This article first appeared in the 2019 Physics World Focus on Instruments & Vacuum under the headline "From field to table"
Vacuum cooling is a fast and energy-efficient method for chilling food and extending its shelf life. Now its success in the food industry is driving wider interest, writes Pierre Lantheaume
Keeping food fresh during its journey to your plate isn’t easy. The clock starts ticking as soon as a lettuce is plucked from the field or a bread roll is removed from the oven, and without some intervention to slow or stop that clock, many food products will become unpleasant or unsafe to eat within days. For consumers who live right next to a farm or bakery, that may be acceptable, but for those of us who live tens or even hundreds of kilometres away from where our food is grown or processed, it is simply not practical.
In the fight to avoid waste and keep food products fresh, cooling is an important weapon. Reducing the temperature of food increases its shelf life, maintains freshness and slows the growth of bacteria that might otherwise cause it to spoil. For this reason, foods are often cooled as quickly as possible after they are produced or harvested, and an entire industry has grown up around meeting this need.
Traditional cooling methods use either air or water to remove heat from food via a combination of conduction and convection. These methods have been around for decades, but they have several drawbacks. It can take hours to cool a pallet of vegetables using forced air circulation or jets of water. During that time, bacteria continue to multiply, and the cooling fluid (air or water) may itself become contaminated with harmful microorganisms unless strict precautions are taken. Conventional cooling also produces an uneven temperature distribution, with food products at the edges of containers being cooled more quickly than those at the centre. And of course, the process is very energy intensive.
An alternative is to cool food by placing it in a vacuum chamber. Vacuum cooling is based on the principle of evaporation: as water evaporates from the product, energy is removed, and the temperature drops. The evaporation process begins as soon as the pressure falls low enough for water to boil, and the desired final temperature can be set by controlling the pressure in the vacuum chamber.
Compared to conventional cooling, vacuum cooling is fast. With the right equipment, a pallet of vegetables that would take several hours to cool via forced air circulation can be chilled within a few minutes. Vacuum cooling is efficient, too, requiring a quarter of the energy of forced-air cooling. Another advantage is that because evaporation takes place on all surfaces at the same time, the spatial distribution of the cooling is homogenous (especially for products with a high surface-area-to-volume ratio). This gives vacuum-cooled foods a significantly longer shelf life. A final benefit of vacuum cooling is safety. Because the flow of air is entirely in one direction, from inside to outside, there is no opportunity for potentially contaminated air to be introduced and to circulate around the food. The speed of vacuum cooling also enhances safety, as the rapid temperature reduction gives bacteria less chance to multiply.
Not all foods are suitable for vacuum cooling. Because the process is based on evaporation, the product must contain sufficient water for cooling to be effective. In addition, leafy vegetables such as lettuce, which have a large surface area, can be cooled more efficiently than solid ones such as tomatoes. But neither of these requirements is as restrictive as you might expect. Many foods that feel relatively dry in the mouth, such as bread, nevertheless contain enough water to be vacuum cooled. And because vacuum cooling typically only removes a few per cent of the product’s water content, the loss of mass is less than you would get with forced-air cooling – minimizing the loss of revenue on foods sold by weight.
The salad challenge
For vacuum experts, the task of designing a system to meet the needs of a customer in the food industry (as opposed to, say, scientific research) poses some interesting challenges. But the basic principles are the same. In particular, the calculation for how big the vacuum cooling system needs to be is based on the law of conservation of energy: the amount of heat released in cooling the food must equal the amount of heat taken up by evaporating the water, Qreleased = Qtaken.
The left side of this equation is calculated by multiplying the mass of the food by its specific heat and the change in temperature before and after cooling, Qreleased = mfood cp ΔT. For example, if we wanted to cool 1000 kg of salad – a material with a specific heat of 3.9 kJ/(kg K), slightly less than that of water – from 25 °C to 5 °C, we would need to dissipate 78,000 kJ of heat. So how much water would we need to evaporate? Well, Qtaken = mwater × Δhvap, where Δhvap, the evaporation heat of water, is 2466 kJ/kg at 15 °C, so the answer is 31.6 kg – a few per cent of the salad’s starting mass.
The next question concerns the flow that the vacuum system needs to handle. If we want the total cooling time for the salad to be 30 minutes, allowing 5 minutes for pumping out between cooling cycles, then we need a system that can pump out msteam = 76 kg of steam per hour. To translate that into an effective volume flow veff, we use the equation veff = msteam × Vm/M × Teff/TN × PN/Peff, where Vm is the molar volume of water (22.4 N m³/kmol); M is its molar mass (18 kg/kmol); Teff and Peff are the effective temperature and pressure; and TN = 273 K and PN = 1013 mbar are the norm temperature and pressure. At Teff = 25 °C (298 K), the vapour pressure of water is 31.7 mbar, so our vacuum system would initially need to pump 3299 m3/hr. At the final temperature of 5 °C, the vapour pressure of water drops to 8.72 mbar, meaning that the system would need to be pumping 11,188 m3/hr.
The right fluff Tests carried out by the vacuum cooling and baking firm Cetravac show that vacuum-cooled bread maintains its volume and structure better than bread cooled using traditional methods.
