Hygiene plus – the versatility of the steel belt

By Sandvik

While the hygienic qualities of stainless steel are widely recognised across the food industry, this versatile material boasts a number of other properties that make it ideal for virtually any belt-based process, from freezing fish to baking bread.

For decades now, the high standards of hygiene achievable through the use of stainless steel tools have been widely accepted. Smooth and easy to clean, it’s the material of choice for everything from knives and tables to equipment cladding and machinery parts. You only have to look at the sterile environment in a hospital to see just how widespread the use of stainless is in situations where absolute cleanliness is essential.

For certain applications however, the inherent safety of stainless steel is but one of its many qualities. It’s also an extremely hard-wearing durable material; it can be used with corrosive materials and it is an excellent conductor of heat. And it is this built-in versatility that makes stainless steel the perfect material for belt-based processing.

Of course, the use of steel as a belt material in food processing isn’t a new concept – a Sandvik steel belt was first used in a continuously operating bake oven as long ago as 1925 – but increasing recognition of the dangers of pathogens such as E. coli, Listeria, Salmonella and Staphylococcus is persuading more and more food processors to take a closer look at how a stainless steel belt might be used in their operations.

Perhaps the most remarkable quality of a stainless steel belt is its ability to be used in processes involving temperatures as widely varied as -80º C and +750º C. In many instances, not only are steel belts capable of operating in these extreme temperatures, but their outstanding thermal properties are actually of benefit.

Freezing
Take freezing for instance. The ability of stainless steel to be used at the core of a continuously running freezing process at temperatures as low as -80º C, without becoming brittle, is a major advantage over other materials and means that processing times can be reduced whilst also improving product quality – particularly important when freezing high value fragile food such as fish fillets.

Other products suitable for fast, air blast contact freezing include meat, shrimps, oysters, plain and breaded hamburger patties, chicken pieces and minced meat for baby food. Furthermore, the smooth, joint-free surface of the belt enables it to be continuously cleaned after discharge of the frozen product, ensuring a clean hygienic surface ready for new product feeding.

Freeze-dried coffee
Another growing application for steel belt-based freezing systems is freeze-dried instant coffee. After roasting, the coffee is milled and turned into a slurry, similar to soft ice. This is then deposited onto a continuously running steel belt system and frozen at a temperature of between minus 40 °C and minus 50 °C.

In the next stage of the process, the frozen material is broken up, milled and transported to the freeze drying vessels where the water component of the coffee mass is extracted, leaving behind pure coffee extract. Around 30 percent of the instant coffee we drink today is freeze dried, and every well-known producer in the world makes use of Sandvik steel belt freezers.

Cooling and solidification
In the same way that the heat transfer properties of steel are used to improve the freezing process, they can also be harnessed for cooling and solidification applications.

One such system is Sandvik’s Rotoform, with which products in liquid or paste form – including chocolate, gum base, fat, emulsifiers and food additives – can be converted into solid pastilles for easier handling, storage, metering and subsequent reprocessing.

The process is simple but efficient. A perforated rotating cylinder is used to deposit drops of molten product across the width of a continuously running steel belt, and the heat of the product is transferred by the stainless steel belt to cooling water sprayed onto the underside of the belt. The excellent thermal conductivity of the belt allows good control of the solidification process, resulting in fast, controlled solidification.

The first Rotoform system was introduced in the early 1980s and since then more than 1400 have been installed around the world. Sandvik extended its Rotoform family with the development of a number of market-specific models including the Rotoform FD, specifically designed for food processing.

Baking
The examples we’ve just looked at make use of the steel belt’s ability to conduct heat away from a product, but it is also equally suitable for the application of heat, such as in a bake oven.

Traditionally, wire mesh belts have been the preferred medium for conveying baked products but growing numbers of users are now turning to solid or perforated steel belts for their superior ease-of-cleaning, heat transfer capabilities and resistance to stretching. Other advantages of a steel belt over wire mesh include faster belt speeds, lower power consumption (due to their lighter weight) and, importantly, the clean product release delivered by its hard, smooth surface.

The recent case of one customer in China is typical. After years of producing thin crackers on a mesh belt, this customer asked Sandvik to look at how production volumes could be increased and, at the same time, how his manufacturing capabilities could be extended to include biscuits and cakes.

The solution took the form of a Sandvik 1300C solid steel belt, 1000mm wide, 1.2mm thick and 170m long, complete with new drums, scraper, spring loaded guiding rolls and an automatic tracking device. As well as the usual cast iron skid bars used to support the belt, Sandvik also recommended the use of graphite skid bars which apply a homogeneous and gradual deposit of graphite, continuously and automatically.

However, once installed the commissioning process revealed a belt tracking problem during the oven heating-up process, although belt tracking during actual production was stable. Sandvik engineers investigated temperature differences during the heating-up stage and discovered too great a variation in temperature across the belt. Once the oven heating system was checked and rectified to ensure that the temperature across the belt was at a constant level throughout the heating-up process, the problem was solved.

The customer achieved his objective, increased his product range and increased production output, with products of a high and consistent quality. Importantly, all this was achieved with a short payback time on the investment

Dehydration systems
Another food process for which steel belts are ideally suited is the drying of fruit and vegetables. Based around perforated belts, these systems enable controlled dehydration by the flow of air through the product. Combined with the ability to adjust the speed of belt travel, this enables Sandvik’s drying systems to deliver reduced process times – as much as 50% lower than competitive systems.

The use of multi-stage drying units allows time/temperature curves to be optimised for products as diverse as potatoes, carrots, apples, orange peel, celery, garlic and tomatoes. Special systems for sensitive products have also been developed, enabling the drying of cut strawberries, blueberries, pineapple, mango and more.

As with all food processes, hygiene is critical in dehydration and this system delivers bacteria, yeast and mould counts far lower than other continuous drying methods. This is achieved through a combination of stainless steel material, belt washing device, simple offloading and easier cleaning than wire mesh or perforated apron belts.

Cutting
Finally, a low-tech but highly significant application: cutting and boning. Hygiene, ease of cleaning and resistance to the corrosive effects of meat and blood are all obvious benefits of stainless steel in this particular ‘process’, but perhaps the most important quality here is its hardness and durability.

It’s no exaggeration to say that no other material comes close in terms of its versatility. The same basic material that’s used at freezing temperatures for the production of instant coffee can be found in mineral recycling operations where temperatures will reach up to 750º C. It can be solid or perforated, it can be polished, grooved, embossed, chrome-plated and Teflon®-coated. It can be used for applications as simple as conveying and storage or at the heart of advanced, multi-stage food processing systems.

Of course, there are different steel grades for different applications, but the vast majority of the benefits of the steel belt are generic: hygiene, strength, thermal conductivity and hardness.

Sandvik invented the steel belt in 1901 and we have been lead the development of this technology ever since, installing the world’s first steel belt-based fish cutting plant, meat cutting tables, ice-cream contact freezer and chocolate conveyor. The development of new steel grades and manufacturing techniques mean that steel belts now be employed in processes that would have been unimaginable a few years ago. If your business involves moving, handling, storing, heating, cooling, freezing or drying foodstuffs, it’s more than likely you could be doing it better with a steel belt.