Application of vacuum in the food industry

Vacuum allows processes to be performed that cannot otherwise be done under atmospheric conditions. Moreover, it offers a huge advantage in the processing of heat and oxygen sensitive materials. There are numerous applications in the food industry that rely on vacuum. The vacuum required in the food industry extends in the range of 1-600 mbar vacuum absolute (vacuum pressure measured relative to absolute perfect zero vacuum), and is applied in the transport, processing, filling and packaging of foodstuffs, in cleaning operations and in the creation of appropriate hygienic conditions.

‘Sous-vide’ is the French word for ‘cooking under vacuum’. This cooking method aims to maintain the integrity of ingredients by heating them for an extended period (usually 8 – 10 hours, sometimes well over 24 hours) at relatively low temperatures (usually between 60-70°C) and pressures of 50 – 250 mbar absolute (40 – 60 mbar lower than the vapour pressure of water at a given temperature, Figure 1, page 68. But, there are cooks that prepare food ‘sous vide’ at temperatures as low as 55°C. However, for food safety reasons, that practice is not really recommended. After vacuum cooking, the food should be held at 55°C or above until served for immediate consumption, or should be rapidly cooled to below 3.3°C. A water ring pump is used to produce the absolute vacuum pressures required.

Vacuum evisceration

In the vacuum evisceration of poultry, a water ring pump generates the required vacuum of about 100-150 mbar absolute, with a receiver tank for the bowls acting as a vacuum tank. The air drawn into the water ring pump is microbiologically heavily contaminated, which implicates that the seal water in the pump must be regularly drained, creating heavy loads to the water purification plant.

Vacuum pumping of fish

For vacuum pumping of fish and water, a rough vacuum, high capacity water ring pump delivering 400 – 500 mbar at high pumping speed is needed to overcome a difference in height of two metres. For reasons of pressure drop, a vacuum level of 100 mbar lower than that required to overcome the difference in height is required.

UHT treatment – vacuum flash cooling of milk

Pre-heated milk of 75°C into an agitated heater feed tank is exposed to culinary steam of 4 – 5 bar (free of chemical contaminants, and saturated to avoid excessive dilution of the product) to achieve a temperature of 140 – 145°C in as few as three to five seconds. At the end of the UHT treatment, the product flows to a high-temperature holding tube, where it is held for a few seconds. The product is then transferred by differential pressure from the high-temperature holding tank to the vacuum chamber. In this aseptic environment, added water is removed by flash cooling (Figure 2, page 70), which involves a sudden reduction in pressure and product temperature down to between 75°C and 80°C. Together with the added water, flash cooling also removes heat-induced volatile components (e.g., hydrogen sulphide and low molecular weight sulphur containing compounds) that are responsible for a cooked flavour in milk. Water vapour and volatile compounds are finally condensed. To avoid entrainment of milk droplets towards the condenser and to let them fall back, the flash chamber should be provided with baffles or wire meshes near the vapour outlet. Milk droplets impinge on these, coalesce into larger droplets and drain back into the milk under gravity. An alternative is a cyclone separator, wherein a mixture of vapour and liquid is directed tangentially, at high velocity, either by natural or forced circulation. In that case, more dense milk droplets impinge on the inner wall of this separator, lose their kinetic energy and drain down into the milk bulk. Successful operation depends upon main – taining a steady vacuum (usually 50 – 100 mbar vacuum absolute), as the flash cooling vessel operates at the boiling point of the UHT treated milk. Once the desired level of product is reached in the vacuum chamber and the product down flow pipe to the flash chamber discharge pump, the flash cooling is completed and the product can be sent to the aseptic filler. Apart from milk, a combined steam-driven cooking (118-121°C) and flash cooling (back to 4°C) process can also be applied for other foods, like sauces, stews, pastas, creams and soups etc. Commonly used vacuum sources in flash cooling operations are steam jet ejectors or water ring vacuum pumps.

