar = 1 pm. See Reference 13 for methodology. G-H, melted ice cream with no surfactant and with added surfactant respectively, as viewed by thin-section transmission electron microscopy. f= fat globule, c = casein micelle, fn = fat network, bar = 1 pm in G and 5 pm in H. See Reference 24 for methodology.

Figure 6.2 Thin section transmission electron micrograph of the structure of ice cream at high magnification, showmg fat droplets (marked f) and casein micelles (c) (Reprinted from A Study of Fat and Air Structures in Ice cream , Copyright 1999, with permission from Elsevier (the scale bar is 1 pm))...

In this chapter we look at each component in turn. However, many properties cannot be explained by simply looking at the separate components, so we next consider their interactions in the composite material. The micro structure of ice cream changes as it warms up and is manipulated by the mouth during consumption. Therefore, to understand the sensory properties, we also have to know how the microstructure breaks down during eating. 

Figure 7.10 shows the fat structure of ice cream that is formed in the manufacturing process. Discrete and partially coalesced fat droplets are present both in the matrix and on the surface of the air bubbles. Although the fat emulsion could lower its energy by coarsening, fat is very insoluble in water so coarsening cannot take place by Ostwald ripening. 

H. Douglas Goff, a professor of food science and an ice cream expert from Ontario, Canada, says, There are no real chemical reactions that take place when you make ice cream, but that doesn t mean that there isn t plenty of chemistry. The structure of ice cream contributes greatly to its taste. Tiny air bubbles are formed in the initial whipping process. These bubbles are distributed through a network of fat globules and liquid water. The milk fat has... 

Caillet, A., Cogne, C., Andrieu, J., Laurent, P., Rivoire, A., 2003. Characterization of the structure of ice cream by optical microscopy Influence of freezing parameter on ice crystal structure. Lebensm. -Wiss. u. -Technol. 36 743-749. 

Figure 3.20 Typical structure of ice-cream revealed in an electron micrograph, (a) Ice crystals, average size 50 pm, (b) air cells, average size 100-200 pm, (c) unfrozen material. [From W. S. Arbuckle, Ice Cream, 2nd Edition, Avi Publishing Company (1972) with kind permission of Springer Science and Business Media]...

Arbuckle has published some excellent work on microscopic studies of ice cream crystal structure and effects of hydrocolloids on this crystal structure. Some interesting examples of this work are shown in some of his papers 1,18). 

A typical characteristic of many food products is that these are multi-phase products. The arrangement of the different phases leads to a microstructure that determines the properties of the product. Mayonnaise, for example, is an emulsion of about 80% oil in water, stabilized by egg yolk protein. The size of the oil droplets determines the rheology of the mayonnaise, and hence, the mouthfeel and the consumer liking. Ice cream is a product that consists of four phases. Figure 1 shows this structure schematically. Air bubbles are dispersed in a water matrix containing sugar molecules and ice crystals. The air bubbles are stabilized by partial coalesced fat droplets. The mouthfeel of ice cream is determined by a combination of the air bubble size, the fat droplet size and the ice crystal size. 

H.D. Goff Formation and Stabilization of Structure in Ice-Cream and Related Products. Curr. Opin. Colloid Interface Sci. 7, 432 (2002). 

Figure 19. Cryo-SEM images of ice cream which, in addition to showing the major structural features of the product, illustrate the impact of variable sublimation times and subsequent learnings, viz., bridging among ice crystals (arrow in D). (x 480). [From 64].

Homogeneous systems, such as cooking oil, exist in thermodynamic equilibrium and the properties of these systems are determined by their chemical composition.1 Heterogeneous systems are not in thermodynamic equilibrium. The properties of these systems are governed by both the chemical composition and the internal framework formed by the spatial arrangement of the individual chemical components present. The formation of steric, electrostatic and covalent forces between these individual components can have a dramatic effect on the properties of the product. Ice cream is a classic example, which is primarily composed of ice cream mix and air. The pure components of ice cream have different structural properties in the mixture than they do in their isolated form. Frozen ice cream has a certain consistency and texture, which is quite different from any of the individual frozen ingredients. 

