The vacuum keeps food fresh and cold from field to table

The vacuum keeps food fresh and cold from field to table

Keeping food fresh during its journey to your plate is not easy. The clock starts ticking as soon as a lettuce is plucked from the field or a muffin is taken out of the oven, and without any intervention to slow or stop that clock, many food products will become unpleasant or unsafe to eat within days. For consumers who live next to a farm or bakery, that may be acceptable, but for those of us who live tens or even hundreds of miles 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. Lowering the temperature of food increases its shelf life, maintains freshness, and slows the growth of bacteria that might otherwise spoil it. For this reason, food is often cooled as quickly as possible after it is produced or harvested, and an entire industry has grown around meeting this need.

Traditional cooling methods use air or water to remove heat from food through 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 coolant (air or water) can become contaminated with harmful microorganisms unless strict precautions are taken. Conventional cooling also produces uneven temperature distribution, and food products at the edges of the containers cool more quickly than those in the center. And, of course, the process is very energy intensive.

An alternative is to cool the 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 drops enough for the water to boil, and the desired final temperature can be set by controlling the pressure in the vacuum chamber.

Compared with conventional cooling, vacuum cooling is fast. With the right equipment, a vegetable pallet that would take several hours to cool through forced air circulation can be cooled in just a few minutes. Vacuum cooling is also efficient and requires 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 cooling is homogeneous (especially for products with a high surface-to-volume ratio). This gives vacuum-cooled foods a significantly longer shelf life. A final benefit of vacuum cooling is safety. Because the air flow is completely in one direction, from the inside out, there is no opportunity for potentially contaminated air to enter and circulate around the food. The speed of vacuum cooling also improves safety, as the rapid reduction in temperature 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 enough water for cooling to be effective. Also, leafy vegetables like lettuce, which have a large surface area, can be cooled more efficiently than solid ones like tomatoes. But none of these requirements is as restrictive as you might expect. Many foods that feel relatively dry in the mouth, such as bread, still contain enough water to cool them in a vacuum. And because vacuum cooling generally only removes a small percentage of the water content from the product, the mass loss is less than what 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 that meets 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 of how large the vacuum cooling system should be is based on the law of conservation of energy: the amount of heat released when cooling food must be equal to the amount of heat absorbed 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, Q Released = Maliment 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 5ºC, we would need to dissipate 78,000 kJ of heat. So how much water would we need to evaporate? Well, Qtaken = Maliment × Δhvap, where Δhvap, the heat of evaporation of water, is 2466 kJ / kg at 15ºC, so the answer is 31.6 kg, a small percentage of the initial mass of the salad.

The next question asks about the flow that the vacuum system needs to handle. If we want the total cooling time for the salad to be 30 minutes, leaving 5 minutes to pump between cooling cycles, then we need a system that can pump mvapor = 76 kg of steam per hour. To translate that into an effective volumetric flow veff, we use the equation veff = mvapor × 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 T N = 273 K and P N = 1013 mbar are normal temperature and pressure. At TN = 25ºC (298 K), the vapor pressure of the water is 31.7 mbar, so our vacuum system would initially need to pump 3299 m3 / hr. At the final temperature of 5ºC, the vapor pressure of the water drops to 8.72 mbar, which means that the system would need to pump 11,188 m3 / hr.

In theory, a vacuum pump should be able to remove these flows. In practice, however, you would need a very large (and expensive) system to do this. The most economical option is often to use a condenser to trap the vapor flow and turn it into a liquid, dramatically reducing the gas flow to the vacuum pump. As a general rule, you need approximately one square meter of condensation surface for every 10 kg / h of steam flow, so to cool our 1000 kg of salad we would need a condenser of approximately 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 calculation of the pumping speed, s = V / t ln (p0 / p1), where V is the volume of the chamber, and p0 and p1 are the initial and desired pressures. Second, the vacuum system must be able to handle the gas flow that remains after the condenser. Assuming a typical leak in the vacuum chamber, around 5 kg of air per hour for a 10 m3 chamber with standard seals, we calculate the flow generated by the non-condensed steam and the leaks left behind the condenser for the initial temperatures and final. The larger of the two calculations above will determine the size of the vacuum system. In the salad example, the results were 570 m3 / hr for pumping speed and 1500 m3 / hr for flow due to leakage and uncondensed steam, much less than would have been required without a condenser.

