Ice cream presents some of the trickiest mixing and blending challenges in food, with lower-fat products nudging up the level of difficulty still further, thanks to their greater use of thickeners and stabilisers.
Mintel research indicates that consumer demand for lower-fat ice cream and alternatives such as frozen yogurt is on the rise globally.
Ice cream consumers have something of a split personality, which leaves the prospects for middle-ground products looking sluggish compared to the drive for luxury and indulgence on the one hand and products with perceived health benefits on the other.
In its newly published report on ice cream and desserts in the UK, Mintel found that healthier options could be a key to boosting sales: “Despite the indulgent nature of the category, health factors emerge as the most likely to encourage more frequent usage. Low-sugar and low-fat are respectively cited by 32% and 28%, providing a clear challenge for brands to balance health and taste.”
Cost is another important reason why producers may be tempted to experiment with lower-fat formulations.
“Fat is always the most expensive bulk ingredient of ice cream and so when you’re talking about premium ice cream, it tends to have a higher fat content and cost more, while the less expensive economy brands tend to have lower fat content,” says John Coupland, professor of food science at Penn State University and one of the authors of a recent report into consumer preference for ice creams with varying fat levels. The research was published in the Journal of Dairy Science.
In a series of taste tests, the food scientists found that participants were unable to distinguish a two percentage point difference in fat levels in two vanilla ice cream samples as long as the samples were in the six to 12% fat-level range.
While the subjects were able to detect a four percentage point difference between ice cream with six and 10% fat levels, they could not detect a four percentage point fat difference in samples between eight and 12% fat. The researchers changed the fat content by adjusting the levels of cream and by adding maltodextrin.
Process challenges (back to top)
All this makes offering a lower-fat option pretty appealing for ice cream manufacturers. However, the processing challenges should not be overlooked, according to Matt Smith, sales director with Silverson Mixers: “In low-fat products, stabilisers act as gelling and bulking agents to replace the texture and body normally provided by the fat content. The thickening or gelling effect of the stabilisers can also contribute to body and texture or ‘mouthfeel’ – to produce a creamier texture associated with more indulgent ice creams.
“With conventional mixing equipment, long processing times are required to complete dispersion and achieve a satisfactory consistency as the powdered ingredients tend to form lumps when added to the base liquid. Stabilisers have an even greater tendency to agglomerate and require special handling.”
It’s not only the more specialised ingredients that lead to mixing challenges for ice cream makers, as Smith explains: “Other ice creams may include a higher cocoa content. Where cocoa powder is used it is very cohesive, making handling and controlled powder additionally difficult. Cocoa – where used – needs to be thoroughly wetted and ‘cooked’ to prevent it imparting a gritty or floury texture to the ice cream.”
High-shear rotor-stator mixers can overcome these problems since they produce enough shear to break down any agglomerates and properly disperse and hydrate even difficult raw materials.
Unlike conventional agitators, these mixers feature a mixing head with a central high-speed rotor that spins and forces material out through perforations in the surrounding stator wall, creating intense shear.
Not only is this good for breaking up agglomerates to create a lump-free mixture when incorporating powders, but it’s also useful in other intensive applications, such as homogenisation and emulsification.
“Silverson offers a number of in-tank and in-line mixer/homogenisers suitable for ice cream manufacture. The exceptional high shear rates generated in the precision machined rotor-stator progressively reduces particle size, quickly resulting in a uniform product, with a fine and uniform droplet size – typically two to five microns. Finer emulsions down to 0.5 microns can be achieved, depending on the formulation,” says Smith.
The most recent addition to the Silverson line-up for forcing powders into the liquid ice cream ingredients is the Flashmix. Smith explains: “A specially modified in-line mixer recirculates liquid from the process vessel through the Flashmix at high velocity.
