Ice Cream Structure

Ice cream structure is both fascinating and confusing. The way we perceive the texture of ice cream when we consume it (smooth, coarse, etc.) is based on its structure, and thus structure is probably one of its most important attributes.

Colloidal aspects of ice cream structure

Please look at

Digram of fat structure in ice cream

Diagram of replacing proteins on the fat surfacewhen reading the following description, and try to put the two together in your mind. Also, please look at the last paragraph of this page for links to electron micrographic images of the structure of ice cream.

Ice cream is both an emulsion and a foam. The milkfat exists in tiny globules that have been formed by the homogenizer. There are many proteins that act as emulsifiers and give the fat emulsion its needed stability. The emulsifiers are added to ice cream to actually reduce the stability of this fat emulsion by replacing proteins on the fat surface (shown on the right), leading to a thinner membrane more prone to coalescence during whipping. When the mix is subjected to the whipping action of the barrel freezer, the fat emulsion begins to partially break down and the fat globules begin to flocculate or destabilize. The air bubbles which are being beaten into the mix are stabilized by this partially coalesced fat. If emulsifiers were not added, the fat globules would have so much ability to resist this coalescing, due to the proteins being adsorbed to the fat globule, that the air bubbles would not be properly stabilized and the ice cream would not have the same smooth texture (due to this fat structure) that it has.

Graph of effect of emulsifier on fat destabilization in ice cream

This fat structure which exists in ice cream is the same type of structure which exists in whipped cream. When you whip a bowl of heavy cream, it soon starts to become stiff and dry appearing and takes on a smooth texture. This results from the formation of this partially coalesced fat structure stabilizing the air bubbles. If it is whipped too far, the fat will begin to churn and butter particles will form. The same thing will happen in ice cream which has been whipped too much.

Ice Cream Meltdown

One of the important manifestations of ice cream structure is its melt-down. When you put ice cream in an ambient environment to melt (as in a scoop on a plate), two events occur; the melting of the ice and the collapse of the fat-stabilized foam structure. The melting of the ice is controlled by the outside temperature (fast on a hot day) and the rate of heat transfer (faster on a hot, windy day). However, even after the ice crystals melt, the ice cream does not "melt" (collapse) until the fat-stabilized foam structure collapses, and that is a function of the extent of fat destabilization/partial coalescence, which is controlled mostly by the emulsifier concentration, for reasons we have just described above.

This is shown in the diagram below, which shows ice cream sitting on a mesh screen at ambient temperature:

diagram showing ice cream sitting on a mesh screen at ambient temperature

You can see above the increased amount of shape retention and slowness of melt that comes from the added emulsifiers, particularly polysorbate 80.

Structure from the Ice crystals

Also adding structure to the ice cream is the formation of the ice crystals. Water freezes out of a solution in its pure form as ice. In a sugar solution such as ice cream, the initial freezing point of the solution is lower than 0° C due to these dissolved sugars (freezing point depression), which is mostly a function of the sugar content of the mix. As ice crystallization begins and water freezes out in its pure form, the concentration of the remaining solution of sugar is increased due to water removal and hence the freezing point is further lowered. This process is shown here, schematically.

The process of the formation of ice crystalsThis process of freeze concentration continues to very low temperatures. Even at the typical ice cream serving temperature of -16° C, only about 72% of the water is frozen. The rest remains as a very concentrated sugar solution. Thus when temperature is plotted against % water frozen, you get the phase diagram shown below. This helps to give ice cream its ability to be scooped and chewed at freezer temperatures. The air content also contributes to this ability, as mentioned in discussing overrun.

 

 

 

 

 

 

 

Graph showing the effects of sweeteners on freezing characteristics of ice cream mixesThe effect of sweeteners on freezing characteristics of ice cream mixes is demonstrated by the plot shown on the ice cream freezing curve.

Also critical to ice cream structure is ice crystal size, and the effect of recrystallization (heat shock, temperature fluctuations) on ice crystal size and texture. A primer on the theoretical aspects of freezing will help you to fully understand the freezing process. Please see the discussion and diagram on ice crystallization rate, as shown on that page, to fully understand this process. Recrystallization (growth) of ice is discussed elsewhere in the context of shelf life.