In theory, a vacuum pump should be able to remove these flows. In practice, though, you would need a very big (and expensive) system to do it. The more economical choice is often to use a condenser to trap the steam flow and convert it into liquid, which dramatically reduces the gas flow to the vacuum pump. As a rule of thumb, you need about a square metre of condensing surface for each 10 kg/h of vapour flow, so to cool our 1000 kg of salad we would need a condenser of about 8–10 m2.
The remaining considerations are, first, that the vacuum system must be able to evacuate the chamber from atmospheric pressure to final pressure in the desired time (25 minutes in the salad example). This can be determined by a simple pumping speed calculation, s = V/t ln (p0/p1), where V is the volume of the chamber, and p0 and p1 are the starting and desired pressures. Second, the vacuum system needs to be able to handle the gas flow that remains after the condenser. Assuming a typical leakage on the vacuum chamber – around 5 kg of air per hour for a 10 m3 chamber with standard seals – we calculated the flow generated by the non-condensed steam and leaks left behind the condenser for both the starting and ending temperatures. The higher of the two above calculations will determine the size of the vacuum system. In the salad example, the results were 570 m3/hr for the pumping speed and 1500 m3/hr for the flow due to leaks and uncondensed vapour – far less than would have been required without a condenser.
Vacuum cooling systems for leafy vegetables, salads and flowers all have a similar design. They are either installed in a trailer placed next to the field where the salad is harvested, or they are integrated into the facilities where salads are cleaned and packed before being shipped. The largest stationary chambers can be loaded with up to 20 pallets simultaneously, and are capable of processing more than 300 tonnes of vegetables every day.
Vacuum cooling is a fast and energy-efficient cooling method with a wide range of applications in food processing and other industrial applications
Before being loaded into the vacuum chamber, vegetables such as lettuce are often sprayed with water to compensate for the loss of weight due to evaporation. As soon as the door closes, the vacuum system starts pumping and the pressure drops from 1000 mbar to 15–20 mbar within 5 minutes. At that pressure, and at a temperature of around 20 °C, water begins to evaporate and the cooling process starts. After 15–20 minutes, the pressure drops further, to 5–6 mbar, and the product reaches a temperature of about 2 °C. During the process, a condenser containing a mixture of glycol and water at a temperature of –6 to –10 °C traps most of the water vapour, protecting the pumps. Then the pumping and cooling systems stop and the chamber is vented back to atmospheric pressure within a few minutes. Afterwards, the salads are stored in a cool chamber where they can be kept for 2–3 weeks without spoilage.
As long as the condenser is doing its job well, the demands that this cycle places on the vacuum pumps are straightforward, because the starting temperature is fairly low (freshly harvested vegetables are seldom warmer than 30 °C) and the amount of water to be evaporated is limited. However, the presence of dirt particles or small plant parts can be a challenge, and there are some trade-offs in designing suitably low-maintenance and cost-effective systems. For example, oil-sealed rotary vane pumps are reliable and cost effective, with good water vapour compatibility and a compact, fully air-cooled design that makes them easy to use in mobile systems. However, they do need inlet filters to protect them against particles, and maintaining them requires a regular exchange of oil, oil filters and exhaust demisters.
Screw vacuum pumps have a higher tolerance for particles, and their small size, low noise level and low energy consumption make them ideally suited for industrial food-processing facilities. On the other hand, most versions require water or air cooling, and their up-front cost is higher than a rotary vane pump. Both types of pump may be used in combination with a roots vacuum pump, which boosts the pumping speed of the system at pressures below 50 mbar.
The success of vacuum cooling in keeping vegetables fresh means that similar techniques are now being applied to other food products. Bread and pastries are one example. In this application, the starting temperature is much higher – up to 90 °C when bread rolls are offloaded from the oven – and the amount of water present in the cycle is therefore dramatically larger than it is for vegetables. Rotary-vane pumps do not have a high enough water-vapour tolerance to do the job, so screw pumps are a better solution. They can swallow large amounts of water without breaking and are also very tolerant of small particles (flour, poppy or sesame seeds, and so on). In addition to saving energy and cooling the bread more quickly, vacuum cooling also bestows advantages for consumers: vacuum-cooled bread has a crispy crust and fluffy crumb, providing more enjoyment when eating.
We are also starting to see some non-food applications of vacuum cooling. For example, the grass on the pitch in top-flight professional football stadiums does not really grow there. Instead, it is produced on special farms, harvested in rolls and transported to the stadium in time for games. Thanks to vacuum cooling, these rolls of grass easily survive the transport process, hanging on until their next watering. The requirements of cooling grass are similar to those for cooling vegetables, except that the amount of water that must be extracted to reach the desired temperature is significantly higher, due to the mass of the product (including soil and mud). It is therefore a more demanding job for the vacuum pump. The combination of rotary vane pumps and roots blowers still works well, but the pumps require more maintenance than is common for cooling vegetables.
In summary, vacuum cooling is a fast and energy-efficient cooling method with a wide range of applications in food processing (and, increasingly, outside it). It enhances food safety and extends the shelf life of food products. The challenges it poses to vacuum systems are both novel and highly dependent on the product being cooled: while oil-sealed rotary vane pumps have proven effective in cooling vegetables, other applications require innovative thinking. Dry pumping technology is creating opportunities for new and more sophisticated processes, including the cooling of sushi rice or food prepared for catering.