SECTION 1 - THE SCIENCE OF ICE CREAM MAKING AND PREPARATION TIPS
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All recipes on the blog are broken into three sections: SECTION 1: The Science of Ice Cream Making and Preparation Tips; SECTION 2: Full Recipe; and SECTION 3: Quick Recipe. To avoid repeating SECTION 1 in each recipe post, we'll be covering it here. We'll be looking at the importance of producing small ice crystals, maintaining these small ice crystals during storage, and the effect of prolonged heating of the ice cream mix on texture.
1. ICE CRYSTALS IN ICE CREAM
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). Ice cream is frozen in two stages: dynamic and static freezing. Ice crystals are formed during dynamic freezing, where the ice cream mix is frozen and agitated in an ice cream machine to incorporate air, and grow during static freezing, where the partially frozen ice cream mix is hardened in a freezer without agitation. The primary aim is to promote the formation of as many small ice crystals as possible during dynamic freezing and then preserve these small crystals during static freezing and storage.
1.1. THE DYNAMIC FREEZING STAGE
In the dynamic freezing stage, the ice cream mix enters the ice cream machine slightly above its freezing point (the temperature at which the water in the mix first begins to freeze). As the freezer bowl absorbs the heat in the mix and brings it below its freezing point, a layer of ice freezes to the wall of the cold freezer bowl causing rapid nucleation (the birth of small ice crystals) (Hartel, 2001). The crystals that form at the wall of the cold bowl are then scraped off by the rotating dasher blades and mixed with the warmer mix in the centre of the bowl, where they grow in size.
1.1.1. NUCLEATION
For smooth and creamy ice cream, it's important to have a high rate of nucleation so as to create as many small ice crystals as possible (Hartel, 1996). Nucleation occurs only at the wall of the freezer bowl where it's cold enough to form new crystals. Drewett & Hartel (2007) found that decreasing the coolant temperatures at the freezer bowl wall caused higher ice crystal nucleation rates. Similarly, Russell et al. (1999) note that the rate of nucleation is determined by the degree of heat removal from the ice cream mix, which is dependent on the freezer bowl temperature, and found that as the freezer bowl temperature was lowered, the nucleation rate increased accordingly.
1.1.2. RESIDENCE TIME
Residence time (the length of time the ice cream mix spends in your machine) has a significant effect on the final ice crystal size distribution (Russell et al., 1999; Goff & Hartel, 2013; Drewett & Hartel, 2007; Cook & Hartel, 2010). A longer residence time means that the ice cream mix is slower to reach its draw temperature (the temperature at which the ice cream is removed from the machine) of -5 to -6°C (23 to 21.2°F) in commercial machines, which gives the ice crystals in the centre of the freezer bowl more time to recrystallise and grow larger (Russell et al., 1999; Drewett & Hartel, 2007). Russell et al. (1999) found that ice creams made with shorter residence times had smaller ice crystals due to a decline in recrystallisation (the general increase in ice crystal size). Russell et al. (1999) also note that ice crystallisation is dominated by recrystallisation and that this mechanism appears to be more important than nucleation in determining the final crystal population.
The freezer bowl wall temperature has a direct effect on the cooling rate, and therefore the residence time, of the mix (Cook & Hartel, 2010). Lower wall temperatures can lower the bulk temperature of the ice cream faster, reducing residence time and improving the ice crystal size distribution (Russell et al., 1999; Drewett & Hartel, 2007).
Tip #1 - THE FREEZER BOWL WALL TEMPERATURETo promote rapid nucleation and a shorter residence time, it's important that your freezer bowl wall temperature falls in the range of -23°C to -29°C (-9.4°F to -20.2°F). If you're using the Cuisinart ICE 30-BC, or any other machine that requires the freezer bowl to be frozen before use, your freezer's temperature will have a direct effect on the freezer bowl wall temperature. The colder you can get your freezer, the lower you will get your freezer bowl wall temperature and the higher the rate of nucleation and the shorter the residence time.
When I freeze my Cuisinart ICE-30 bowl, I use the 'super freeze' function on my freezer to get the temperature down to around -30°C (-22°F). This results in a freezer bowl wall temperature of -27°C (-16.6°F) and a residence time of 32 minutes for a 700 ml (0.74 quarts) batch of ice cream. This compares to a freezer bowl wall temperature of -15°C (5°F) and a residence time of 38 minutes for a 700 ml (0.74 quarts) batch when my freezer temperature is set to around -18°C (0.4°F).
When freezing your removable bowl, cover the top with cling film. This will help prevent water vapour in your freezer, as well as any ice that may fall in, from freezing to the inside of the bowl. Any water that freezes at the bowl wall will likely be incorporated into your mix during dynamic freezing, with possible implications for texture if a sufficient amount is incorporated.