Gelin, J.-L., Poyen, L., Courthadon, J.-L., Le Meste, M., Lorient, D. 1994. Structural changes in oil-in-water emulsions during the manufacture of ice cream. Food Hydrocoil. 8, 299-308. 

Goff, H.D. 2002. Formation and stabilization of structure in ice cream and related products. Curr. Op. Coll. Interface Sci. 7, 432-437. 

Goff, H.D., and Clarke, C.J. (2003). Effects of structural attributes on hardness and melting rate of ice cream. Ice Cream 2 2nd IDF Symposium on Ice Cream, Thessaloniki, Greece, International Dairy Federation, Brussels, 2004. 

The creep-compliance technique has been used extensively by Sherman and co-workers for the study of ice cream, model emulsions, margarine, and butter (Sherman, 1966 Shama and Sherman, 1969 Vernon Carter and Sherman, 1980 Sherman and Benton, 1980). In these studies, the methodology employed was similar to that for ice cream, that is, the creep-compliance data on a sample were described in terms of mechanical models, usually containing four or six elements. Attempts were made to relate the parameters of the models to the structure of the samples studied. However, with increased emphasis on dynamic rheological tests and interpretation of results in terms of composition and structure, the use of mechanical models to interpret results of rheological tests has declined steadily. 

Increasing levels of emulsification significantly depleted protein from the fat globule in the mix. The adsorbed protein content in the mix (mg m of fat surface area) correlated with major characteristic analyses describing the fat structure in ice cream (fat agglomerate size, fat agglomeration index, solvent extractable fat Fig. 6). Thus, the measurement of protein load in the mix can be used to predict ice-cream-fat stability and related structure. Structural analyses indicated enhanced interaction between fat and air as protein adsorption decreased. It was also observed that the fat content in the dripped portion collected from a meltdown test correlated well with other indices of fat destabilization. 

Gelin, J.-L., L. Poyen, J.-L. Courthadon, M. Le Meste, and D. Lorient, Structural Changes in Oil-in-Water Emulsions During the Manufacture of Ice Cream, Food Hydrocoil. 8 299-308 (1994). 

Difference in structure generally implies differences in properties. Take, for example, ice cream. It is made (by freezing and agitation) of ice cream mix and air, and the two systems are very different, as is illustrated in Figure 9.1. Nevertheless, they have exactly the same chemical composition. Also the properties are very different, as we all know when we let ice cream melt, we obtain a product of very different appearance, consistency, and eating qualities. However, melted ice cream is not quite the same as ice cream mix as, for instance, larger fat globules or clumps of them will be... 

Milk fat helps to give body to ice cream, produces a smooth texture, and increases the richness of flavor. Texture is the attribute of a substance relating to its finer structure - the size, shape, and arrangement of small particles. Body is its consistency and firmness, and in the case of ice cream, its melting resistance. Higher amounts of milk fat in ice cream enhances both the texture as well as the body. Insufficient amounts result in a coarse or icy texture. 

To describe the science of ice cream, it is first necessary to describe some of the physical chemistry and colloid science that underpins it these are laid out in Chapter 2. Chapters 3 and 4 cover the ingredients and the ice cream making process respectively. Chapter 5 focuses on the production of various types of ice cream product. The physical and sensory measurements used to quantify and describe it are discussed in Chapter 6, and the micro structure, and its relationship to the texture, is examined in Chapter 7. Finally, Chapter 8 describes a number of... 

Both the total amount of each component and the microstructure i.e. the sizes, shapes and connectivity of the particles) are important. Together they determine the properties of the composite, i.e. the physical and sensory properties of ice cream. The amounts of the structural components are different for different types of ice cream. Table 7.1 shows typical volume fractions of each component at — 18 °C in standard, premium, low fat and soft scoop ice cream, and water ice. Premium ice cream contains more fat than a standard ice cream, whereas a low fat ice cream (obviously) contains less. Soft scoop ice cream contains less ice than the standard. Water ice is a composite of two materials, ice crystals and matrix, and contains no air or fat. 

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