Thus the structure of ice cream can be described as a partly frozen foam with ice crystals and air bubbles occupying a majority of the space. The tiny fat globules, some of them flocculated and surrounding the air bubbles also form a dispersed phase. Proteins and emulsifiers are in turn surrounding the fat globules. The continuous phase consists of a very concentrated, unfrozen solution of sugars. One gram of ice cream of typical composition contains 1.5 x 1012 fat globules of average diameter 1µ m that have a surface area of greater than 1 square meter (in a gram!), 8 x 106 air bubbles of average diameter 70 µ m with a surface area of 0.1 sq. m., and 8 x 106 ice crystals of average diameter 50 µ m with a surface area of another 0.1 sq. m. The importance of surface chemistry becomes obvious!

Microscopy

Before we leave ice cream structure, I want to draw your attention to the following address: "Foods Under the Microscope". This is a link to an absolutely marvelous website developed by my good friend Dr. Milos Kalab, with many high-quality images of the structure of milk and dairy products obtained during Dr. Kalab's long and outstanding career as a food microscopist with Agriculture and Agri-Food Canada in Ottawa. Dr. Kalab asked me to contribute microscopic images of ice cream structure as a guest microscopist. You can find my (Doug Goff) first contribution under "Guest microscopists", and I have also copied it here. Subsequent to that submission, I have prepared another one for D. Kalab that focuses on the use of cryo-fixation and TEM for visualization of fat and air structures in ice cream. One of my graduate students, Alejandra Regand, also made a contribution, based on her M.Sc. thesis work, focusing on the structure of polysaccharides in frozen solutions.

Ice cream structure is an active area of our research here at the University of Guelph. Please see my publications for more details of our research.

Foods Under the Microscope

Alejandra Regand, Ph.D.

Alejandra Regand is currently a Post-doctoral fellow at Ryerson University, Toronto, Canada. Information about H. D. Goff may be found in his earlier contributions in this series of Guest Food Microscopists 1 
Ken Baker owns his company, Microscopy, Imaging and Analysis in Acton, Ontario, Canada.

Effects of stabilizers on ice recrystallization in sucrose and sucrose-skim milk solutions: A light microscopy study

Alejandra Regand1, Douglas Goff1, Ken Baker2
1Department of Food Science, University of Guelph, Guelph, ON, Canada N1G 2W1 
2Ken Baker, Microscopy, Imaging and Analysis, 4943 Fourth Line, Acton, ON, Canada, L7J 2L8

Polysaccharide or protein hydrocolloids are known to retard ice recrystallization in frozen systems during storage at fluctuating temperatures, but the mechanism is not clear. Hydrocolloid stabilizers were labeled with rhodamine isothiocyanate (RITC) and incorporated into solutions of sucrose (24%) and sucrose (16%) with skim milk powder (SMP) (14.7%). Solutions contained either no stabilizer or 0.3% of carrageenan, carboxymethyl cellulose (CMC), xanthan gum, sodium alginate, locust bean gum (LBG), or gelatin. For light microscopy, a small drop of the solution was placed on a glass microscope slide, cover slipped and secured within a cold stage (Linkam Scientific Instruments, UK) mounted on an Olympus BX-60 microscope. The solutions were quench frozen to -50°C, precycled in order to get similar ice crystal size at t=0 (p<0.05) and cycled between -3.5°C and -6°C, 5 times. Samples were observed using either transmitted yellow/green light or epifluorescence illumination with an Olympus rhodamine filter set. Two images per field, one transmitted and one epifluorescence, were acquired at t=0 and at -3.5°C of each cycle. Images were acquired using a Photometrics SenSys 1401E monochrome camera. Adobe PhotoShop and the Image Processing Tool Kit (1) were used for all image processing and image analysis. Quantitative image analysis was used to measure the equivalent circular diameter (2) of ice crystals in all brightfield images. Recrystallization rate was then calculated as the slope of the linear regression of the ice crystal median diameters obtained from the brightfield data.

Figure 1 Brightfield (a) and fluorescence (b) images collected from the same field for locust bean gum in sucrose solution

Fig. 1. Brightfield (a) and fluorescence (b) images collected from the same field for locust bean gum in sucrose solution. 
1: Before freezing at 22°C, 2: t=0, 3: Cycle 1, 4: Cycle 3, 5: Cycle 4, 6: Cycle 5, 7: After cycling at 0°C.

