Most of the people would probably say ''Heat'' as the answer. Although heat is the external factor to directly penetrate into the ice cream and break the structure faster, ice cream itself is formed by 3 main phases through which an ideal melting rate can be reached : air phase, fat phase and ice phase. The air phase is about the air cells and overrun interaction; the fat phase is about dairy or vegan fat content, the interaction between fat globules distribution and partial coalescence, also known as fat destablisation; the ice phase is about ice volume and size distribution. The coexistence of these 3 phases are of the foundation of shape retention (slow melting rate). Other than these 3 phases, there are a few minor factors that also contribute to prolong the melting rate, such as the usage of synthetic stabiliser and emusifier.
1. Air phase
1A. Overrun
Overrun refers to the air content whipped in the ice cream during agitation by rotated dasher in ice cream machine. Ice cream with higher overrun - though it still has to be within a reasonable range - melts slower than those with lower overrun. (Sakurai et al. 1996) Air cells are as if a bubble bag in a kraft box to protect the item so that heat cannot penetrate into the ice cream and melt the ice cream so fast and easily.
1B. Air cells distribution
Air cells' size and its distribution also contribute to the desirable melting rate. These small bubbles are generated during the dynamic freezing process in the ice cream machine by the in-built shear stress, and are initially big entities yet later turned into small under agitation. A high volume but small sized air cells distribution profile is preferred to achieve a decent melting rate.
Regarding to air cells and overrun, these two subjects are in conjunction with the residence time and rotating speed of the ice cream machine. Residence time refers to the time of pasteurised mix sit in the ice cream machine during dynamic freezing and in general the shorter the time, the better. Since commercial ice cream machines have a very advanced refrigerant system and well- established cooling tubes to absorb heat, the wall of the in-built cooling bowl can reach a very low temperature at around -30 to -40°C, and this low temperature enables greater nucleation rate (ice crystallisation) and shortens the residence time. The mechanism behind is that a more powerful nucleation rate is prone to obtaining more smaller size ice crystal. And the faster the speed, the harder the free water is to diffuse to other unfrozen water molecules to form a bigger ice crystal. However, short residence time to produce smaller ice crystals has to deal with a competing phenomenon with long residence time producing smaller air cells as both results are beneficial to having a decent melting rate.
Rotating speed is also claimed to affect ice crystal size as there is heat generated by the rotating dasher. Hartel (1996) argues that changing the agitation speed of the dasher has significant effect on ice crystal formation during the dynamic freezing process. It is believed that the output of heat driven by friction during the high speed rotating slows down the ice crystal nucleation. However, even the rotating speed of commercial ice cream machines happens to be much stronger than domestic ones and likely to project extra heat, the freezing power is just so strong to absorb the heat from pasteurised mix.
In my experiments, in order to achieve a short residence time as well as a desirable melting rate, it is primarily based on total solids (fat content, milk-solid-non-fat, application of sweeteners, fibre or natural emulsifier like egg yolks or SMP). Low total solid profile is prone to less frozen water during dynamic freezing and thus more ice recrystalisation will be driven. In this point, if the ice crystal size is not small enough (under 40 to 50 μm) to build up a strong small ice crystal network between fat globules, protein and air cells, the rest of these entities will be likely collapsed easily and it is also this mechanism to speed up melting rate. Also, low total solid profile means there is more free water in the aged pasteurised mix, which also take longer residence time, even though a longer residence time is claimed to generate smaller air cells that contribute to have a decent melting rate, yet I don't find this mechanism too influential in my experiments.
2. Fat Phase
2A. Fat content
Ice cream carrying higher fat content melts slower. Li(1997) studied the correlation between fat content and melting rate and drew a conclusion that ice cream with higher fat rate melts slower than those with lower fat content. The mechanism is attributed to partial coalescence or fat destablisation, a protruding fat crystal from one fat globule pierces the interfacial film of another globule, forming a largely irreversible connection between the internal phases of the globules (Walstra, 2003).
