The structure of ice cream is comprised of fat, protein, air cells and ice crystal. A high quality, creamy, delicious ice cream requires numerous factors to fulfill the standard - balanced mix composition, well-functioned and powerful freezing machines. In this post, I'll try to explain the mechanism behind ice ctystalisation formation with the freezing process, which is the latter - the mechanic factors: nucleation rate, draw temperature, temperature of refrigerant fluid, rotating speed of the ice cream machine and residence time.
1. Standard
Ice crystal size, around 20-30 μm in average, delivers a smooth mouthfeel. The size will finally grow up through migration and fuse with others and it is the time that shelf life is ended. Small one, around 10-20 μm is an ideal standard size that no coarseness will be detected; with the large one greater than 50 μm is considered as an product whose shelf life is surpassed and the tongue will notice its existence during consumption.
2. Dynamic Freezing
Dynamic freezing is the only process in which the ice crystalisation will occur, that the ice cream mix aged in the fridge at below 5°C for overnight is poured in the machine where the refrigerant fluid will start absorbing the heat from the mix and start forming a very thin layer of ice crystal in the barrel wall within the freezing section of the machine. This birth of layer is called nucleation, and the greater the nucleation rate, the smaller the ice crystal size. Soon after the birth of the ice crystal on the layer, the sharp scrapper blade, together with the rotating dash will scrape off the ice crystal that some will melt in the centre of the barrel where the temperature is relatively higher, whilst some that did not melt will immediately fuse together and grow in size carrying on recrystalisation. Recrystalisation is a process that the total number of ice crystals will drop but the size of the them will be bigger through different forms of enlargement.
3. Nucleation & Recrystalisation
High nucleation rate will give birth to a lot of small ice crystals and this will results in a lower recrystalisation rate during static storage and more smooth and small ice crystal can be preserved.
If the ice cream freezer cannot provide a strong nucleation rate, the nucleation rate is weak and more unfrozen ice cream will be transferred to the static storage section and start growing up in size with other big crystals, and thus finally affect the quality of the final product due to the coarseness.
Relationship of in-betweens
4. Refrigerant temperature affects nucleation:
Ice cream freezer is the machine that agitates the mix. With aid of air incorparation, the mix finally becomes ice cream with full body. There must be a powerful refrigerant behind that enables the mix freeze to a certain low temperature, and it is this refrigerant that plays the paramount role. The refrigerant enables the temperature itself fall onto the range between -25°C to -30°C and the cold transferred to the barrel wall will be responsible for freezing the ice cream mix, though the temperature at the barrel wall is slightly higher because of the distance from the coldest spot i.e the refrigerant fluid. If the barrel is cold enough, the heat of the mix will be able to be transferred rapidly and to keep dropping the temperature to the level at which the small ice crystals can be formed and thus results in a lower rate of recystalisation in both dynamic agitation and static storage.
4A. Nucleation affects residence time.
Residence time is the duration during which the ice cream mix resides in machine agitation. The shorter time the mix resides in, the smaller the ice crystals will be found in the product. A longer residence time means that the ice cream mix takes longer time to reach the draw temperature that is around -5°C to -8°C, at which temperature the ice crystals grow up very rapidly in the centre of the barrel and cause recrystalisation. The greater extent the recystalisation is undergone, the longer is the residence time. So the key to manufacture high quality ice cream in freezing sector is to have a fully commercial ice cream machine with a powerful refrigerant that promote high nucleation.
4B. Rotating speed affects residence time
When scrapper blade is rotating, heat generated from it will be transmitted to the mix and this hinders more or less the heat movals from ice cream mix itself and thus to reach the targeted low draw temperature. The heat melts some of the nucleations (newly formed ice crystals) from the wall and these melted concentrates stays longer in the centre of the barrel and trigger a greater extent of recystalisation, and the residence time is then also longer. Research has shown that a high rotating speed will directly afftect the temperature of the ice cream mix to be higher, hence resulting in a larger ice crystal mean.
4C. Draw Temperature
A higher draw temperature will generate a larger ice crystal size . The factors of affecting the final draw temperature are attributed to refrigerant temperature, nucleation rate and rotating speed. As previously stated, refrigerant is the origin and the first place of the cold supply. The colder it gets, the better to reach a lower temperature at the barrell wall, so is the nucleation rate greater because of the faster heat removal and the ice crystal to be smaller. Research found that ice crystals were larger at draw temperatures from -3°C to -6°C, and were smaller when the temperature is below -6°C.
5. Contradictory phenomena bwtween residence time and draw temperature
Since shorter residence time is attributed to the high nucleation rate that the majority of the ice crystals were small and freezed very rapidly, it is preferred that an ice cream mix stay shorter time during dynamic freezing process. However, longer residence time can enable the mix be drawn at lower temperature which is also preferrable. In my experiments, longer residence time has indeed lowered the draw temperature but the texture of the ice cream that freezes quicker in the machine is still better in general in terms of ice crystal size.
Summary
Ice crystal size must be within 10 to 20 μm to deliever an ideally smooth texture, and there are different key factors behind the freezing process that affects the final ice crystal size, including refrigerant fluid temperature, nuclearation rate, rotating speed, residence time and draw temperature. Each of them are highly interrelated and all may affect the recystalisation rate. In short, refrigerant is of the paramount importance to supply powerful cold and thus a high nucleation rate, shorter residence time and low draw temperature. Some research might have shown the heat from rotating friction transmitted to the mix will affect the heat removal of the mix, however the key factors outweigh this minor point and still play the predominante role in affecting the final ice crystal size.
Reference
Goff & Hartel Ice Cream 7th Edition
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