Figure 1 shows both the brightfield and fluorescence microscopy images collected from the same field for the LBG + sucrose sample. The source of fluorescence is the labeled polysaccharide, which enables its location in the unfrozen phase to be seen. After cycling, the formation of a gel-like fluorescent structure surrounding the ice crystals was observed. Once the crystals were melted (-2°C), the LBG network remained intact.

Figure 2 Comparison between fluorescent images at 0°C from stabilizers in sucrose solutions after cycling and ice crystal melting, in the absence (a) or presence (b) of skim milk solids.

Fig. 2. Comparison between fluorescent images at 0°C from stabilizers in sucrose solutions after cycling and ice crystal melting, in the absence (a) or presence (b) of skim milk solids.  
1: No stabilizer, 2: Locust bean gum, 3: Xanthan gum, 4: Carboxymethyl cellulose, 5: Gelatin, 6:
Carrageenan, 7: Alginate.

Figure 2 shows the structures from stabilizer-sucrose solutions resulting from cycling, in the presence or absence of SMP. The only sucrose solutions in the absence of milk protein that developed a gel-like structure after cycling were those that contained locust bean gum. The recrystallization rate in these solutions was, however, similar to that in the control samples. In contrast, the recrystallization rate was significantly reduced by alginate and xanthan (p>0.05) (Table 1). Conversely, gelatin was the only stabilizer tested which did not retard ice recrystallization in sucrose-skim milk solutions (Table 1). It was observed to form distinctive gels with milk proteins. Similar gels were also formed in the presence of locust bean gum or carrageenan in sucrose solutions which contained milk solids (Figure 2).

Table 1. Ice recrystallization rates in sucrose solutions and in sucrose solutions containing skim milk powder (SMP) solids as affected by various stabilizers

Table 1
Stabilizer1 Sucrose solutions Sucrose solutions with SMP 
None 4.02a 4.59a
LBG 4.05a 3.17c
Carrageenan 3.87a 3.02c
CMC 3.32a,b 2.83c
Gelatin 3.65a,b 4.13a,b
Alginate 2.77b 2.78c
Xanthan 2.82b 3.58b,c

1Stabilizer concentrations used: Carrageenan at 0.05% w/w. All others at 0.3% w/w. 
a,b,c Values with the same letter in the same column do not differ (a=0.05).

It is therefore suggested that steric blocking of the interface or inhibition of solute transport to and from the ice interface caused by the gelation of the polymer, is not the only mechanism of stabilizer action. However, a structural molecular arrangement that keeps the melting water in close proximity to the ice crystal surface and ensures that this water refreezes onto the original crystal rather than migrating to the surface of a larger crystal elsewhere, must be present. The molecular interactions between polysaccharides and proteins appear to be key factors in retarding ice recrystallization.

References

  1. Russ, C. and Russ, J. C., Image Processing ToolKit, http://www.reindeergraphics.com
  2. Russ, J. C. 1998. In "The Image Processing Handbook" 3rd edition, CRC Press, Boca Raton, FL, US

© Alejandra Regand 2001

Foods Under the Microscope - Ice Cream Structure

H. Douglas Goff, Ph.D.

Professor of Food Science Dr. Doug Goff has studied a great variety of dairy foods at the Department of Food Science, University of Guelph in Ontario, Canada. One of the foods that he examined by microscopy is ice cream. It is very popular with consumers but has received little attention from the microscopists. Dr. Goff used cryo-SEM and TEM to produce the micrographs below, and he provides us with the following description of what he sees.

Ice Cream Structure
by H. Douglas Goff

TEM of 3 fat globules

TEM of 3 fat globules (orange) showing only little crystallinity (lighter lines) inside the globules. Proteins can be seen as black spheres.

TEM of 1 large and 1 small fat globule

TEM of 1 large and 1 small fat globules (orange) showing their contents almost completely crystallized (lighter lines inside the globules).

