Ice crystal size

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. 

Microscopic analysis is the only method available for estimating ice crystal size in ice cream. Light microscopy, equipped with cold stage and image analysis, may be used for this purpose54. Low temperature scanning electron microscopy may also be used55. 

An NM R protocol for determining ice crystal size distribution during freezing and pore size distributionn during freeze-drying. Appl. Magn. Reson. 17, 537-556,1999... 

Stabilizers are macro-molecules which prevent the formation of large ice crystals. The average ice crystal size should be between 40-50 pm. Stabilizers increase viscosity, improve the smoothness and resistance to melting. The average content depends on the kind of stabilizer, but it is typically between 0.01% and 0.2%. The use of stabilizers corresponds to the content of fat and solids. Stabilizers are important when the fat content is below 12% or the total content of solids is below 40%. 

Freezing forms an amorphous solid of the protein and excipients with associated water in crystalline form. Annealing, an optional step, increases ice-crystal size and allows crystallization of bulking agents (such as glycine or mannitol), removing... 

Ice Crystal Size Determination. Frozen green beans were taken from storage at times which corresponded to the biochemical assay points and put into the Isothermal freeze fixative, Asquith and Reid (1). 

Martino, M. N. and N. E. Zaritzky. 1988. Ice crystal size modifications during frozen beef storage. Journal of Food Science 53 1631-1637, 1649. 

Figure 13.10 Ice crystal size distributions in ice creams made with different total solids content. The initial 40% total solids mix was diluted with water to produce mixes of 37% and 34%. Ice cream frozen in a batch freezer at constant conditions. (From Hartel 1998c with permission.)...

Figure 13.12 Ice crystal size distributions in ice cream (a) before and (b) after storage for three months at -15 °C. Ice creams correspond to photomicrographs shown in Figure 13,2,...

Cooling rate Nucleation rate Number of ice crystals Size of ice crystals Ice sublimation time... 

Figure 2.7 Energy required to form a ice crystal nucleus as a function of ice crystal size at —10, —20 and —30 °C...

Figure 4.15 shows both the mean ice crystal size and total number of ice crystals as a function of ice content (which is directly related to temperature by the ice curve) in a standard ice cream. Samples were taken from the centre of a block of ice cream, and measured... 

Figure 4.15 Changes in ice crystal size and number in ice cream during hardening...

The ice crystal size distribution as well as the ice content affects the properties of ice cream. Figure 7.2 shows the ice crystal size distribution... 

Ice crystal size distribution in a sample of ice cream before and after thermal abuse... 

Both accretion and ripening of ice crystals occur in ice cream. Ripening is more important when the ice content is low, whereas accretion is generally the dominant mechanism when the ice content is high. Recrystallization leads to a deterioration in the quality of the ice cream. Figure 7.5 shows a plot of the sensory smoothness of ice cream as a function of ice crystal size. As the ice crystal size increases the texture of the ice cream becomes less smooth. When the ice crystals become very large ( 100 pm) they can be individually detected in the mouth and the texture becomes icy and gritty. 

Figure 7.6 Effect of ISP on ice crystal size and shape (a) ice crystals in the absence of ISP and (b) in the presence of ISP (The images are 125 Lim wide)...

Despite these difficulties, connections can be made between the microstructure and the physical and sensory properties. We have already discussed some of these, for example, the correlation between ice crystal size and sensory smoothness shown in Figure 7.5. Analysis of physical and sensory data by PCA and other statistical methods is an important tool for example, we saw in Figure 6.18 how changes in formulation and storage conditions (which alter the microstructure) affect the sensory attributes. Another example is the relationship between the ice and air microstructure, the thermal conductivity and the perception of coldness in the mouth. The high thermal conductivity of water ice means that heat is rapidly removed from your mouth when you eat the product. This is one of the reasons why an ice lolly feels colder than an ice cream even if they are actually at the same temperature. Table 7.3 summarizes established links between microstructure, physical and sensory properties. 

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