Browning and blistering of Mozzarella during high temperature pizza baking : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemical and Bioprocess Engineering at Massey University, Manawatū, New Zealand
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Date
2022
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Massey University
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Abstract
Mozzarella and pizza baking are large and growing industries, and manufacturers and commercial
operators need to know how their cheese will function during baking. The aim of this research was to develop a kinetic model describing the Maillard browning reaction occurring at the surface of Mozzarella during high temperature cooking, and to develop a conceptual model for how blister formation and growth is influenced by baking conditions.
Image analysis using lightness, L*, from the CIELAB colour space (L*a*b*) colour scale was used to
measure the development of brown colour during baking. An experimental method was developed
using a DSLR camera to record colour data and an Infrared (IR) camera to study the Maillard browning reaction. A first order kinetic model was fitted to the lightness data using non-linear regression, with the temperature dependence found to follow the Arrhenius Law. The activation energy (Ea) and the kinetic rate constant at 120°C (k120), were fitted to two data sets. One based on an average region of interest (Average ROI kinetics, Eₐ = 124 kJ.mol⁻¹ & k₁₂₀ = 5.0x10⁻⁴ s⁻¹); and the other based on a discrete localised region where blistering occurred (P1 kinetics, Eₐ = 101.3 kJ.mol⁻¹ and k₁₂₀ = 3 .65x10⁻⁴ s⁻¹). Predicted L* was used to create a visual representation of predicted browning for the whole cheese surface which matched well with the shape and the outlines of the browned regions of the experimental data. The goodness of fit was better based on the Average ROI kinetics with an R2 of 0.88, compared to 0.66 for the P1 kinetics. A modified commercial pizza oven (Lincoln Impinger II 1100 Series) was characterised to provide information and data for the development of experimental techniques and analysis of baked pizzas. The modifications to the oven allowed for accurate temperature measurement, bake time, mass logging, and variable air flow. Air flow measurements showed that most of the air comes from below, based on the impinger layout, and there was minimal change to air flow within the control
limits. The heat transfer coefficient (HTC) within the oven was measured using two methods; heating an aluminium block (29.1 to 37.1 Wm⁻²K⁻¹), and water evaporation (43.4 to 45.7 Wm⁻²K⁻¹).
To analyse pizzas baked in the modified oven, an image analysis method was developed to process
post baking images. MATLAB was used to process images of pizzas baked under a range of conditions, which involved colour conversion and correction, thresholding and determining characteristics. A dynamic thresholding method was developed to account for the variation in overall surface colour and provided consistent and accurate identification of blistering and browning of baked pizzas. The blister analysis process was applied to the experimental and predicted data from the kinetics experiment to determine blister characteristics. A general trend was identified, where blister numbers increased to a maximum and then reduced over time. Blister size was found to increase with time, which can be attributed to blister growth and also the joining of adjacent blisters as they continue to grow. Overall, the P1 kinetics gave a better fit across all of the experimental trials with R2 values ranging from 0.94 to 0.99, compared to the fit for the average kinetics at 0.90 to 0.98.
The overall process of cheese baking can be categorised into several stages; melt and flow, steam formation, bubbling, drying and blistering and browning. The conceptual model developed,
proposed that low heat flux conditions trend towards fewer but larger blisters and high heat flux conditions trend towards a greater number of small blisters. Blister nucleation was found to be likely more affected by bake time and temperature, than from initial conditions. It was found that the initial form factor (shreds vs slices) of Mozzarella did not affect the nucleation of blisters. The effect of baking conditions on blister characteristics was studied by baking pizza at a range of time and temperature combinations, with temperatures from 220 to 288°C and bake times from 6 to 15 minutes. Baking repeatability was found to be reliable and reproducible, with the % mass loss consistent with a standard error of 0.2% and the level of browning with a standard error of less than 3% for each set of operating conditions assessed.
Blister characteristics were assessed from binary images based on the dynamic thresholding process.
Observations from the images showed that the size of blisters decreased as oven temperature increased and baking time decreased. It was found that a higher oven temperature, and hence higher heat flux, resulted in greater blister numbers where blister coverage was less than 50%. Above 50%, it was found that further blister growth and characteristics were not sensitive to oven
temperature.