Concentration of milk by vacuum evaporation

Concentration of liquid food (milk, sugar juice, etc.) by evaporation under vacuum in a batch vacuum pan or a falling film evaporator is common practice. Evaporation under vacuum reduces the boiling point and hence heat damage to the food product. A batch vacuum pan evaporator is the simplest type of evaporator, consisting of a steam jacketed vessel provided with an agitator and an exhaust vapour line (in the head of the vacuum pan) that channels the water vapours out to a condenser connected to a vacuum source. The heat transfer characteristics of a batch vacuum pan are rather poor, requiring long product residence times. In a falling film evaporator, food liquid moves by gravity in a thin liquid film downward in 8 – 12 metre long tubes with a diameter of 30-50 millimetres. Due to the short residence time of only 20 – 30 seconds, a falling film evaporator allows the concentration of heat sensitive products in continuous operation with little to no cooked flavour. The food product concentrate is separated from the water vapours in a vapour separator located at the bottom and beyond the falling film evaporator. An exhaust vapour line in the head of the vapour separator carries the water vapours to a condenser, or to the next effect of a multieffect evaporator (series of evaporator units running in sequence) where they may act as a heat source. Evaporation of milk is usually performed at 40 – 45°C, requiring an absolute vacuum of less than 75 mbar. Water ring pumps or steam jet ejectors with a high capacity usually operate at 50 mbar vacuum absolute, and are suitable for that purpose.

De-aeration of process water and liquid foodstuffs

Carbonated beverages are commonly prepared in a beverage mixer/carbonator which com – bines the de-aeration of water with blending and carbonation of the beverage, being finely sprayed by special nozzles. De-aeration of water for beverage preparation under 50 mbar vacuum absolute causes a large proportion of the air and dissolved oxygen to be released and sucked out. The lower the oxygen content in the beverage, the better its quality can be preserved and the more effective the carbonation process afterwards. The required vacuum for the de-aeration of water can be generated by means of a water ring pump or a dry pump. For the de-aeration of milk, only a water ring pump is suitable; while for the de-aeration of vegetable oils, steam jet ejectors are the most appropriate vacuum source

Deodorisation of vegetable oil

In the processing of edible oils and fats, a deodorisation step is required to remove undesirable compounds such as free fatty acids and volatiles that affect flavour, odour, stability and colour of the final product. Before that deodorisation process, the oil is de-aerated under vacuum to protect the quality of the product by preventing oxidation. Moreover, the removal of non-condensable gases from solution is recommended, as if not removed, they often bubble out of solution on surfaces, lowering heat-transfer rates. After de-aeration, the oil is heated and transported to the deodorisation column, and is subjected to a vacuum steam distillation process in which steam is passed through oil. To prevent the stripped off free fatty acids and volatiles from being carried over in the vacuum system, they must be recovered in a condenser. The vacuum system usually consists of a first stage, a second stage steam ejector and finally a water ring pump, operating at 3 mbar inlet and 15 mbar outlet, 15 mbar inlet and 50 mbar outlet, and 50 mbar inlet and atmospheric pressure outlet, respectively.

Fractional distillation

Fractional distillation is applied to separate components whose boiling points lie close to one another. For example, fractional vacuum distillation is applied to separate fatty acids or their esters. A multistage steam jet ejector system or a combination of steam jet ejectors and a water ring pump is used to generate the required vacuum level of 10 mbar absolute. The steam condensate or the water of the water ring pump that is drained to the water purification plant usually contains high amounts of fatty acids. Dry vacuum pumps are less appropriate because oil components may start to carbonise.

Vacuum filtration

The vacuum levels required to filtrate product depends on the thickness and porosity of the filter cake. The pressure drop is higher over a thick filter cake, requiring absolute vacuum levels of 300 mbar. The pressure drop over a porous filter cake is rather small, making absolute vacuum levels of 600 mbar already appropriate. The water ring pump is commonly used for vacuum filtration.

Vacuum drying

For vacuum drying, initial absolute vacuum levels of 20 – 50 mbar are required, further ending with a final absolute vacuum of about 1 – 4 mbar. As the filtration process proceeds, the moisture must be sucked away out of the pores of the filter cake deeper and deeper, and the pressure drop over the filter cake increases. The condenser (plate or shelland- tube heat exchanger) positioned up-stream of the water ring or dry vacuum pump must have sufficient capacity with large dimensions and a deep cooling facility to remove the maximum of (water) vapours. A plate or shelland- tube heat exchanger is commonly applied for that purpose.

Freeze drying of herbs, spices, coffee, fruit and vegetables

To prolong the shelf life of food and to maintain the basic nutrients, freeze-drying (where water is frozen and then sublimed) with the aid of vacuum is applied. The start vacuum should be about 20 – 50 mbar absolute to avoid sudden formation of a thick ice layer that makes further sublimation nearly impossible. The absolute vacuum can then gradually be lowered by increasing the capacity of the commonly used dry vacuum pump (screw pump).

