Download Influence of bacterial ice nucleators on characteristics of food

April 19, 2002
Department of Food Science
Final Ph.D. seminar
Rutger van Sleeuwen
Thesis Advisor: Dr. Tung-Ching Lee
Influence of bacterial ice nucleators on characteristics of food freezing: Design of a predictive
computer model
Foods preserved by freezing retain much of the texture, taste and nutrition of the original
fresh product. This explains why large quantities of food products are frozen around the world.
However, it is imperative to thoroughly control the freezing process to ensure that quality loss
during freezing is minimized. In a typical commercial freezing process, food samples are rapidly
frozen to create a large number of small ice crystals. Slow freezing can lead to the formation of
small numbers of large ice crystals, which can result in lower product quality1. However, for certain
food applications, such as freeze-concentration, freeze-texturing and freeze-drying, large ice
crystals are desirable 2.
When pure water is cooled below its equilibrium freezing temperature (0°C), phase change
does not necessarily occur and the water remains supercooled. A process, known as nucleation, is
needed, which provides a seed for ice crystals to grow upon3. Ice-nucleation active (INA) bacteria
or Extracellular Ice Nucleators (ECINs) derived from these bacteria have been shown to increase
the temperature at which nucleation occurs in model food systems, reduce freezing times and
change the patterns of ice formation, altering the texture of frozen foods 4;5. Most of this research,
however, is focused on small samples, whereas it is known that the diameter of ice crystals in, for
instance, large commercial frozen meat samples varies from surface to center 6.
The goal of this research is to study the effect of INA on temperature history profiles at
various locations in a food model for beef (Tylose®, a 77% Moisture Methylcellulose7). Whether
INA affects ice crystal size and its distribution throughout the sample is determined. Α numerical
heat transfer model 8 is adapted to incorporate supercooling as suggested by Miyawaki et al. to
simulate these experiments 9. A method is developed for indirect visualization of ice crystals and its
distribution in Tylose®, using freeze-drying and Scanning Electron Microscopy (SEM), followed by
image analysis.
The results show that supercooling is mainly observed on the surface of the large samples,
and that the amount of supercooling is a function of sample-size. Addition of bacterial ice
nucleators results in an almost complete elimination of supercooling in large samples. However, no
reduction in freezing time is observed. The numerical model correctly simulates the observed
temperature profiles in Tylose® samples and only predicts a slight reduction in freezing time, when
supercooling is eliminated. The SEM images show a spongy structure, revealing variation in ice
crystal sizes from surface to center. A clear difference in crystal size is observed between samples
frozen at different freezing rates, which validates the use of this method in this research.
In future work, the potential effect of INA on ice crystal size distribution will be quantified,
to find conditions that will yield a frozen food product with a desired size (large or small) of ice
crystals and more uniform size distribution.
References
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George RM: Freezing Systems, in Erickson MC, Hung Y-C (eds): Quality in Frozen Food, New York, NY, 1997, pp 3-9
Watanabe M, Arai S: Applications of Bacterial Ice Nucleation Activity in Food Processing, in Lee Jr RE, Warren GJ, Gusta LV
(eds): Biological Ice Nucleation and its Applications, St. Paul, Minnesota, 1995, pp 299-313
Reid DS: Overview of Physical/Chemical aspects of Freezing, in Erickson MC, Hung Y-C (eds): Quality in Frozen Food, New
York, NY, 1997, pp 10-28
Li J, Lee T-C: Bacterial Extracellular Ice Nucleator Effects on Freezing of Foods. Journal of Food Science 63:375-381, 1998
Li J, Izquierdo MP, Lee TC: Effects of Ice-Nucleation Active Bacteria on the Freezing of some Model Food Systems. International
Journal of Food Science and Technology 32:41-49, 1997
Bevilacqua AE, Zaritzky NE, Calvelo A: Histological Measurements of Ice in Frozen Beef. Journal of Food Technology 14:237251, 1979
Riedel L: Eine Prüfsubstanz für Gefrierversuche. Kältetechnik 8:222-225, 1960
Mannapperuma JD, Singh RP: Prediction of Freezing and Thawing of Foods Using a Numerical Method Based on Enthalpy
Formulation. Journal of Food Science 53:626-630, 1988
Miyawaki O, Abe T, Yano T: A Numerical Model to Describe Freezing of Foods when Supercooling Occurs. Journal of Food
Engineering 9:143-151, 1989