Analysis of the time for the onset of moisture loss and blistering relative to oven temperature found
that as oven temperature increased, the time difference between the initiation of moisture loss and
blister formation reduced from approximately 5 minutes at 220°C down to approximately 2 minutes at 280°C. This indicated that heat flux has a greater effect on blister initiation than overall moisture
loss which suggested that heat flux has a greater effect on surface moisture loss in comparison to
bulk moisture loss. This connection suggested that the pizza surface dried out faster relative to the
overall moisture loss as oven temperature increased, and as moisture is lost, the viscosity of the cheese increases. At high heat flux, there is limited time for bubbles to form before a stable skin
forms and the surface dries out. In addition, an increased viscosity means that there would be more
resistance to growth. The resulting blisters would be smaller than for low heat flux conditions. If the
cheese surface dries slower, there is more time for bubble growth before viscosity increases and a
stable skin and hence larger blisters are formed. This suggested that there is a critical moisture
content where the cheese surface forms a stable skin where temperature can rise above 100°C.
To further study the effects of baking conditions on blister formation and growth, a benchtop oven
was modified to measure and record surface colour, surface temperature and mass loss in real time
during baking. A series of trials were carried out to bake cheese within the modified oven at temperatures of 168°C, 175°C, and 185°C for a minimum of 25 minutes each. The 10th percentile,
90th percentile, minimum and maximum values showed a pattern where initially the overall range is
wide but reduces as the average temperature approaches 100°C, and then variation increases
beyond 110°C. This pattern was similar for the L* data. Across all oven temperatures the 90th
percentile and maximum L* values were similar, which agreed with the observations of lower
surface temperatures in the 10th percentile and minimums across all oven temperatures. In
contrast, there was a significant decrease in 10th percentile and minimum L* values as oven
temperature increases, due to the increased surface temperatures relative to oven temperature.
It was found that the original dynamic threshold was not suitable for analysis of the benchtop oven data as a result of different lighting conditions, so a new dynamic threshold equation was developed based on sensory evaluation. The analysis of binary images created using the new equation showed
that blisters formed in less time at higher temperatures and blister coverage increased linearly
relative to bake time, which agreed with the earlier analysis. Heat flux was found to be significantly
lower for the benchtop oven, with the time to achieve 10% blister coverage at the highest oven
temperature taking approximately 6 minutes longer compared to the lowest oven temperature from
the commercial oven trials. The heat flux to the cheese layer for the commercial oven experiments
was estimated to compare the effect of heat flux on the average blister size between the two
experiments. It was found that there was a linear correlation between heat flux and average blisters
size for the combined results, which suggested that this correlation may hold true irrespective of
differences in oven setup and operating conditions. For blister initiation, a similar trend was observed from the benchtop data analysis as for the commercial oven data. Combining the blister
initiation data suggested that the onset of browning is highly correlated to oven temperature, which
is consistent with the temperature dependence of the Maillard reaction. The developed kinetic parameters were applied to the integrated kinetic equation to assess the performance of the model in a different application. It was found that the predicted images and subsequent blister characteristics matched well with the experimental data, especially for blister coverage, with and R2 of 0.99.
The findings from these experiments and analysis indicate that surface drying dynamics are
fundamental to browning and blister development. Blister growth and size is governed by the
moisture content and therefore viscosity, at the cheese surface. Oven temperature (and therefore
heat flux) dictates how fast the surface will dry and therefore the general blister characteristics. Bake time determines the desired level of baking based on blister/browning coverage. Hence heat flux and cheese surface dynamics, in regard to the rate of moisture loss and viscosity, are the critical factors in determining browning and blistering characteristics.
The experiments and methods developed, and the understanding of cheese behaviour and influential factors during baking have potential benefits for the high temperature food processing industry, especially cheese manufacture and applications. Benefits include improved accuracy and consistency of product assessment, control over baking characteristics and reduced production costs. The image analysis process developed provides an objective measure of blister characteristics, which are otherwise subject to individual interpretation. This process could be applied in a commercial setting give a robust, repeatable and consistent means of product quality and performance assessment. Given the developed understanding of the fundamentals that drive blister formation and browning it is possible to manipulate parameters that affect surface drying dynamics. This could be through external baking conditions, such as a multi-stage baking process, or adjusting the cheese composition itself to achieve a desired result.
The findings from this study resulted in a kinetic model describing Maillard browning of Mozzarella
during baking as well as a good insight into the fundamentals that affect blister formation and
growth to better understand the Maillard reaction and its application in high temperature food
processing.
Description
Fig 2.2A&B is reproduced under a CC Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) license.
Keywords
Mozzarella cheese, Baking, Mathematical models, Pizza, Analysis