Ice cream generally contains seven categories of ingredients: milk fat, milk solids-not-fat (the lactose,proteins, minerals, water-soluble vitamins, enzymes, and some minor constituents), sweeteners,stabilizers,emulsifiers, water, and flavours ([^1]). The fat in an ice cream mix can either come from dairy sources (e.g. milk or cream), or from vegetable sources (e.g. coconut oil, palm oil, palm kernel oil, sunflower oil, peanut oil, olive oil). In this post, I'll discuss 4 important factors to consider when choosing a non-dairy fat for the production of vegan ice cream. If you're short on time, you can jump straight to the summary. :)
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- Using a blend of 60-80% solid fat (i.e. coconut oil or palm kernel oil) and 20-40% liquid fat (i.e. sunflower oil, peanut oil, olive oil, soybean oil, corn oil, or safflower oil) produces ice cream with the highest rates of partial coalescence.
- An ice cream mix that contains coconut oil or palm kernel oil as the solid fat source will benefit from reduced ageing times of only 1-2 hours at 4°C to 5°C (39°F to 41°F).
- Fats with a tendency to crystallise in a form that enhances partial coalescence include soybean oil, peanut oil, corn oil, olive oil, coconut oil, sunflower oil, safflower oil, and cocoa butter.
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Ice cream is a multiphase system composed of an unfrozen serum phase, air bubbles, ice crystals, and fat globes ([^1]).During the whipping and freezing process, some of the fat globules in the mix stick together during collisions caused by the rotating scraper blades, forming clumps and clusters of fat globules. This process is known as partial coalescence ([^2]).Partially coalesced fat globule clusters are responsible for surrounding and stabilising air cells and for creating a semi-continuous network or matrix of fat throughout the ice cream. This structure has a significant influence on dryness when the ice cream is extracted from the ice cream machine (aids in noveltymolding), smooth and creamy mouthfeel, resistance to shrinking during storage, and resistance to meltdown or good shape retention (necessary for soft-serve operations) ([^3] [^4] [^5]).
When selecting a non-dairy fat for the production of vegan ice cream, 4 factors are important to consider for optimal partial coalescence: 1. the amount of solid fat; 2. the rate at which the fat crystallises, which determines, in part, the ageing time; 3. the crystal structure of the fat; and 4. the temperature dependent melting profile of the fat ([^1]).
Fats and oils are part of a group of compounds known as lipids. This group of compounds contains a large number of different types of molecules, including triacylglycerols, diacylglycerols, monoacylglycerols, free fatty acids, sterols, and phospholipids. Triacylglycerols are by far the most common lipid in foods, and it is this type of molecule that is usually referred to as a fat or oil ([^6]).
Triacylglycerols can be either solid (crystalline) or liquid at a given temperature depending primarily on the nature of their constituent fatty acids. Triacylglycerols rich in saturated fatty acids are solid at room temperature (25°C (77°F)), whilst triacylglycerols rich in short-chain and/or unsaturated fatty acids are liquid at room temperature.
For optimal partial coalescence during freezing, it is essential that an ice cream mix contains an optimum ratio of solid:liquid fat at 4°C to 5°C (39°F to 41°F). Too much solid fat leads to insufficient partial coalescence, as the liquid oil component is thought to hold globules together when they collide, but too much liquid oil will result in coalescence, rather than partial coalescence, which will not build up the desired aggregated structure ([^7] [^8] [^9]).
2.1 What is the optimum amount of solid fat?
The optimum amount of solid fat for optimal partial coalescence has been suggested to be between 30% and 70% at 4°C to 5°C (39°F to 41°F), with the remaining 60% to 30% being liquid oil ([^1]). The liquid fat content of common vegetable oils used in the production of vegan ice cream is illustrated below.
Replacing milk fat (around 70% solid fat at 4°C to 5°C (39°F to 41°F)) with palm kernel oil (around 80% solid fat at 4°C to 5°C (39°F to 41°F)) or coconut oil (around 90% solid fat at 4°C to 5°C (39°F to 41°F)) will result in ice cream that is much harder, melts quicker, has a wet appearance, and a reduced creaminess perception due to the high solid fat content and the subsequent reduction in partial coalescence ([^10]). Blends of vegetable oils are, therefore, often used to obtain the optimum ratio of solid:liquid fat. Studies have shown that using mixtures of 60% to 80% solid fat (i.e. coconut oil or palm kernel oil) with 20% to 40% liquid oil (i.e. sunflower oil, soybean oil, peanut oil, olive oil etc.) results in ice cream with the highest rates of partial coalescence, the highest creaminess values, and the lowest rates of meltdown ([^9] [^14]). Palm oil, however, may have sufficient solid fat (around 60% 4°C to 5°C (39°F to 41°F))) to be used as the single vegetable fat source.