Bottling of beer and soft drinks

Vacuum is used in different steps during the bottling of beer, soft drinks, mineral and sparkling water by means of a filling carousel. On a rotary rinser, bottles are turned 180 degrees and a nozzle enters the neck to either spray water or air into the bottle. When bottles are cleaned with dry, compressed sterile air, vacuum is applied to remove dirt particles more effectively. As a second step on the rinser, the bottle may be sparged with an inert gas such as carbon dioxide or nitrogen (case of mineral water) to reduce the amount of oxygen in the bottle. On a rotary filler, as a first step, vacuum is filled on the bottle which extracts the ambient air or air / carbon dioxide (air / nitrogen) mix through a separate channel of the filling valve to the outside environment; after which the bottle is filled with inert gas. The liquid (beer, soft drinks, sparkling, mineral water, etc.) to fill pushes the carbon dioxide or nitrogen from the bottle back through a separate channel into the filler bowl, where it serves as a blanket to avoid air contact in the bowl as well. After the bottle is released from the filling valve, it is conveyed to either the corker or capper. For cap applications, a headspace vacuum can be pulled before the cap is placed on the neck of the bottle. This reduces the amount of air from the bottle headspace, and also reduces head – space pressure when the cap is placed. Pulling a vacuum at the capper may potentially result in a contamination issue, if at times beer or soft drink is sucked into the vacuum system. Usually, a vacuum tank is installed between the filler application and the vacuum source, but even then, the vacuum suction line and the vacuum source can become contaminated with beer or soft drink residues. To generate the required vacuum absolute of 50 – 60 mbar, dry vacuum pumps are not recommended because beer/ soft drink residues entering the vacuum pump have the tendency to carbonise within these dry pumps that usually operate at temperatures of 60 – 80°C. But for bottling of water in glass bottles, due to the lack of carbonisable food components, they are suitable.

Only water ring pumps are applicable in the bottling of beer and soft drinks but even then, up-concentration of beer/soft drink residues in the seal water occurs, creating an optimal culture medium for bacteria if the temperature is in the range of 18 to 28°C. Therefore, CIP-able vacuum systems (Figure 3), where separator (used to separate liquids and foam from the drawn gas on the suction side), vacuum pump, heat exchanger (used to remove the compression heat of the vacuum pump), pipes and fittings can be ‘cleaned-in-place’ are recommended. Because plastic bottles contract under the influence of vacuum, vacuum filling can’t be applied in the filling of soft drinks in plastic bottles (e.g., PET). Milk bottling is a no contact bottom-up filling process, where no pressure or vacuum is applied.

Figure 3: Hygienic vacuum system (SIHI Sanivac)

Vacuum packaging of food in plastic bag film

For vacuum packaging operations, the required absolute vacuum may not be too high (maximum 100 mbar). A too high vacuum increases the risk of food being drawn into the vacuum system. A vacuum tank, between the vacuum pump and the vacuum packaging machine, helps to minimise fluctuations in vacuum and to maintain a constant vacuum level as speed is essential in vacuum packaging operations. The commonly used vacuum source for food packaging operations is an oil lubricated rotary vane pump, an oil ring pump or a venturi-system with compressed air or nitrogen as the motive fluid, that are integral to the packaging machine.

Dry and wet vacuum cleaning

For both dry and wet vacuum cleaning, the required vacuum capacity is situated between 600-900 mbar absolute, depending on the frequency of cleaning, the maximum expected number of simultaneous users, the pressure drop over separators and filters, etc. A dry vacuum cleaning system makes use of tubular bags or a centrifugal separator to remove solid particles in the air stream, produced by a dry vacuum pump or an oil-lubricated rotary vane pump (with a filter in front). In a wet vacuum cleaning system, a dry/wet separator and a water ring pump as vacuum producer (exhauster) is required.

Opening of bags and lifting of loads

Lifting and displacement of loads occurs via suction cups that adhere to non-porous surfaces thanks to the vacuum created between the cups and the surface of the loads to displace. The design of the suction cups largely determines the weight that can be displaced, and the required vacuum for that job depends on the load weight, the properties of the suction cups and the quality of the packaging material (e.g., surface roughness and strength). The surface roughness of the packaging material determines the air leak rate; and the strength of the packaging material must be sufficient to avoid tearing during the lifting and transfer process. Vacuum levels of 200 – 300 mbar absolute produced by a dry pump (in dusty environments) or an oil-lubricated rotary vane pump (with a filter installed just in front of the pump) are suitable. The required vacuum for the opening of bags is determined by the same parameters as those applicable for the lifting of loads.

