Goff et al. (1999) describe ice cream as a complex food colloid, containing fat globules, air bubbles, and ice crystals dispersed in a freeze-concentrated solution of proteins, salts, polysaccharides and sugars. In this post, we will be looking at the role of air bubbles in ice cream.
THIS POST WAS FIRST PUBLISHED ON 13TH FEBRUARY 2012 AND UPDATED ON 12th FEBRUARY 2016
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The production of ice cream starts with formulating, pasteurising, homogenising, and ageing an ice cream mix, followed by aeration, freezing, hardening, and storage. During aeration and freezing (also known as dynamic freezing), air is incorporated to about 50% of the volume (Goff & Hartel, 2013) through the folding and mixing action of the rotating dasher and scraper blades. Careful control of the amount of air incorporated into ice cream, or overrun, and the air cell size distribution is critical for ice cream texture, meltdown, and hardness (Sofjan & Hartel, 2003; Xinyi et al., 2010).
1. AIR CELL SIZE DISTRIBUTION
During dynamic freezing, air cells start out as large entities but are continually reduced in size by the shear stress, or the imposed force, of the rotating dasher and scraper blades (Goff & Hartel, 2013). Smaller dispersed air cells produce a creamier mouthfeel during consumption (Eisner et al., 2005).
To break air cells down into smaller ones, a high local shear stress is required. This shear stress is governed by the rotational speed of the dasher and scraper blades and by the viscosity, or the resistance of a liquid to flow, of the ice cream slurry as it is forming (Sofjan & Hartel, 2003).
1.1. ROTOR SPEED
An increase in the rotational speed of the dasher and scraper blades results in a smaller mean bubble diameter (Den Engelsen et al., 2002). In a study on the size of bubbles in foam in a dynamic mixer, Kroezen (1988) found that the mean bubble diameter decreased with increasing rotational speed of the mixer. This was ascribed to an increase of the shear stress in the mixer at higher rotational speeds. Similarly, Gido et al. (1989) found that increasing the rotational speed of a dynamic mixer caused a decrease in the average bubble diameter.
Increased rotor speed, however, appears to have a detrimental effect on ice crystal size, although there appears to be conflicting research on the extent of this effect. Ice crystal size is a critical factor in the development of smooth and creamy ice cream (Donhowe et al., 1991). Smooth and creamy ice cream requires the majority of ice crystals to be small, around 10 to 20 µm in size. If many crystals are larger than this, the ice cream will be perceived as being coarse or icy (Drewett & Hartel, 2007; Goff & Hartel, 2013).
Hartel (1996) argues that increasing the agitation speed or the number of scraper blades has significant effect on ice crystal formation during the freezing of ice cream. This is because the increased agitation speed causes an increase in the input of heat, which may result in larger ice crystals.
Russell et al. (1999) also found that increasing dasher speeds resulted in an increase in ice crystal size. Drewett & Hartel (2007), however, found only a slight increase, Koxholt et al. (2000) found no effect, and Inoue et al. (2008) found mixed effects on ice crystal size.
Cook & Hartel (2010) argue that it is possible that the dasher speed itself is not a direct predictor of ice crystal size. Instead, heat generation by the dasher may give a better correlation with ice crystal size.
1.2. VISCOSITY
Hirt et al. (1987) found that an increase of the viscosity of the liquid phase, or a thicker ice cream mix, resulted in a smaller mean bubble size. Similarly, Den Engelsen et al. (2002) found that a small increase of the viscosity of the liquid ice cream mix yields a decrease of the bubble size. They found, however, that above a certain viscosity, an increase of the large bubbles is observed and the process of bubble formation is apparently retarded.
Stabilisers are known to increase the viscosity of the aqueous phase. Chang & Hartel (2002b) found that the addition of stabiliser caused an increase in viscosity and this led to smaller air cells during the early stages of freezing. However, after about 10 minutes of freezing, slurry viscosities were approximately the same regardless of stabiliser level and air cell size distributions were smaller.
The formation of ice during dynamic freezing is also necessary for air incorporation. This is because during freezing, a slurry of ice crystals is formed, which, together with the freeze-concentrated continuous phase, causes the viscosity of the ice cream to increase dramatically (Goff & Hartel 2013). This increased viscosity enhances stabilisation of air cells and allows air cells to be reduced to smaller sizes. Similarly, Chang & Hartel (2002b) found that freezing was required to break down air cells incorporated during mixing, since whipping alone did not lead to small air bubbles. As freezing commenced, the apparent viscosity increased, which caused a reduction in maximum air cell size due to the increased shear stress applied to disrupt the air cells.