After pasteurisation and hom*ogenisation, an ice cream mix is quickly cooled to below 4°C (39°F) and aged at that temperature for anywhere from 4 to 24 hours. It is during this cooling and ageing process that the fat crystalises. Insufficiently aged mixes will have less crystallised fat and, therefore, less than optimal partial coalescence during the whipping and freezing stage.
Different fats crystalise at different rates and to different extents. Therefore, the time a vegan ice cream mix needs to be aged will, in part, be dependent on the rate of crystallisation of the vegetable fat. A standard ice cream mix containing milk fat takes 4-5 hours at 4°C (39°F) for about 70% of the fat droplets to crystallise([^15]). Coconut and palm kernel oil, however, crystallise at a faster rate than milk fat, being nearly complete after 6o minutes of ageing at 5°C (41°F) ([^13]). The rapid crystallisation of these two oils can, therefore, allow for shorter ageing times, which is beneficial from a commercial standpoint. Palm oil, on the other hand, crystallises at a slower rate and will, therefore, require longer ageing times than coconut or palm kernel oil. Despite their varying rates of crystallisation, all three vegetable oils will have crysallised sufficiently during typical ageing times of 4 hours or more [^13].
Oils and fats can crystallise in different polymorphic forms (crystal forms). The three commonly observed fat crystal forms are the α (alpha), β′ (beta prime), and β (beta) forms. These forms of crystallisation (polymorphism) are an important characteristic of oils and fats because they influence the rate of partial coalescence ([^16]). Alpha crystals usually grow as spheres, giving much less surface area/unit fat compared to needle-like beta prime crystals. Beta crystals, on the other hand, usually grow as plates. For a given volume, plates have a larger area than needles (factor of 10). The more plate-like the structure, the more surface area the fat has and the easier it becomes to make contact with the rest of the free fat to enhance partial coalescence ([^16]).
The polymorphic behaviour of an oil or fat is determined to a large extent by the fatty acids within the triacylglycerols. Fats and oils that are composed of fatty acids predominantly of a single chain length are most likely to be stable in the beta form ([^17]). Fats with a tendency to crystallise in the beta form include soybean oil, peanut oil, corn oil, olive oil, coconut oil, sunflower oil, safflower oil, and cocoa butter, whereas palm oil, cottonseed oil, rapeseed oil, and milk fat produce β′ crystals ([^18] [^19]).
During consumption, the fat in ice cream should melt slowly and almost completely at, or just above, the temperature in the mouth (around 37°C (99°F)). Vegetable fats that melt above this temperature may not melt sufficiently, thus giving rise to a waxy or greasy sensation in the mouth.
Edible fats and oils contain a complex mixture of many different types of triacylglycerol molecules, each having a different melting point. They therefore usually melt over a wide range of temperatures, rather than at a distinct temperature ([^6]). Milk fat, for example, has an extremely complex triacylglycerol composition with hundreds of different types identified and thus melts over a broad range of temperatures, typically from -40°C to 40°C (-40°F to 104°F), resulting in a gradual melting profile ([^11]).Coconut oil, on the other hand, has a narrow melting range, typically 23°C to 25°C (73°F to 77°F), meaning that it melts quickly and completely in the mouth ([^12]). Similarly, palm kernel oil has a very narrow melting point of 27-28°C (81°F to 82°F), meaning it will melt quickly and completely in the mouth. Palm oil, on the other hand, has a slightly broader and higher melting point range of between 32°C and 40°C (90°F and 104°C), meaning that it will melt slower than coconut oil and palm kernel oil and may not melt completely, thus giving rise to a waxy or greasy sensation in the mouth ([^12]).
Partial coalescence of fat globules has a significant influence on parameters such as dry appearance, creamy mouthfeel, resistance to shrinkage during storage, and resistance to meltdown during consumption. For optimal partial coalescence, it is essential that an ice cream mix contains an optimum amount (between 30% and 70% solid fat at 4°C to 5°C (39°F to 41°F)) of solid fat. Blends of vegetable oils (60% to 80% coconut oil or palm kernel oil with 20% to 40% of a liquid oil such as sunflower oil, soybean oil, peanut oil, or olive oil) are often used in vegan ice cream production to obtain the optimum amount of solid fat. Palm oil may have sufficient solid fat to be used as the single vegetable fat source. Vegan ice cream mixes should be aged for between 1 and 4 hours at 4°C to 5°C (39°F to 41°F) to ensure that sufficient fat crystallisation (solidification) has taken place. Vegetable fats including soybean oil, peanut oil, corn oil, olive oil, coconut oil, sunflower oil, safflower oil, and cocoa butter will crystallise in a form that is likely to enhance partial coalescence. During consumption, coconut oil will melt completely but quickly in the mouth, as will palm kernel oil. Palm oil, on the other hand, will melt slower and may not melt completely, thus giving rise to a waxy or greasy sensation in the mouth.
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