Vacuum pumping of liquid

For vacuum pumping of liquid, a rough vacuum, high capacity water ring pump delivering 400 – 500 mbar at high pumping speed is most frequently needed to overcome a difference in height of two metres. For reasons of pressure drop, a vacuum level of 100 mbar lower than that required to overcome the difference in height is required. Rotary vane pumps are also applicable, but they can’t take a slug of water or liquid. They require locks and check-pots to avoid contamination of the oil within the oil-lubricated rotary vane pump. Notice that for the transfer of liquids, pumps are the most appropriate for that duty, while blowing and vacuum are less recommended.

Vacuum transport of solids

In the transport of solids, usually at 200 – 500 mbar vacuum absolute, a storage vessel to store these solids is placed between the original container containing that solid material and the vacuum source. For reasons of clogging risk, water ring pumps should not be applied for this application. Commonly applied vacuum resources for vacuum transport of solids are oil-lubricated rotary vane vacuum pumps for clean powder transport (where there is little risk for particles to be entrained within the rotary vane pump) or dry pumps (such as the hookand- claw and screw pumps) that withstand a little bit more particle dust. The cheapest investment is the oil-lubricated rotary vane pump, followed by the hook-and-claw pump (twice the cost of a rotary vane pump) and finally the screw pump (three times the cost of a rotary vane pump).

With a rotary vane pump, a filter in front of the pump should always be applied, but the oil of the rotary vane pump still needs filtration to remove powder particles. In contrast to dry hook-and-claw pumps where the small tolerances (100th of a millimetre) between the rotor claws and the pump casing limit their use, screw pumps having higher tolerances (one tenth of a millimetre) between the screws and these screws and the pump housing can better tolerate small dust particles moving with the air stream through the pump. Where conveying of very fine solid material (e.g., flour) is required, pneumatic transport by means of a vacuum source should be replaced by blowing (e.g., a roots blower). Blowers operating at a maximum of 4-6 bar overpressure should always be selected for long distance transport (500 metres up to 2000 metres maximum), to overcome differences in height of 30 – 100 metres, and to transfer solid particles with an equivalent diameter of 20 – 60 millimetres. Vacuum transport of solids is only possible where particles have an equivalent diameter smaller than approximately 20 millimetres, to transfer these solids over long horizontal pipe trajects of ca. 200 metres up to 500 metres, and to overcome maximum differences in height of 30 metres. The required vacuum level for that purpose amounts to about 500 mbar. Roots pumps are not appropriate for vacuum transport of solids, because they warm up very quickly.

Dry/wet vacuum cleaning

Dry/wet vacuum cleaning can be considered as a specific type of vacuum solid/liquid transport. A dry/wet vacuum cleaning system consists of a vacuum producer (exhauster), a dry/wet separator and tubing to convey the air and material to that separator. In dry vacuum cleaning, a rotary vane pump provided with a filter in front of the pump is acceptable. In wet vacuum cleaning, a water ring pump is recommended. A system com – bining both dry and wet vacuum cleaning also requires the application of a water ring pump. In dry vacuum cleaning systems, tubular bags or a centrifugal separator are used to remove solid particles in the air stream generated by the vacuum producers. In wet vacuum cleaning systems, a separator that can separate water from the air stream and that can discharge the waste to a drain is used. The required vacuum capacity (between 600 – 900 mbar absolute) depends on the frequency of cleaning, the maximum expected number of simultaneous users, the pressure drop over separators, filters, etc., and the loss of vacuum pressure due to friction of the air in the piping system.

About the authors

Frank Moerman graduated with an MSc in bioengineering from the University of Ghent in Belgium. After a short academic career, he worked as a chemical process engineer in the pharmaceutical and fine chemical division of Ajinomoto, Co. in Belgium. He was then a researcher at the Catholic University of Leuven, a scientific journalist and an instructor giving training in the field of ‘process equipment cleaning’. In 2002, he became member of the European Hygienic Engineering and Design Group, acting as the chairman of EHEDG Belgium. He participated in the European 5th framework project ‘HYFOMA’ (2002 – 2004) giving support in the development of EHEDG guidelines and promoting the subject of ‘hygienic design’ in Belgium. He is the author of several articles and book chapters dealing with hygienic design; and is an active member in several EHEDG subgroups. For more about the ‘European Hygienic Engineering & Design Group’, please visit: www.ehedg.org.

Nico Desanghere graduated in 2000 as a marketer at the High School of Bruges in Belgium. Starting as an external sales engineer for STERLING SIHI in 2001, he became Deputy Benelux Sales Manager in the Industry division in 2006. After reorganising the SIHI sales structure in 2011, he became the Deputy Sales Manager of the SIHI BELUX organisation.