1.3. RESIDENCE TIME
The amount of time a mix spends in a machine, or residence time, during dynamic freezing affects air bubble size, with longer residence times resulting in smaller air bubbles. This is because extra whipping, provided by the increased residence time, breaks the air cells into smaller bubbles (Chang & Hartel, 2002b; Thakur & others, 2005). Kroezen (1988) found that a shorter residence time increased the mean bubble diameter.
However, Koxholt et al. (2000) note that the dynamic freezing step must account for competing phenomena as shorter freezing times are needed to produce small ice crystals, but longer freezing times give smaller air cells. For this reason, they suggest that pre-aeration may be a good choice to better control ice cream structures.
1.4. PRE-AERATION
Pre-aeration involves first whipping the ice cream mix to incorporate air and begin destabilising the fat before the mix is frozen in an ice cream machine. The effects of pre-aeration are to produce slightly smaller air bubbles and improve perceptions of creaminess and smoothness, especially in low fat ice cream (Tharp & Young, 2007; Burmester & others, 2005).
However, Kusumaatmaja (2009) found little correlation between pre-aeration and air cell size. Air cell size likely did not decrease significantly with pre-aeration due to relatively low shear before the mix was frozen. Windhab & Wildmoser (2002) also note that without the ice crystals present to provide viscosity and stability to the ice cream structure, air bubbles are prone to coalescence, which reduces the impact of pre-aeration.
2. STABILISING AIR BUBBLES IN ICE CREAM
During aeration and freezing, the small, newly formed air bubbles are not stable and need to be stabilised to prevent coalescence. Coalescence involves the coming together of two or more bubbles and results in larger air bubble sizes (Ronteltap & Prins, 1989).
Dynamic freezing involves numerous physical changes which affect air bubble stability. Goff (2002) summarised these as: the partial coalescence of the fat emulsion causing both adsorption of fat at the air interface and formation of fat globules clusters that stabilise the lamellae between air bubbles; the action of proteins and surfactants in forming and stabilising the foam phase; and freeze concentration of the premise by the removal of water from solution in the form of ice.
2.1. THE PARTIAL COALESCENCE OF THE FAT EMULSION
During aeration and freezing, the ice cream mix undergoes partial coalescence, where clumps and clusters of the fat globules form and build an internal fat structure or network into the frozen product by trapping air within the coalesced fat. These fat globule clusters are responsible for stabilising the air cells, thus preventing them from recombining (Walstra, 1989; Chang & Hartel, 2002a, b). This results in the beneficial properties of dryness, smooth texture, and resistance to meltdown (Lin & Leeder, 1974; Buchheiim et al., 1985; Berger, 1990).
2.2. PROTEIN
Proteins play an important role in forming and stabilising the foam in ice cream (Turan et al., 1999; Zhang & Goff, 2004; Patel et al. 2006). Proteins adsorb to and help to stabilise the air bubble interface, along with adsorbed fat globules, during dynamic freezing.
2.3. EMULSIFIERS
Emulsifiers enhance fat destabilisation, incorporate more and smaller air bubbles, and form thinner lamellae between air bubbles (Marshall & others, 2003). If too much emulsifier is present, however, or if an ice cream mix is subjected to excessive shearing action, the formation of objectionable butter particles can occur as the emulsion is churned beyond the optimum level (Goff, 1997).
Flores & Goff (1999) found that added emulsifier resulted in more air incorporation and air was more finely dispersed. Zhang & Goff (2005) found that added emulsifiers increased air bubble stability through promoting partial coalescence of fat, but too much partially coalesced fat led to unstable air bubbles.
3. OVERRUN IN ICE CREAM
The amount of air incorporated during freezing, or overrun, affects the size of the ice crystals, with larger ice crystals observed at lower overrun (Arbuckle, 1977). Flores and Goff (1999) suggested that overrun below 50% does not influence ice crystal size because it does not affect overall microstructure. They held that the amount of air cells at 70% overrun is just enough to prevent collisions among ice crystals and to disperse the serum phase around each crystal. Sofjan & Hartel (2003) found that increasing the overrun in ice cream (from 80% to 100% or 120% led to formation of slightly smaller air cells and ice crystals, probably due to the higher shear stresses exerted in the freezer barrel due to the higher air content. They also found that ice cream with 80% overrun had larger air cells and ice crystals after hardening than ice creams made with 100% and 120% overrun.