If you're using a domestic machine with a self-refrigerating compressor, switch the compressor on and leave the machine running for about 15 minutes before you add the mix. This will ensure that the freezer bowl wall temperature is as low as possible when the mix is added.
Tip #2 - FREEZE YOUR EQUIPMENTFreeze a 1 litre (1.06 quarts) plastic container and the ice cream dasher overnight. Freezing the plastic container will remove any stored heat. Heat stored in the container causes the ice cream that contacts the side and bottom to melt, resulting in an increase in ice crystal size.
It’s also important to freeze enough water in some ice trays to make an ice bath. We'll be using an ice bath to quickly cool the ice cream mix once it's been heated, minimising the time it spends in the ‘danger zone’, between 5°C (41°F) and 65°C (149°F), where bacteria likes to multiply.
1.1.3. DRAW TEMPERATURE
Draw temperature, between -9°C and -12°C (15.8°F and 10.4°F) in domestic machines, refers to the temperature at which ice cream is removed from the freezer bowl once dynamic freezing is complete. Draw temperature has a significant influence on mean ice crystal size (Drewett & Hartel, 2007). In general, a lower draw temperature results in smaller ice crystals (Arbuckle, 1986). Hartel (1996) argues that of all the factors affecting ice crystallisation that can be controlled, the draw temperature of the ice cream freezer bowl is probably the most significant.
To obtain a low draw temperature, the freezer bowl temperature must be as low as possible to give rapid heat removal. Heat transfer must also be as efficient as possible. This means using dasher scraper blades that contact the wall of the freezer bowl as they rotate to prevent a build up of ice. A lower draw temperature also reduces recrystallisation during the static freezing stage as harding times are reduced (Drewett & Hartel, 2007).
Longer residence times are usually required to obtain a lower draw temperature. 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 and a lower draw temperature.
Tip #3 - PROMOTE EFFICIENT HEAT TRANSFERIf you're using the Cuisinart ICE-30, you can limit ice build up on the freezer bowl wall, thus promoting efficient heat transfer, by using your thumb to push the dasher firmly against the side of the bowl during dynamic freezing. I've used this technique for the past 6 years with my ICE-30 and haven't had any issues with the added stress that is placed on the motor.
1.2. ICE CRYSTAL GROWTH DURING THE STATIC FREEZING STAGE
No new ice crystals are formed during the static freezing stage but the existing small crystals grow in size until the temperature decreases to -18°C (-0.4°F), or ideally -25°C to -30°C (-13°C to -22°F), to halt this growth. Faster cooling of the partially frozen ice cream during static freezing, therefore, results in smaller ice crystals (Donhowe, 1993; Goff & Hartel 2013).
TIP#4 - FAST COOLINGTo promote faster cooling of the partially frozen ice cream during static freezing, place your ice cream at the back of your freezer where it's coldest. The size of your batch also affects the cooling time, with larger batches usually taking longer to cool to below -18°C (-0.4°F).
1.3. ICE CRYSTAL GROWTH DURING STORAGE
Storage conditions, namely temperature and temperature fluctuations, influence ice recrystallisation and shelf life, with colder storage temperatures better for minimising recrystallisation and extending shelf life. The ideal storage temperature to reduce recrystallisation would be below the glass transition temperature, below about -32°C (-25.6°F) (Goff & Sahagian, 1996; Roos, 2010). Below the glass transition temperature, recrystallisation occurs very slowly, but storage below about -25°C (-13°F) gives a sufficiently slow recrystallisation rate to give extended shelf life (Goff & Hartel, 2013).
Temperature fluctuations due to automated defrost cycles on home freezers contribute to higher rates of recrystallisation during storage. Witting and Smith (1986) showed that ice creams stored in a ‘supermarket-type frost/defrost freezer’ with temperature cycles between -9.4°C and -15°C (15°F and 5°F) became detectably icy in 4 weeks and objectionably icy in 3-10 weeks.
TIP#5 - TEMPERATURE FLUCTUATIONS DURING STORAGETo minimise recrystallisation during storage, try to minimise temperature fluctuations by reducing the number of times you take your ice cream out of the freezer, leave it out at room temperature, and then re-freeze it. The more times this cycle is repeated, the more recrystallisation is likely to occur. Also, try to limit the time that your tub of ice cream is left at room temperature by returning it to the freezer as soon as you've finished scooping.
TIP#6 - DISABLE AUTO DEFROSTIf possible, disable the auto defrost setting on your freezer to minimise temperature fluctuations during storage.
2. REDUCTION
For all recipes on the blog, the starting weight of the ice cream mix will be either 1000g or 1200g, with a target reduction of 15% and 13% respectively after heating for 25 minutes at 72°C (162°F). These target reduction figures are to ensure that we have the necessary mix composition after heating. For a recipe with a starting weight of 1200g and a 13% target reduction after 25 minutes heating at 72°C (162°F), you should be left with 1044g of mix after heating. For a recipe with a starting weight of 1000g and a 15% target reduction, you should be left with 850g of mix after heating.
