Key points
Starch is a key functional ingredient in a wide range of prepared food and drink products around the world, including soups and sauces, ready meals, juices, plant-based alternatives, dairy products and baked goods.
The functional properties of starch, including its ability to maintain or enhance texture and mouthfeel and improve the shelf-life and visual appeal of products, make it highly valued and carefully studied.
Starch manufacturers modify starch, either chemically, or physically, to enhance these functional attributes, thus making it suitable for specific processing and storage scenarios and specific recipes or formulations. Understanding how different starches behave under processing is key to creating optimal end products that consumers will enjoy.
Researchers at AgroParisTech, a French public institute of technology for life, food and environmental scientists (UMR SayFood, Université Paris-Saclay, AgroParisTech, INRAe) sought to better understand the swelling behaviour of starch – a key factor in determining its functional role in end products.
This swelling, or, gelatinisation, happens when starch granules undergo an irreversible transformation when subjected to thermomechanical treatment in the presence of water. During this process, starch granules swell by imbibing water, thereby increasing the overall viscosity of the suspension – this process causes starch to modify the texture of end products.
In recent research by AgroParisTech [1, 2], a novel kinetic model for individual starch granule swelling was developed, based on swelling kinetics time series data, using time-lapse microscopy and varying hydro-thermal treatments.
Ms. Gabrielle Moulin, part of the research team explained: “Previous work in this area has attempted to model the swelling kinetics of the population mean, but not at the individual granule scale. In our work, we propose a novel kinetic modelling at the individual granule scale rather than the population mean. This model takes into account the effect of temperature evolution and variability between granules on their swelling. It can therefore contribute to robust multi-scale simulation of industrial processes involving starch suspensions. It can help provide a better understanding of the mechanisms driving and affecting starch swelling, so producers will also be able to design more reproducible processes and better understand and predict starch gelatinisation.”
To provide new insights on starch granule swelling, the team studied the kinetics and dimensional changes affecting individual starch granules by developing a new particle detection procedure using particle image tracking principles generally used in cell biology.
Time-lapse light microscopy observations were made of individual starch granules placed on a temperature-controlled stage, to describe both the kinetics of individual granules and the evolution of granule populations.
The team developed an automated algorithm to track a significant number of individual granules through the complete heating period. They were able to estimate dimensional and kinetic parameters of individual granules under heat treatment. Statistical analysis of the data enabled them to quantify the effect of factors such as the heating rate and final temperature and to evaluate the impact of the initial granule size on swelling.
The method they developed enables modelling at the granular scale, offering a better understanding of the dynamics for multi-scale modelling and translating that into plant equipment design, optimisation, differentiated product development, simulation, robust process development and robust model-based process control systems.
Time-series data obtained from image processing of hot stage coupled microscope time-lapse images was used for building and fitting the model.
Chemically stabilised cross-linked waxy maize starch (acetylated distarch adipate, C*Tex 06205), provided by Cargill (Baupte, France), cross-linked via adipates, was used in 0.5g.kg-1 suspensions of starch in water.
The suspension was preheated to 50°C under gentle stirring with a magnetic pellet. Using a dropper, sampling was performed and the sample was transferred to the microscopic glass slides with adhesive spacers of 250 µm. In order to assess the contours of each granule, observations were conducted under white light.
Samples were observed under 50x magnification using an Olympus BX-51 microscope (Olympus Optical Co. Ltd., Tokyo, Japan). Images were acquired using a Basler A102fc digital camera (Basler AG, Ahrensburg, Germany). Microscope glass slides with samples were placed on a Linkam LTS120 stage (Linkam Scientific Instruments, Surrey, UK). The temperature was controlled with LinkSys32 software.
In the first treatment (A), the temperature was increased from 50 °C to 90 °C with a rate of 5 °C per minute, in the second treatment (B) the temperature was increased from 50 °C to 90 °C with a rate of 10 °C per minute, and in the third treatment (C) the temperature was increased from 50 °C to 70 °C with a rate of 5 °C per minute.