Field work

Vacuum cooling systems for leafy vegetables, salads and flowers are similar in design. They are installed in a trailer located next to the field where the salad is harvested, or integrated into the facility where the 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 tons of vegetables per day.

Vacuum cooling is an energy efficient, rapid cooling method with a wide range of applications in food processing and other industrial applications.

Before being loaded into the vacuum chamber, vegetables like lettuce are often doused with water to compensate for weight loss due to evaporation. As soon as the door is closed, the vacuum system starts pumping and the pressure drops from 1000 mbar to 15–20 mbar in 5 minutes. At that pressure, and at a temperature of around 20°C, the water begins to evaporate and the cooling process begins. After 15 to 20 minutes, the pressure drops further, to 5-6 mbar, and the product reaches a temperature of approximately 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 vapor and protects the pumps. Then the pumping and cooling systems are stopped and the chamber is vented back to atmospheric pressure in a few minutes. Subsequently, the salads are stored in a cold room where they can be kept for 2 to 3 weeks without spoiling.

As long as the condenser is doing its job well, the demands this cycle places on the vacuum pumps are straightforward, because the starting temperature is quite low (freshly harvested vegetables are rarely above 30°C) and the quantity of water to be consumed. evaporated is limited. However, the presence of dirt particles or small plant parts can be challenging, and there are some downsides to designing suitable cost-effective, low-maintenance systems. For example, oil-sealed rotary vane pumps are reliable and cost-effective, with good compatibility with water vapor 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 from particulates, and their maintenance requires regular changes of oil, oil filters, and exhaust mist.

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 cooling water or air, and their UP-front cost is higher than a rotary vane pump. Both types of pump can be used in combination with a vacuum root pump, which increases the pumping speed of the system at pressures below 50 mbar.

Beyond vegetables

The success of vacuum cooling to keep vegetables fresh means that similar techniques are now being applied to other food products. Bread and pastries are an example. In this application, the start temperature is much higher, up to 90°C when the buns are unloaded from the oven, and the amount of water present in the cycle is therefore dramatically higher than that of the vegetables. Rotary vane pumps don't have a high enough water vapor tolerance to get the job done, so screw pumps are a better solution. They can swallow large amounts of water without breaking down and are also very tolerant of small particles (flour, poppy or sesame seeds, etc.). In addition to saving energy and cooling bread more quickly, vacuum cooling also provides benefits for consumers: vacuum-cooled bread has a crisp crust and a fluffy crumb, which provides more pleasure when eating.

We are also beginning to see some non-food applications of vacuum cooling. For example, the grass on the field of top-notch professional soccer stadiums doesn't really grow there. Instead, it is produced on special farms, harvested in rolls, and transported to the stadium in time for the games. Thanks to the vacuum cooling, these rolls of grass easily survive the transport process and are kept until the next watering. The requirements for 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). So it is a more demanding job for the vacuum pump. The combination of rotary vane pumps and root blowers still works well, but the pumps require more maintenance than is common for cooling vegetables.

In short, vacuum cooling is an energy efficient, rapid cooling method with a wide range of applications in food processing (and increasingly beyond). Improves food safety and extends the shelf life of food products. The challenges it poses to vacuum systems are new and highly dependent on the product being cooled - while oil-sealed rotary vane pumps have proven effective for cooling vegetables, other applications require innovative thinking. Dry pumping technology is creating opportunities for new and more sophisticated processes, including cooling sushi rice or ready-to-eat foods.

Source: Physics World

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