Ice Crystals in Ice Cream
One of the most important goals of an ice cream maker is to make ice cream with many small ice crystals, resulting in a smooth texture, and to preserve that ice crystal size distribution until consumption.
Although many factors influence the eating quality of ice cream, it is generally understood that smooth ice cream requires the majority of ice crystals to be smaller than about 50 um in size. Ice crystals larger than 40 to 55 p result in coarse or grainy texture if present in sufficient number (Arbuckle, W. S. 1986).
Factors Affecting Ice Crystal Size
So, how can we make ice cream with many small ice crystals at home? Total solids content, fat content, sugar content, air bubbles, residence time, and hardening time all play and important role in forming small ice crystals and, in turn, a smooth and creamy texture.
(i) Total Solids
An increase in the total solids (TS) (the percentage of milkfat, non-fat milk solids, sugar and egg solids) level will decrease the amount of water in the mix, thereby reducing the total amount of ice that can form. However, a total solid percentage over about 47% will result in a heavy, chewy ice cream.
Flores and Goff, 1999, held that the percent of TS of the ice cream mix is directly related to ice crystal size distribution; lower TS ice cream contains larger ice crystals (Donhowe et al., 1991). Variation in solids content of just a few percent greatly influences ice crystal growth (Keeney, 1979).
The recipes I present in this blog contain around 41% total solids. Heating the mix for 60 minutes reduces the water content by roughly 19%. This increases the total solids content to around 47% and concentrates flavour. Economy ice cream contains around 35-37% total solids , whilst super premium contains around 42-44% (Marshall et al., 2003). The high total solids content in homemade ice cream contributes to the formation of small ice crystals.
Arbuckle suggested that the fat content can also influence the size of the ice crystals in that fat globules could mechanically impede the ice crystal growth. It is important to keep the milkfat percentage high in homemade ice cream in order to develop a smooth and creamy texture. I try and keep the milkfat content in my ice cream around 19% before the mix is concentrated.
Studies have clearly shown that sucrose reduces ice crystal growth rate (Buyong and Fennema, 1988). In ice cream, higher sucrose content produces smaller ice crystals and an increase in sugar content from 12 to 18% decreases ice crystal size by about 25% (Arbuckle, 1986).
However, an ice cream that is too sweet will suffer from rapid melting. When ice cream is too sweet, other flavours tend to be subdued.
The recipes presented in this blog contain 14% sucrose before the mix is heated and concentrated by 19%.
(iv) Air Bubbles
When an ice cream mix is frozen in an ice cream maker, air bubbles are whipped into the mix. Hartel has stated that low overrun (a small amount of air whipped into the ice cream mix in the ice cream maker) induces the formation of coarser ice crystals in ice cream compared with the same formulation made at higher overrun, because air cells may provide a physical impediment to ice crystallisation during freezing.
So, whipping more air into an ice cream mix appears to result in the formation of smaller ice crystals. Since most of us cannot adjust the speed of the ice cream dasher (unless you are fortunate enough to own an industrial ice cream machine), the only way to whip more air into a mix is to leave it in the machine for longer. However, as we will see, a long residence times (the time the ice cream spends in the machine) significantly contribute to an increase in ice crystal size.
(v) Residence Time
Residence time (the amount of time an ice cream mix spends in the ice cream maker) was found to have the most pronounced effect on mean ice crystal size (Drewett and Hartel, 2007). Russell et al. (1999) clearly demonstrated the inverse relationship between residence time in the freezer and ice crystal size.
C. Clarke, 2004, held that the total number of ice crystals depends mostly on the residence time. Short residence times produce many crystals with a small mean size, whereas long residence times and high rotation speeds result in fewer, larger crystals (C. Clarke, 2004).
To obtain small ice crystals, it is necessary to have the shortest residence time possible so as to minimise the amount of ripening that occurs within the freezer barrel itself (Marshall et. al. 2003).
The manufacturers instructions on my Cuisinart ICE30 ice cream machine states that I should leave the mix in the machine for 20-25 minutes. However, I have found that enough air is incorporated into the mix after about 16 minutes. The longer you leave your mix in the ice cream machine, the bigger the ice crystals will be.
(vi) Hardening Time
To achieve small initial ice crystals, the ice cream mix must be rapidly frozen in a freezer, after it has been churned in the ice cream maker, to promote nuclei formation and minimise ice crystal growth (Hartel, 2001).
Temperatures as low as -30°C are used in order to promote rapid nucleation in the freezer (Marshall et al., 2003). Several ideas involving liquid nitrogen have also been developed to promote rapid nuclei formation.
When ice cream is removed from the ice cream maker, it must be immediately hardened in a freezer. The longer the ice cream spends at room temperature once it has been churned in the ice cream maker, the more ice will melt and the larger the ice crystals will be. You must ensure that you empty the ice cream into a plastic tub and freeze as quickly as possible.
The temperature and rate of hardening determine the final ice crystal size and the physical sensory properties of the product (Sutton and Bracey, 1996). Time to accomplish hardening has been assumed to be the time for the temperature at the centre of the package to drop to -18°C or lower, preferably -25 to -30°C.
Factors affecting hardening time:
- Heat stored in the container at filling causes melting when cold ice cream is removed from the freezer bowl. To remove any heat stored in the container, chill it over night.
- The smaller the surface to volume ratio, the slower heat will be transferred. Try to use a long, or rectangular, plastic tub instead of a small, deep one. Ice cream at the centre of the tub will take longer to freeze in small, deep containers.
- The colder you can set your freezer, the faster the rate of hardening. Try to set your freezer’s temperature to between -25 to -30°C. Hagiwara and Hartel, 1996 found that storage of ice cream at -20°C resulted in very little increase in mean ice crystal size, but, at -5°C, mean ice crystal size increased from 40 pm to about 220 pm during 5 d of accelerated storage.
- It is the circulating air within your freezer that cools the ice cream. If the ice cream tub is surrounded by other items in your freezer, it will take longer for the cold air to chill the ice cream. Try to ensure that there is sufficient space within your freezer for the air to circulate freely, thus minimising the hardening time.
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