1. Why Japan's Ice Cream "Types" Are Based on Milk Solids, Not Melting Speed
In Japan, the bag of an ice cream product has a section called "Type." The name is decided by the amount of milk solids (milk components excluding water) and the milk fat within those solids [1]. The Japanese categories are: Ice Cream (15.0% or more milk solids, including 8.0% or more milk fat), Ice Milk (10.0% or more milk solids, including 3.0% or more milk fat), and Lacto Ice (3.0% or more milk solids). Products with less than 3.0% milk solids, or none at all, are classified as Ice Confectionery [2]. This classification is strictly a scale for how much milk ingredient is present. It is not a scale for how easily the ice cream melts. The reasons for melting resistance are explored in the following sections.
2. What Is Overrun, the Amount of Air in Ice Cream?
Ice cream is made of fat, protein, sugar, water, and air. The ratio of air included is called "overrun." If you mix 1 liter of mix with 1 liter of air to make 2 liters total, the overrun is 100% [3]. Typical ice creams have an overrun of 40–100%. According to the Japan Ice Cream Association, lower overrun results in a rich taste, while higher overrun gives a light, fluffy texture [3]. At 100% overrun, half the volume is air (1 liter of contents plus 1 liter of air equals 2 liters total). At 40% overrun, 0.4 liters of the 1.4 liters total is air, which is about 30%. When magnified, ice, fat, and air bubbles are scattered finely and evenly. This structure creates a mouthfeel that melts quickly in the mouth [3].
3. Does the Air in Ice Cream Slow Melting? Sources Disagree
One view is that because air does not conduct heat well, more air bubbles make it harder for outside heat to reach the center, slowing melting. The Japan Ice Cream Association states that air bubbles and fat particles make it harder to transfer coldness, but this is part of an explanation about mouthfeel, not a direct statement about melting speed [3]. A blog-style site summarizing ice cream science describes bubbles acting like insulation, citing an example where a mix with 100–120% overrun melted slower than one with 80% [5]. However, a 2025 paper comparing ice creams made with different dairy fat bases explains it the opposite way. In their comparison, ice creams with less air intake melted slower. The authors attribute this to increased heat capacity and a reduced specific surface area (the surface area relative to volume), which makes heat transfer harder [6]. This result may not apply under other conditions. Additionally, a 2019 study by Wu et al. varied stabilizers, emulsifiers, and overrun. An excerpt from the abstract suggests that overrun only affected melting when no stabilizer was added [7]. The full text was not verified. In summary, the amount of air relates to melting ease, but the direction of the effect changes depending on the recipe and conditions. It is best not to assume that "more air means slower melting."
4. How Fat Particles Form a Mesh While Ice Cream Is Frozen and Stirred
When making ice cream, the liquid mix is stirred while freezing. During this process, fat particles (fat globules) in the liquid partially stick together. This is called "partial coalescence." It is not a complete merging into one, but a state where several particles connect while keeping their individual shapes [8]. Research by Goff et al. (2004) states that this partially coalesced fat stabilizes air bubbles and forms a fat mesh that separates bubbles in the unfrozen liquid [8]. Several studies explain that this mesh acts as a framework that holds the shape, making the ice cream melt slower. Think of it like a loose net wrapping balloons (the bubbles). However, the actual particles are microscopic, different from real balloons or nets. The type of emulsifier and the amount of protein affect this mesh formation. Reports indicate that emulsifiers (like polysorbate 80) promote partial coalescence and lower melting speed, while more protein can hinder partial coalescence [5]. These are summaries from a blog, so results depend on specific conditions.
5. How Stabilizers and Ice Crystals Help Ice Cream Keep Its Shape
Thickening stabilizers bind with water, reducing the amount of free water that can move freely. This suppresses the growth of large ice crystals during freezing and also during storage [9]. Stabilizers also relate to adjusting air content and maintaining shape [9]. Saneigen FFI, a company that makes stabilizers for frozen confections, states that stabilizers are used to adjust texture, maintain shape, and stabilize quality. They explain that stabilizers can make ice cream melt slower and provide resistance to temperature changes during storage and transport [10]. Regarding ice crystals, reports suggest that larger crystals melt faster. The reason may be that larger crystals take longer to melt completely. Other reports note that if there is more dissolved sugar, the freezing point drops [5]. These are blog summaries, so results are conditional. Melting ease is determined by a combination of type, air amount, fat mesh, stabilizers, and ice crystal size. It is important not to judge based on just one factor.
6. How to Investigate Ice Cream Melting Yourself
Here are two ways to explore the topic. With an adult, choose two different ice creams with different types. Compare the "Type" and "Ingredients" lists on the bags. Look for names of stabilizers, such as thickening polysaccharides. Place similar amounts on plates of the same size in the same location. Observe the shape every 10 minutes. Record the dripping pattern with sketches or words. One experiment alone cannot prove the effect of type or stabilizers. You can experience the difficulty of controlling conditions by trying multiple times and keeping the location and amount consistent. If you want to read original sources, the Japan Ice Cream Association Q&A [3] is in Japanese. The 2025 review paper abstract [4] is in English, but the abstract alone shows an organization of what factors are considered important.