8 minute read
Campden BRI: Technical challenges and opportunities with vacuum cooling
One of the popular uses of vacuum cooling is for laminated pastry products. Campden BRI carried out a research project on vacuum cooling for bakery products to investigate the scientific issues that arise when cooling bakery products including laminated pastries, sandwich bread and cake.
By Gary Tucker, Fellow, Campden BRI
+Vacuum cooling is a rapid evaporative technique that can be used for various foods and vegetables, including bakery products (Bradshaw, 1976). The major characteristic of vacuum cooling is that products are cooled extremely fast by evaporation of water under vacuum. The theory is that pure water boils at 100°C at 1-atmosphere pressure but a reduction in pressure causes water to boil at a lower temperature. In practice, it works by placing hot baked products into a vacuum chamber and evacuating the air with a vacuum pump. The chamber pressure falls to the point where the water starts to evaporate or flash off. Large quantities of energy are involved with this phase change resulting in a rapid decrease in the product temperature. The water vapor is condensed back to water for discharge through a drain.
The major advantage of vacuum cooling over conventional cooling is the shorter time required to cool a product to a given temperature. This can reduce cooling times for bakery products by around 90%. Benefits are a reduction in equipment footprint and decreased production costs through increased throughput. Energy savings are also claimed, which are achieved by reducing the baking time and allowing the vacuum cooling process to ‘finish off’ the bake in the cooler. This will be discussed.
Suitability for bakery products
One of the popular uses of vacuum cooling is for laminated pastry products where the sudden conversion of water into steam helps with separating the pastry layers. By placing the vacuum cooler immediately after the oven, it is possible to shorten the bake time and allow layer separation to occur at lower temperatures. As well as saving oven time and energy, improved layer separation increases the product volume.
Vacuum cooling is suited to products with a large surface area and a structure that allows the steam to escape quickly. Products with a close porous structure such as sandwich bread present challenges. Sandwich bread is designed with a network of small bubbles that are important to confer softness and increase the whiteness of the slice. Towards the end of baking, all the intact gas bubbles must break from the gradually increasing internal pressure. Steam finds its way out of the crumb through the interconnected pathways between the gas cells. With vacuum cooling, water turns suddenly to steam, and there is a much more rapid increase in gas volume than with conventional baking. The steam must escape otherwise the volume increase damages the soft and delicate bread structure by creating large holes at weak points in the structure.
Vacuum cooling schedules must be designed specifically for these products and are not easily transferable to other products.
Soft bread-like goods such as malt loaves can be included in other products suited to vacuum cooling. They tend to collapse during conventional cooling as the large gas cells contract. Vacuum cooling causes the water to leave the product so rapidly that the product structure is effectively frozen in time. The crust goes hard through water loss, and this keeps the product shape similar to that at the end of baking. There will be some crust contraction but not as much as with a slower cooling rate.
Technical challenges
Campden BRI carried out a research project funded by Innovate UK on vacuum cooling for bakery products (Tucker et al., 2021). It used a purpose-built vacuum cooler developed by C-Tech (Figure 1) with a 50-liter vacuum chamber. It was used to investigate the scientific issues that arise when cooling bakery products including laminated pastries, sandwich bread and cake. The experimental work plans included the extent to which starch was fully gelatinized, whether residual enzyme activity remained, the uniformity of the moisture distribution, and various crust and crumb textural properties.
The results reported here are for bread products. The experiments used a control baking time of 25 minutes at 220°C for 800g sandwich bread. Samples cooled at ambient temperature were compared with those using reduced bake times followed by vacuum cooling.
Micro kill
Baking for a reduced time needs to be done with food safety in mind. Microbiological safety is not likely to be an issue until core temperatures approach as low as 75°C. This is required to kill pathogens such as Salmonella, Listeria and E. coli. Starch gelatinization and residual enzyme activity both require a few minutes at around 85°C to ensure completion of either process. Bread structure will break down over the shelf life if temperatures do not reach this value in baking. Previous work (Tucker, 2013) showed that core temperatures of 85°C in white sandwich bread were sufficient to gelatinize wheat starch and inactivate amylases present in flour or improvers. This is similar to core temperature targets used for part-baked bread and should be considered the minimum values that must be achieved.