For the three treatments, observation started one minute before temperature increase and lasted 2 minutes after reaching the final temperature for treatments A and B, and 10 minutes in the case of treatment C. The final step of the heat treatment was chosen in order to assess the residual swelling under constant temperature.
The starch granules initially exhibited uniform texture which changed during the heat treatment and became non-uniform after swelling, as shown in Fig. 1.
Figure 1: Raw image of starch granules after 0s, 180s, and 540s at 5°C/min, corresponding to 50°C (A), 75°C (B), and 90°C (C).1
The image processing procedure was used to track the evolution of individual granules over time and detect granule evolution.
Results of the study confirmed that above a threshold temperature, starch granules swell until they reach an equilibrium size several times larger than their initial size. The growth factor is 2.35 on average, but subject to variability from granule to granule.
Fig. 2A-D presents the evolutions of the size distribution for treatment A, comparing one individual evolution of starch granule with the global evolution of the population.
Figure 2: Distribution of starch granule size (A, B) during heat treatment A. Selected granule size evolutions during treatment A (C). Typical individual kinetic and global kinetic during heat treatment A (D). The grey area represents the proportion of granules in the process of swelling as a function of time.1
It was found that the granules do not swell with the same kinetic and do not grow with the same ratio [Fig 2C]. The team observed that individual granules can start swelling at different temperatures and have different swelling rates.
The characteristic swelling time of the population is much larger than the characteristic swelling time of an individual granule. Swelling continues until an equilibrium size is reached once a granule has been heated above a threshold temperature. The beginning of swelling is specific to individual granules and occurs in the range of 62°C to 74°C and 68°C on average.
The same samples and equipment used in the first study [1] outlined above were used in subsequent work [2], in which the samples were subjected to different temperature profiles, starting with a holding at 50 °C for 1 min. Some samples were observed under the microscope at different constant temperatures from 55°C to 62°C for 1 hour to determine the minimum temperature at which swelling starts.
The team observed a gradual increase in fraction of granules swollen as temperature increased from 57 °C to 62 °C [Figure 3].
It required at least around 60°C to observe swelling in the majority of the granules, however, some granules swelled at 58 °C and some were still not swollen at 62 °C, even after 1 hour. Nevertheless, in industrial processes, the maximum temperature is much higher than 60°C and residence time in heat exchangers is usually brief. To ensure relevance to industrial processes, the assumption was made that the minimum swelling temperature was 60 °C for all granules.
Figure 3: starch granules before and after 1 hour at different temperatures.2
Using the insights from the research, the team developed a novel individual granule scale model to predict the swelling kinetics of modified waxy maize starch granules under heat treatment. The model parameters were calibrated with data from one ramp rate and hold temperature and validated with data from other cases.
From the experiments, it was shown that there is granule-to-granule variability both in ratio of swelling and latency to swell when a critical temperature is reached. These phenomena are independent of the diameter of the granules.
The rheology of the suspension results from the hydrodynamic interactions and collisions between the granules which is a function of the volume fraction of granules and granule particle size distribution.
To design, control, scale up and operate complex processes for liquid food products simulation techniques such as Computational Fluid Dynamics (fluid flow, heat transfer) and Discrete Element Method (particle evolution and interaction) are used. The new granule scale model can inform these techniques for multi-scale modelling and simulation of modified starch suspensions undergoing thermal treatment.
The AgroParisTech team’s new model takes into account the effect of temperature evolution and variability between starch granules on their swelling. This model can be used to support hypothesis in the mechanistic modelling of starch swelling, as well as improving the general understanding of starch interactions in food products.
It can contribute to robust multi-scale simulation of industrial processes involving starch suspensions and could also be modified to determine other sizes of particles or air bubbles in any kind of matrix.
The Linkam LTS120 Peltier stage played a key role in this work and is used in many temperature-dependent experiments across a broad range of sectors. Sample temperature can be controlled from 0.1°C to 30°C/min in the range of -30°C* to 120°C. Temperatures down to -40°C can be achieved with an added chilled liquid circulator.
References
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