Freezing’s main purpose is to preserve product quality over extended periods of time. When done correctly, the process locks in the moisture that is vital for the product’s taste and texture and prevents microbial growth, while at the same time avoiding the formation of ice crystals that could damage the molecular structure of the goods.
By Tudor Vintiloiu, senior editor European Baker & Biscuit/Asia-Pacific Baker & Biscuit
Bakeries have notoriously hot ambient temperatures, mainly due to the large baking ovens. In this high temperature environment, cooling down and freezing a product with clockwork precision can prove to be quite challenging. For typical frozen baked bread processing lines, when it exits the oven, bread is cooled down from +95°C to +45°C and then frozen from +45°C to -12°C to reach 80% product level of frozenness. The remaining temperature decrease from -12°C to -18°C takes place in cold store.
First cooling after the oven
The first cooling step after the oven is generally the most problematic and is achieved in an ambient spiral cooler, according to specialists. Ambient temperature could be too high for effective cooling, particularly if the oven is close to the spiral cooler. In such case heat treatment of the spiral area is provided by air sockets blowing cold air with controlled temperature above the spiral conveyor. And if air treatment is affected too much by oven area temperature then an insulated enclosure is erected around the spiral cooler.
Mathieu Nouhin, product manager freezing, GEA, explains how this delicate balance is achieved: “In proofing process for pastries, exact temperature and moisture control is crucial in the proofer for product quality. Typically, air temperature is controlled at +28°C +/- 1°C and moisture is controlled at 75% +/- 2%. This is achieved with air handling units blowing air with an air socks design and positioned in the spiral proofer for even air distribution on all products. Accurate temperature and moisture control take place in the air handling units prior to distribution through air socks. Basic configurations with fans blowing air directly into the proofer enclosure does not provide accurate air distribution resulting in product quality issues.”
Types of freezing equipment
Air temperature settings are key for product quality in bakery processes which include highly sensitive products. For instance, air should not be too cold in the baked bread freezing step to prevent damaging the crust. Same constraints apply for the proofing process described above where a small temperature variation can damage the product heavily.
Freezing too quickly will impact yeast activity but too slow will result in large water crystal formation and moisture loss. It is critical to find a moderate freezing rate, depending on the product, and a uniform airflow across the product zone. Cryogenic systems are not adapted to bread and more generally to bakery processes as Nitrogen or CO2 generates very low temperatures below -70°C which burn product surface. In addition, temperature control is challenging with cryogenic freezing.
“The solution to get the right product quality is still mechanical refrigeration allowing for a wide range of temperature levels and accurate temperature control. Refrigerant and refrigeration systems are designed to ‘feed’ the process equipment heat exchangers with the right temperature and with minimum energy consumption. Inline solutions provide the best quality in regards to accurate temperature control versus batch solutions where door openings during product loading and unloading are generating temperature and moisture variations”, Nouhin added.
Experts from Food Process Solutions (FPS) describe the most common types of freezers suitable for the baking industry:
- Box freezers (carton freezers) are great for products that have a rigid exterior surface like a box or tray and can be pushed around. Usually this correlates with longer retention time items since product is insulated by the surrounding box with retention times upwards of 24 hours. Due to these long holding times, spirals would require a large footprint and many linear meters of belt which becomes very expensive.
- Spiral freezers are good for general range of products that have to be naked, individually handled or on trays with relatively shorter retention times (up to 3-4 hours) than box freezers.
- Tunnel freezers can be separated into a few types: fluidized bed/IQF, impingement high velocity jet tunnel and multipass tunnels. Fluidized bed freezers are for handling bulk products which can be piled on top of each other with an initial crust freeze belt and a secondary finish freeze belt. Impingement tunnels implement high velocity jets from top and bottom typically used for thin products or for crust freezing products. Multipass tunnels come in various arrangements dropping from 1-4 passes allowing for lots of retention time in a short space. This is good for products which can be crust frozen and piled up.
- Plate freezers having refrigerant circulating through plates sandwiching the product to directly cool and freeze. Product in boxes and rectangular shapes implement this technology for conductive cooling which is more effective than convective cooling.
You can read the entire article in the Jan-Feb print issue of Frozen Food Europe magazine.