Thomas (1981) notes that an increase in air-cell dispersion results in limiting the size of ice crystals. Smaller air cells pack more tightly, which leaves smaller spaces between bubbles for ice to grow in, and the ice crystals, consequently, are smaller in size (Barfod, 2001).
4. CHANGES DURING HARDENING
Changes in air cell size distribution also occur during the hardening stage, where ice cream is hardened in a freezer without agitation to -18°C (0.4°F), preferably -25°C (-13°F). Once hardening is complete, changes in air bubble size are greatly reduced.
Goff & Hartel (2013) summarise the changes that take place during hardening as disproportionation (Ostwald ripening), coalescence (fusion of neighbouring bubbles), drainage (leading to an uneven distribution of air as bubbles rise, especially at warmer temperatures when ice cream is still soft), and distortion of air bubbles by growing ice crystals.
Changes in air cell size distribution during hardening can be minimised by reducing temperature as quickly as possible and ensuring that ice cream does not remain at elevated temperatures for an extended period of time (Goff & Hartel 2013). Increasing serum viscosity through addition of stabilisers decreases rates of drainage and slows air cell growth. Addition of emulsifiers also reduces air cell changes during hardening, most likely by increasing the extent of fat destabilisation.
References
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Barfod, N. M., 2001. The emulsifier effect. Dairy Ind Int. 66(1). 32–3.
Berger, K. G., 1990. Ice cream. In Larson, K., and Friberg, S., Food Emulsions, 2nd ed. New York: Marcel Dekker Inc.
Buchheim, W., Barfod, N. M., and Krog, N., 1985. Relation between microstructure, destabilization phenomena and rheological properties of shippable emulsions. Food Microstructure. 4. 221-232
Burmester, S. S. H., Russell, A. B., and Cebula, D. J., 2005. The evolution of ice cream technology. New Food. 2.42–45.
Chang, Y., and Hartel, R. W., 2002a. Measurement of air cell distributions in dairy foams. International Dairy Journal. 12:463-472.
Chang, Y., and Hartel, R. W., 2002b. Development of air cells in a batch ice cream freezer. Journal of Food Engineering. 55, 71-78.
Cook, K. L. K., & Hartel, R. W., 2010. Mechanisms of Ice Crystallisation in Ice Cream Production. Comprehensive Reviews in Food Science and Food Safety. 9 (2).
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Flores, A. A., and Goff, H. D., 1999. Ice crystal size distributions in dynamically frozen model solutions and ice cream as affected by stabilizers. Journal of Dairy Science. 82. 1399–1407
Gido S. P., Hirt, D. E., Montgomery S. M., Prud’home, R. K., and Renbenfeld, L., 1989. Foam bubble size measured using image analysis before and after passage through a porous medium. J. Dispersion Science and Technology. 10. 785-793.
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Kusumaatmaja, W., 2009. Effects of mix pre-aeration and product recirculation on ice cream microstructure and sensory qualities [MSc thesis]. Madison, WI: University of Wisconsin – Madison.
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Russell, A. B., Cheney, P. E., & Wantling, S. D., 1999. Influence of freezing conditions on ice crystallisation in ice cream. Journal of Food Engineering. 29.
Sofjan, R. P., and Hartel, R. W., 2003. Effects of overrun on structural and physical characteristics of ice cream. International Dairy Journal. 14.255-262
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I run a small-batch ice cream start-up based in Manchester, U.K.
Hi Ruben, I bought a vitamix earlier this year to help me increase my produce intake, and I couldn’t be more pleased with it. I am especially eating many more vegetables by hiding them in “milkshakes.” For instance, I make a great mint chocolate chip milkshake that includes lowfat milk, chia and flax (healthy fats), fresh mint, figs and grapes to sweeten, guar gum, and quite a bit of broccoli and green cabbage (there is also a little sugar and chocolate chips but not that much really). I also do carrot cake, mocha, chocolate coconut, and blueberry cheesecake milkshakes with the same base. My idea is to add the milkshakes to an ice cream maker, and I have been reading on your site about the science of making ice cream (as well as your machine recommendations, thanks!). I’m wondering how adding this already frozen product to the ice cream maker might affect the quality of the final product. I would guess that the fiber from the fruits and vegetables would raise the viscosity, which sounds like a positive. But I’m concerned that the heat from the high-speed Vitamix and the higher overrun from all the blending might increase ice crystal formation. Also, I’m guessing the mixture would need less time in the machine, since it is partially frozen, and this might have a negative effect on the overall quality. So what I’m wondering is: is this an overall reasonable idea to try to implement, and do you have any specific things you would recommend given the formulation? Thank you
Hi again Tori!