“The powder feed valve is opened and unique pumping rotor forces the powders into the liquid stream where they are mixed on contact in the high-shear zone as they are subjected to intense mechanical and hydraulic shear. The resultant mix is passed back to the vessel by the self-pumping Flashmix.”
Although a high-shear mixer is sufficient for most food industry process requirements, many large-scale manufacturers also pass their ice cream premix through a high-pressure homogeniser, which breaks down fat globules in order to help form the ideal microstructure during freezing.
Even here, Smith argues that getting a head start by using a high-shear mixer to incorporate and disperse the ingredients allows the homogeniser to operate more efficiently and increase throughput.
High-pressure homogenisers (back to top)
Simply put, high-pressure homogenisers work by forcing material through a tiny orifice, which is very energy intensive.
One of the latest mixing innovations from Tetra Pak – the R370-1000D high-shear mixer – reduces the need for downstream homogenisation and may even eliminate it entirely in some cases by producing a much finer, more-consistent and more-stable emulsion.
The ability to eliminate the homogenisation step could cut energy consumption by up to 50% in the overall process. Soren Steffensen, commercial product manager for high-shear mixers, says that Tetra Pak is still running the tests needed to determine precisely how big the benefits are likely to be in different ice cream formulations.
A flexible powder introduction system and a newly-designed mixing head are among the innovations that allow the new machine to handle viscosities up to 2,000 centipoise, which Tetra Pak says is the highest available for a recirculation mixer.
It also breaks up droplets down to one micron, which is significantly smaller than the industry average of seven microns.
Another crucial element in the new system is the integration of a built-in deaerating system. It is important for the ice cream mix preparation process not to incorporate air into the mix, because it can lead to problems downstream.
During pasteurisation it can cause burning on the plates, for instance, and it can lead to separation of the mix during the ageing process, when it’s is given time for stabilising agents to hydrate and fat to crystallise.
It can also cause problems during freezing, because it leads to greater variability and inconsistent levels of air being incorporated into the final product – a parameter known as overrun.
Standard ice creams typically feature one part air to every two parts cream, or 50% overrun.
“The deaerator cone generates a larger surface area and a thinner product layer, which enables faster and more complete deaeration,” says Steffensen.
Inlet nozzles distribute the product smoothly over the deaerator cone. The vacuum causes any air bubbles to expand while bubble density decreases. This allows air bubbles to rise to the surface more quickly.
Extrusion line boosts flexibility (back to top)
Medium-sized ice cream producers face increasing pressure to be more responsive to market demand by offering a greater variety of products at smaller volumes. Tetra Pak has responded by launching an ice cream extrusion line to maximise flexibility.
“There are three ways of producing ice cream on a commercial scale: moulding, filling or extrusion,” explains Charlotte Harhoff, business intelligence co-ordinator for ice cream solutions. “In volume-terms it is split evenly between the three production methods globally. But in most of the western world including the UK we estimate the split to be moulding, filling, extrusion 20/40/40%.
“An extrusion line operates with a colder ice cream mix, and therefore it can be shaped and extruded and cut in slices, before being coated, decorated, hardened and packed. Having the complete process at a lower temperature means a smoother texture of ice cream can be achieved.”
All this means that extrusion offers an ideal approach to meet the needs of the trend towards greater indulgence, confirms Harhoff: “The majority of premium ice cream products are produced on extrusion lines, because producers have more options to make indulgently finished products.”
The new line uses an independently controlled horizontal cutter to slice the ice cream as it emerges from the extruder. This patented technology means the cutting speed stays consistently fast, irrespective of the line speed, ensuring a precise, clean cut even at low rates of production.
By programming the cutter to operate at different speeds and movements, ice cream producers can also switch between multiple products on the same line, such as sticks, sandwiches, cones, wafer cups, candy bars or cakes. With a capacity range of 5,000 to 18,000 products an hour, Tetra Pak says the line offers much-needed volume flexibility to medium-sized producers, allowing them to increase or decrease output without compromising efficiency.