Ice cream structure under the microscope is a marvelous thing to see. While most consumers see it as a cold, creamy, smooth, delicious dessert, it is no easy task to produce and maintain a structure that will deliver these attributes! Information on the manufacture of ice cream can be found at the University of Guelphs Dairy Science and Technology Education website . Briefly, the first step of ice cream manufacture is to combine the ingredients (cream, milk, milk solids, sugars, and <0.5% of stabilizers and emulsifiers) into a mix, which is pasteurized and homogenized. This creates a milkfat emulsion, comprised of millions of tiny droplets of fat dispersed in the water phase, each surrounded by a membrane of proteins and emulsifiers. The proteins can be seen in the top image as the black spheres adsorbed onto the fat globule surface. The sugars, also added to the mix during processing, are dissolved in the water phase. After the mix is cooled, the milkfat partially solidifies (as does butter when you cool it), so that each droplet consists of solid fat crystals cemented together by liquid fat. The two images at left, produced by TEM, show low crystallinity (top image) and a high crystallinity (lower image) of fat droplets in an ice cream mix.

The ice cream mix is then whipped and frozen, a process that creates two more discrete structural phases, millions of tiny ice crystals and air bubbles dispersed in the concentrated unfrozen mix. The water, which comes from the milk or cream, freezes into ice, and the dissolved sugars become increasingly concentrated in the unfrozen phase as more ice forms. Ice crystals should be 30-50 µm in diameter - the larger they are from manufacture or become due to temperature fluctuations in storage, the more coarse/icy the ice cream will taste. The whipping processs helps keep the ice crystals small and discrete. The colour image below, produced by cryo-SEM, shows a cross section of frozen ice cream, illustrating the four microscopic phases of frozen ice cream: ice crystals (blue - 'C'), air bubbles ('A'), fat droplets ('F' - for details see the micrograph at right), and the unfrozen phase ('S' - yellow).

cryo-SEM phases

The whipping process also helps to incorporate air in the form of tiny bubbles 50-80 µm in diameter. Approximately one half of the volume of ice cream is air (without it, ice cream could not be scooped or chewed in the mouth), but the fact that it is dispersed in tiny bubbles means that the ice cream tastes smooth and the air is not noticeable. All of the fat droplets play an important role at the air interface, helping provide that smoothness. The process of freezing and aeration of the mix causes the milkfat emulsion to undergo a process called partial coalescence, in which the fat droplets form clusters and aggregates of fat that surround and stabilize the air bubbles. This same process is what creates structure in whipped cream (the structure of the fat and the air in whipped cream and ice cream are very similar). The colour image below, also produced by cryo-SEM, shows another cross section of frozen ice cream, illustrating an air bubble lined with the agglomerated fat and individual droplets (yellow).

cryo-SEM air bubble

I have prepared another narrative of our visualization of ice cream structure by freeze-substitution (cryo-fixation) transmission electron microscopy (TEM) for Dr. Miloslav Kalab, and it can be found here.

The next time you enjoy a cone of ice cream, pause for a moment and marvel at its structure!

© D. Goff 1998

Foods Under the Microscope - Ice Cream Structure by Freeze-Substitution

Thin-Section Transmission Electron Microscopy of Ice Cream after Freeze Substitution

by H. Douglas Goff, Ph.D.

This narrative of our visualization of ice cream structure by freeze-substitution (cryo-fixation) transmission electron microscopy (TEM) was prepared for Dr. Miloslav Kalab, Foods Under the Microscope". It is my second contribution for Dr. Kalab. The first can be seen here.
 
A great deal is known about ice cream structure and the contribution of fat to that structure, through fat destabilization, but one element of structural information that has been missing is the nature of fat structures within the frozen system itself, and the interaction of these fat structures with air. It has not been clear whether the appropriate schematic model for destabilized fat networks is one of partially coalesced (clustered) fat globules adsorbed to air bubbles, or fat clusters primarily in the bulk, or both. The role of coalescence in fat destabilization is not clear. There has been debate about the presence of liquid fat at the air interface, the origin of which might be non-crystallized triglyceride escaping from the fat globule during rupture. It has also not been clear what happens to the structure of fat at air bubble interfaces during the process of expansion of the bubbles, which must happen in the transition from imposed back pressure to atmospheric pressure upon drawing ice cream from the swept-surface freezer.
 