The consistency of your milkshake ice cream will very much depend on the composition (fat, protein, sugar, total solids) of your milkshake. It’s difficult to say what the texture will be like without knowing how much fat, total solids etc. is in your milkshakes. Do you freeze your milkshakes after you blend them in the vitamix?
What you could do is blend your milkshake, place it in the freezer for about 4 hours so that it partially freezes, blend the partially frozen milkshake in your vitamix to break down the large ice crystals, and then put it back in your freezer to fully harden. This is the same technique I use for making ice cream without an ice cream maker but again, the quality of the finished product will very much depend on your mix composition.
I hope that helps. Let me know if you have any other questions and good luck!
All the best,
Ruben
Wow, those are some serious scientific references. You rock, and are clearly serious about your craft and the science behind it!
🙂
Using a Pola 5030 with a measured overrun for vanilla at 32%. Would like to increase the overrun, as a test, to about 50%.
From a mechanical standpoint, it would be nice to up the dasher speed near the end of the processing cycle to incorporate more air. However, that would require a motor change out, etc.
A more practicable approach would seem to be the addition an air injection tube off center from the dasher. A very small hole at the bottom or several along the sides would inject air into the mix as it is being churned. One could control the volume to perhaps regulate the resultant overrun. Also it might be possible to determine the best time within the processing cycle to start the air injection process. i.e. once the mix becomes viscous enough to support small air bubble formation and retention.
With the Pola, it would be easy to attach a temporary bracket from front to back to hold the injector. The clear plastic cover could be left off for the test.
What are your thoughts on this approach and if the overrun could be increased to the desired levels.
Hi again Jim!
Very interesting to see that you got 32% overrun from your 5030. A lot of people have been asking about overrun on both the 5030 and 4080 but I haven’t had time to test them. I’ll use your 32% figure as a reference if anyone asks again, if you don’t mind. How long did the mix spend freezing in the machine and what size batch did you use?
Wow that is a very interesting approach and I think above my pay grade! I know that continuous freezers, usually used in larger operations, have air injected into the mix but this is in a closed cylinder; I don’t know how that would work on a batch freezer. I’m guessing there might be an issue with uniformity/ensuring the same amount of air is dispersed into all of the product. Do you have an engineering background? Trying to build different rotational speeds into the 5030 would be interesting, along with redesigning the dasher to ensure that it scrapes the side and bottom of the bowl.
All the best,
Ruben
Rubin:
The overrun (32.6%) was measured using a 1200 gram batch of vanilla that was aged overnight and then processed in the Pola after a 6 minute pre cool. Since then, have changed the formula which may result in different findings. Found that a mix with a normal amount of sugar (vanilla ice cream 12% BF) takes 16 minutes. i.e. like the product a little on the firmer side.
The new process of using a blender at 20,000 rpm (last blend) creates a bit of foam and even overnight still is present as a top layer. Thus, in my opinion, making an accurate overrun measurement a bit difficult. However, find that mix puffs up in the Pola toward the end of the freezing cycle.
Other batches are larger due to added ingredients. As an example, the Oreo Crunch was 1428 grams (mix plus all additions) and produced around 2 quarts of finished product. The Strawberry ice cream at 1554 grams made close to 2-1/4 quarts. So far no spillage outside the Pola bowl even at 1554 grams.
Have not taken the Pola apart so not sure if there is additional room inside the box for mechanical alterations. If a different motor could be located, with the correct fractional horse power, shaft size and physical size then mods could be made.
Thinking that a three phase 110 VAC or 220 VAC small motor at 3600 RPM could be used with a VFD (variable frequency drive) to control the speed. The unknown, would be how well built is the gear reduction box as one would not want to damage that component with too much torque or speed. Or something more substantial (gear box) could be substituted depending on available space.
Speed could be varied from the nominal 80 – 84 RPM to perhaps 40 – 170 RPM. The only production concern is that one would have to start out no faster than 84 RPM or the mix could splash out of the bowl. Once the mix is partially frozen, the speed could then be increased, toward the end of the run, to perhaps achieve a higher overrun.
May give the air injection idea a try. Perhaps using a hand held silicon tube with a very small hole/s would work. If the beater hits the tube no problem as it is most flexible even at ice cream temperatures.