Starch gelatinization in bread
At the end of baking and cooling, all the starch in bakery products needs to be gelatinized. Ungelatinized wheat starch has a characteristic peak on a differential scanning calorimeter (DSC), with the area under the peak giving a measure of the extent by which the starch is gelatinized. This is known as the enthalpy of gelatinization for amylopectin in wheat starch.
For the full baking times, the analysis by DSC showed the starch to be fully gelatinized. Samples baked for much reduced times of 12 and 14 minutes showed starch gelatinization to be incomplete. With low bake times, there was insufficient heat to gelatinize the starch and it did not matter if cooling was slow or fast. As the bake time increased to 16 minutes and more, the temperature gradients within the bread enabled starch gelatinization to start during baking and continue during cooling. Vacuum cooling suddenly stopped this happening as the heat was removed quickly from the bread.
Melting temperatures for amylopectin were found to be lower in vacuum-cooled bread than in ambient-cooled bread. This suggested the reaction involved with starch gelatinization had changed because of the vacuum cooling effect. This could be related to the lower amount of water present with vacuum-cooled bread.
Enzyme activity
A normal baking time for 800g malted grain sandwich bread (25 min at 220°C) completely inactivated the enzyme activity. This was evaluated using Rapid Visco Analysis (RVA) on diluted bread samples baked at different times and against a control starch paste. Differences in the final viscosity
Figure 1. Prototype vacuum cooler developed by C-Tech for the INNOVBAKE project
© Campden BRI
between ambient-cooled and vacuum-cooled products were small when products were fully-baked for 25 minutes. When the baking time was very short (e.g., 12 minutes), enzyme activity remained, but when products were baked for 14 or 16 minutes, the ambient-cooled breads showed less enzyme activity than when vacuum-cooled. This suggested the vacuum cooling effect of lowering the water boiling point did not have a significant effect on enzyme activity. However, cooling at ambient allowed the core temperatures to continue rising slowly after the breads were removed from the oven. More heat was retained within the ambient-cooled bread than with the sudden temperature drop associated with vacuum cooling, and this continued inactivating enzymes such as amylase.
Issues with residual enzyme activity are not found with all vacuum-cooled products. Bakery products such as laminated pastries are successfully vacuum cooled and do not experience a structural breakdown of starches from residual amylase activity. The main reason for this is the higher product temperatures during baking, which inactivate enzymes before vacuum cooling starts. Typical core temperatures of 100°C are required to generate steam to force apart the laminated pastry layers. By comparison, bread and cake products tend to operate with a target temperature of 94-96°C. It is also likely that the lower water content of laminated pastries slows enzyme activity to the extent that it would not happen within the short shelf life of these products. Packaged sandwich bread has a water content of around 40 to 43% and a five to seven-day shelf life, whereas unpackaged pastries tend to contain only 20-23% water with a one-day shelf life. After one day, the pastries will lose a substantial amount of water but the packaged bread will be close to its original water content. Enzymes are more active in high water systems.
Crust hardness
Measurements on crust hardness and moisture levels were made using white Bloomer loaves because this is a product in which the crust properties are important. Vacuum cooling was shown to increase the hardness of the crust compared to ambient cooling (Figure 2). This was partly because the moisture in the crust was lower than with conventional cooling (Figure 3). Using a shorter baking time helped to decrease the hardness of the crust and increase the moisture, however, the crusts were still hard compared to ambient cooled products.
An alternative solution to overcome the increased dryness with vacuum-cooled bakery products could be to spray a fine mist of water onto the products, either before or after vacuum cooling. This is a similar approach to that used with conventional cooling technology deploying humidified air to reduce water loss. This was investigated in the project as a ‘look-see’ trial but not through detailed experiments. There was an increase in the moisture content of the crust and a decrease in the firmness (data not presented).
Figure 2. Crust hardness of ambient cooled and vacuum cooled products with different baking times. The error bar shows standard error (n=48)
Figure 3. Moisture content of crusts of ambient-cooled and vacuum-cooled white sandwich bread with different baking times. Data were mean values from 12 samples
Discussion and conclusion
Vacuum cooling is a commercially successful technology for laminated bakery products that relies on steam generation to help separate the pastry layers. It offers the economic benefits of reducing the baking time and quality improvements by separating the pastry layers during cooling. With these products, there are no issues with residual enzyme activity or incomplete starch gelatinization.
With larger products, vacuum cooling causes the crusts to dry out more than with conventional cooling. It is possible to counteract this drying effect by reducing the time the products spend in the oven. This is where the claims for