Here is how to check the level of reduction after heating for 25 minutes at 72°C (162°F) of a mix with a starting weight of 1200g:
First weigh your pan and record its weight. My 23cm diameter pan weighs 1606g.
1606g pan + 1200g starting mix = 2806g starting weight.
After 25 minutes of heating, my total weight (1606g pan + 1044g 13% reduced mix) should be around 2650g.
If my total weight after 25 minutes heating is greater than 2650g, I will continue heating until the weight falls to 2650g or less.
3. THE SIZE OF YOUR PAN
The size of the pan you use will affect the rate of evaporation and, therefore, the heating time. I use a large pan with a 23cm diameter, which results in either a 13% or 15% reduction after 25 minutes of heating. If your pan is smaller than 23cm, you'll likely need to continue heating your mix for a further 2-5 minutes to achieve the target reduction. If you don't achieve the target reduction, you may be left with too high a water content in your mix, which may result in coarse texture.
4. THE IMPORTANCE OF HEATING TIME AND TEMPERATURE
There are three principal reasons why we'll be heating our mix to 72°C (162°F) and holding it there for at least 25 minutes: 1. to pasteurise the mix, 2. to improve foaming and emulsification, and 3. to improve body and texture.
4.1. PASTEURISE THE MIX
If you’re running a business and making ice cream to sell, you need to ensure that you are in compliance with food safety legislation. Here in the U.K, the Dairy Products (Hygiene) Regulations 1995, Schedule 6, part v 1 (a) states:
1. Pasteurised ice-cream shall be obtained by the mixture being heated—
to a temperature of not less than 65.6°C (150.1°F) and retained at that temperature for not less than 30 minutes;
to a temperature of not less than 71.1°C (160°F) and retained at that temperature for not less than 10 minutes; or
to a temperature of not less than 79.4°C (174.9°F) and retained at that temperature for not less than 15seconds.
Ice cream needs to be pasteurised in order to destroy all pathogens and the enzyme phosphatase that may be harmful to health. This is just as important for those of us making ice cream to sell as it is for you guys making ice cream at home.
4.2. IMPROVE FOAMING AND EMULSIFICATION
The second reason we'll be heating our mix to 72°C (162°F) and holding it there for at least 25 minutes is to improve whey protein foaming and emulsification. Foam formation and its stability is important for texture and for the retention of air that is incorporated into ice cream during dynamic freezing. Heating milk so that the whey proteins undergo partial protein unfolding yields a more voluminous and more stable foam and improves the emulsifying characteristics of the proteins (Philips et al., 1990). Similarly, Damodaran (1996) found that denatured proteins have better foaming properties, attributed to increased hydrophobicity, and greater interfacial contact. Sava et al (2005) found that surface hydrophobicity increased considerably at temperatures between 70°C and 77.5°C ( 158°F and 171.5°F) when whey protein was heated for 45 minutes, with greater increases noted after longer heating times.
Sava et al. (2005) note that thermal denaturation of whey protein involves 2 steps: an unfolding step at 70 to 75°C (158 to 167°F), and an aggregation step at 78 to 82.5°C (172.4 to 180.5°F), that mostly follows unfolding. Phillips et al. (1990) note that foaming and emulsifying characteristics may be impaired if protein undergoes aggregation
4.3. IMPROVE BODY AND TEXTURE
The third reason we'll be heating our mix to 72°C (162°F) and holding it there for 25 minutes is that heating milk also improves ice cream texture because of the denaturation of proteins and the consequent increase in their water-holding capacity (Goff & Hartel 2013).
TIP#7 - HEATING TIME AND TEMPERATUREStudies point to an optimal heating temperature for whey protein at somewhere between 70°C (158°F) and 75°C (167°F). In this temperature range, whey proteins undergo reversible unfolding, which improves foaming, emulsification, and texture. Holding whey protein at between 70°C (158°F) and 75°C (167°F) for an extended period of time significantly increases surface hydrophobicity with only a minimal loss of solubility, which improves foaming.
5. WHY IS SKIMMED MILK POWDER ADDED TO ICE CREAM?
Skimmed milk powder’s primary role in ice cream is to increase the milk solids-not-fat (MSNF), namely the protein. Flores & Goff (1999) demonstrated that milk proteins had a large impact on texture by limiting ice crystal size and enhancing their stability.
I hope this helps. I'd love your thoughts on how this post could be improved so do get in touch and say hi! All the best, Ruben :)
REFERENCES:
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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).
Damodaran, S., 1996. Functional properties. In: Nakai, S., Modler, H.W. (Eds.), Food Proteins – Properties and Characterization. VCH Publisher, New York, pp. 167–234.
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