2B. Partial coalescence
Fat globules are supposed to be stable but unstable enough to form a cluster and demonstrate partial coalescence during dynamic freezing process. The partially-coalscenced fat globules in this cluster act as a very important link to bond up with air cells, ice crystals, protein and other minor milk-solids-non-fat as well as a resistance to meltdown and promoting a dry surface that looks neat and clean upon withdrawal. Gelato is an classic example to explain why it melts faster. This category usually carries 6-10% fat content profile, which is relatively lower compared to conventional ones which carries 11-18%, depending on various factors such as cost, flavour and ingredients nature. Gelato tends to melt faster as its composition carries lower fat content and lower total solids and that is why gelato is prone to be icy if turnover is not high enough. Artisans producing this type of ice cream normally use sucrose and other low sugar to sugar free solids contents such as planted fibre or maltodextrin to compensate the solids insufficiency, yet the products tend to be sweeter that might be a turn off for some people.
Regarding to partial coalescence, milk-solid-non-fat, sweeteners, optional natural or synthetic emulsifier and stabiliser, along with aging phase are all taken into factors consideration affecting how partial coalescence can demonstrate.
Milk-solid-non-fat (MSNF)
The milk-solid-non-fat content can be obtained from dairy products and the main function is to absorb the thick layer of fat globules and prevent them from getting too closed through its stabilising property. The absence of MSNF will lead to these fat globules wholly-coalescenced instead of being proper partial coalescenced, which is crucial to achieve the ideal 3-phase structure and thereby slow down the melting rate.
3. Ice Phase
Sweeteners
Sweeteners such as sucrose, which is the table sugar we use on daily basis, is one of the most economical solid options to fill up the ice cream composition. Noy only does it just provide sweetness, but also a viscosity enhancer to the ice cream mix. Viscosity acts as a stablising agent to trap water coming together so that big ice crystals are not easily formed. However, sweeteners affects the freezing point at the same time, which is encountering a competing status. Sweeteners drop the freezing point and trigger recrystalisation, speeding up melting rate. Application of sweeteners is based on flavours, targeted hardness at certain scooping temperature, and the balance with other solids contribution. In this point, trial and error is needed to get the freezing point right. A normal freezing point range will benefit the ice cream to be creamy instead of icy as icy ice cream melts much faster.
Emulsifier & Stabiliser
Emulsifier and stabiliser are widely used in economic ice cream production, which are the ones holding up a year of consumption before expired. Emulsifiers, either synthetic or natural, such as polysorbate 80 or the lecithin from egg yolk may also promote partial coalescence (Goff, 1997) by displacing adsorbed proteins from the fat layers interface. A good shape retention and dryness of products as consequences can be seen as visible evidence to show a resistance to meltdown. Synthetic stabilisers such as locust bean and guar gum also enhance the viscosity of the serum phase, which is effective in binding the movement of water and other MSNF concentrations such as mineral and vitamins. However, emulsifer and stabiliser are not necessary in artsian category, depending on the business scale and turnover.
Aging
Partial coalescence can only occur if fat globules contain solid particles (Goff, 1997). Aging is the process where these fat particles start to become crystalised and approximately 70% will have become solid at 4 to 5°C after 5 hours, yet preferrably aged over 12 hours. Ice cream mixes which have not been aged do not undergo much partial coalescence and tend to end up being wet in surface and losing shape retention, which are all disadvantages to melting.
Summary
Ice cream melting is not just because of natural heat that penetrates into ice cream but also the structure itself plays a bigger role in deciding how fast it melts - a poor structure built up by an unbalanced composition will produce final products that melts quickly even it's not exposed to heat. Ice cream is structured in 3 phases: the air, the fat and the ice. The fat phase is about the crystalised fat globules undergoing partial coalesence and the structure with the rest of two, which are air cells and ice crystals. The latter ones should be great in terms of quantity but small in terms of size to resist meltdown. In order to promote partial coalesence, there are various factors such as the usage of sweeteners, msnf, emulsifier and stabiliser. Aging is necessary for crystalising fat globules or the one skipping this process will likely suffer from rapid meltdown.
Reference
Sakurai, K., 1996. Effect of production conditions on ice cream melting resistance and hardness.
Li, A.,1997. Effect of milk fat content on flavor perception of vanilla ice cream. Journal of Dairy Science.
Walstra, P., 2003. Physical chemistry of foods
Goff, H. D., 1997. Instability and Partial Coalescence in shippable Dairy Emulsions.
Goff, H. D., and Hartel R. W., 2013. Ice Cream. Seventh Edition. New York: Springer.
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