Have a non oil based compressor that is used in the health care industry that will supply the needed pressure. For the Pola, this would work as a test. In other machines not so much as there is no open beater top access. i.e. most machines have a bar across the top of the beater for added strength which will get in the way of trying to insert an air injection tube. The Pola does not.
Regards,
Jim
Hi Ruben! Thank you so much for your reply. In the article you researched that Blades speed inside the machine is important to make different types of ice cream (with different amounts of overrun). I’m about to purchase a professional industrial machine but the are machines with fixed RPM (much cheaper) and machines with variable rotation blades (much much much more expensive). If i purchase a fixed RPM machine, do you know what is the recommended RPM to make a smooth ice cream, like a gelato or near in density?
Hi again Bruno!
That’s a good question. Which industrial machine are you going with? Dasher speeds on commercial machines are usually 100-200 rpm. I don’t know the rpm necessary for gelato but the dasher design, as well as the rpm, will also affect air incorporation.
I hope that helps. Let me know if you have any other questions.
All the best,
Ruben
Overrun is influenced by many factors. Including those cited by Ruben. In addition, the amount of sugar in the mix, type and amount of stabilizer, added ingredients such as cookies and aging all will change overrun.
In your case, if the machine is designed to make gelato then its beater speed will be lower to add less air to the product. A machine, that has only one fixed speed and designed for ice cream will add too much air IMO. Good for ice cream, but not gelato.
As an example, the Pola 5030 (not a production machine) runs at 84 RPM and makes a nice low overrun dense product. A large commercial batch freezer, of a different design, suggests 140 RPM for gelato.
If your intention is to only make gelato, then purchase a machine specifically designed for gelato. Or one that has RPM choices that can make gelato or higher overrun products. If moving toward a variable speed machine, be sure to ask owners of gelato shops how well that type of machine works for their production of gelato.
Hi Ruben, Perrin and friends,
i will open a Paletas Shop (Mexican big popsicles made of cream and fruits) and i’m looking for a Mixer/Blender to blend the ingredients of the popsicles (including emulsifiers, stabilizers and also fruits), and i came across several machines to accomplish this task. Some are Home Blenders and others are Industrial. But, one in particular got my attention because of the different blade it uses (please see picture): http://postimg.org/image/xpneb1em3/
In the company website they say that this Mixer and it’s different blade (cowles blade) can:
– Emulsify and homogenize the ingredients, specially with the presence of fats;
– Incorporates air in the mix, leaving the ice cream and popsicles smoother and profitable;
Do you know if this blade really helps doing all the above? And, most important that i need to know, whats the correct RPM (rotation per minute) of this Mixers?
Because, if the RPM is too high, for example 18.000 RPM, the friction will transfer heat to the mix and blew the flavors, maybe this happened with Perrin… and if the RPM is too low, it will not incorporate air…
I have searched a lot, but could not find any information and what different RPM and for how long (in minutes) could cause on the ice cream mix.
Any help appreciated.
Thank you
Hi Bruno!
Good to hear from you. I’ve never actually used an industrial blender myself so might not be the best person to offer advice. I looked into buying an industrial mixer before to make nut butters but decided against it because of the cost. I looked at http://www.ika.com when I was looking. I got my magnetic stirrer from those guys and am very happy with that. Have a look on their website and see if that helps.
Yes I agree that if the RPM is too high, the heat will effect delicate flavours, especially fruit flavours. Also have a look at a homogeniser as this may be able to blend liquids without heat. I would recommend that you have a look at http://modernistcuisine.com/ as those guys are very good at describing industrial machinery for the kitchen.
I hope that helps. Let me know if you have any more questions.
All the best,
Ruben
Hi Ruben,
Thanks for your other replies.
I’m having a problem with not enough overrun in my ice cream. The texture is smooth, but dense, and I like it lighter and fluffier.
I’m using a Kitchen Aid mixer bowl attachment to freeze my mix. The mix is low-fat dairy, with a combination of corn syrup and cream cheese to thicken and emulsify, using somewhat higher proportions than the standard Jeni’s base to compensate for the lack of butterfat.
I tried increasing shear strength by running my mixer’s beater at a slightly higher speed, but this just resulted in the mix not freezing well and incorporating even less air. I was thinking I might try this approach again but with a colder starting temperature for the mix. It’s already at the temperature you suggest after an overnight rest in the fridge, and the freezer bowl is as cold as I can get it in my freezer.
My other idea was to use more thickeners to increase the mix viscosity, in hopes that it would hold air bubbles more effectively.
What do you think?