What was thus needed was an electron microscopic technique that would allow the visualization of fat and air structures in the natural frozen state. The first reported electron microscopy methods for the study of ice cream structure utilized thin-section transmission electron microscopy (TEM) of highly fixed samples of melted ice cream, or freeze-fracture, metal shadowing, and examination of replicas of frozen ice cream with TEM. The development of low temperature (cryo) scanning electron microscopy (LT-SEM) has allowed further detailed studies of ice cream structure, particularly the ice phase. However, both the metal replication TEM technique and LT-SEM are limited to the features of fractured surfaces, and in the latter case, magnification and resolution are not high enough to discern interactions between fat globules. Thin-sectioning for TEM after sub-ambient temperature fixation has been used to view ice cream mix and melted ice cream, however, the use of aqueous fixatives has limited the use of thin-section techniques to non-frozen samples. We have recently used a TEM thin-section method through application of a freeze-substitution technique for sub-zero sample fixation. There has been no reported use of cryo- sectioning of ice cream for viewing with cryo-TEM, but this may be forthcoming with the presence of new, sophisticated instrumentation, and may provide the ultimate in structural information.
 
In the freeze-substitution TEM method, ice cream specimens were taken from the inner bulk of the hardened samples at -25∞C with a surgical blade and immediately placed into liquid nitrogen (-196∞C), where they
were broken into <1.0 mm3 pieces with the surgical blade under slight force. Frozen specimens of ice cream were transferred into vials that contained a mix of fixatives consisting of 2.15 % (w/v) uranyl acetate, 3.23 %(v/v) glutaraldehyde and 1.0 % (w/v) osmium tetroxide in absolute methanol, which had been kept at -196∞C in liquid nitrogen (the fixative mixture is solid at this temperature). The vials were then placed in a -80∞C freezer for 4 days, where after warming up from -196∞C to -80∞C the fixative mixture melted and the gradual freeze-substitution of ice with methanol took place. Samples were then transferred to -40∞C for 1 day where some fixation with glutaraldehyde, osmium tetroxide and uranyl acetate took place (there was evidence of staining after treatment at -40∞C). Samples were then transferred to -20∞C for 2 days where fixation proceeded at a faster rate and further staining was accomplished. Then the fixative mixture was replaced by washing the specimens in precooled (-20∞C) 100% methanol followed by repeated washing (3x) with absolute ethanol. Specimens were placed into gelatin capsules and the infiltration with low temperature embedding resin Lowicryl HM20 (Marivac Ltd., Halifax, NS, Canada) was performed in the following way: one wash with ethanol:Lowicryl 1:1 at -20∞C for 10 min, one wash with ethanol: Lowicryl 1:3 at -20∞C for 10 min and three washes with 100% Lowicryl at -20∞C. The resin-infiltrated samples were polymerized under 360 nm UV light at -20∞C. Resin blocks were sectioned at a thickness of 90 nm using an LKB ultramicrotome. Sections were mounted immediately on formvar-coated grids (Marivac Ltd., Halifax, NS, Canada). No post-staining was carried out, as the applied fixation process provided a sufficient contrast for TEM imaging. The sections were observed and photographed in a TEM at 75 kV.
 
cross sections of ice cream at low magnification
Figures 1 and 2 are cross sections of ice cream at low magnification as viewed by TEM after freeze substitution, showing the unfrozen serum phase (s), air bubbles (a), and an ice crystal (i). In the unfrozen serum, dispersed fat globules and casein micelles are just discernible. At higher magnification (Figures 3 and 4), fat globule (f) adsorption to the air serum interface (a) and fat clustering (fc) from partial coalescence in the serum phase can be seen. Highly freeze-concentrated casein micelles (c) can also be seen in the serum phase.

What was concluded from a detailed study of ingredient and process variables was that the structures created by increasing levels of fat destabilization in ice cream were observed as an increasing concentration of discrete fat globules at the air interface and increasing coalescence and clustering of fat globules both at the air interface and within the serum phase. However, air interfaces at the highest levels of fat destabilization were not completely covered by fat globules, nor was there evidence of a surface layer of free fat. Air interfaces from continuous and batch freezing were similar.
 

Reference:

Goff, H. D., E. Verespej, and A. K. Smith. 1999. A study of fat and air structures in ice cream. International Dairy Journal 9: 817-829.