SELECTED ASPECTS OF SELF-HEATING BEHAVIOUR OF FAT CONTAINING FOOD POWDERS

The self-heating of milk powder deposit in drying devices has been identified as having
the potential to cause a serious thermal hazard. The consequences of self-heating can
result in disruption to equipment and installation, lost production time, degradation of
product quality and that of the prime importance, ie. the danger to life. The preventive
and protective measures have been designed and implemented in most dairy plants. In
order to aid such efforts, the process conditions which could lead fires have been further
investigated. The thermal kinetics of the dairy powders have been studied previously by
some researchers but a number of related issues still require more in-depth study. A series
of mathematical models simulating the self-heating behaviour of reactive materials have
been developed and modified over time but experimental validation is not substantial.
This project presents a study evaluating the thermal ignition kinetics of whole milk
powder. It is an extension of a previous work in which a novel measurement procedure
and a numerical simulation model were proposed (Chong, 1997). In this work, several
new aspects has been addressed and experimental validations made. In particular, a much
larger sample size was used, so that the effect of heat accumulated inside the sample was
more pronounced to make the measurements more accurate.
Previously, two reaction regime for whole milk powder were identified in the
temperature range of 125°C to 160°C. However, the relationship drawn was not 100%
convincing. The number of points plotted at low temperature range were perhaps
insufficient to illustrate a definitive trend. In the current work, a similar test was
conducted at ambient temperatures ranging from ll6°C (a much lower ambient
temperature) to 150°C. The experimental data from this work were plotted together with
the previous results which prove the reaction regime discovered by Chong(l997). Hence,
the existence of at least two reaction mechanisms during self-heating of whole milk
powder have been validated .

The experimental conditions employed in this were then simulated using a numerical
simulation programme. The experimental results recorded were compared with the
simulation data. Some modifications were made to the program, based on the recent
available information. The model was able to predict the self-heating behaviour of the
milk powder in a larger sample size. From the sensitivity analysis, a more reliable
thermal conductivity of milk powders was identified. Hence, the model produced more
precise results compared with the experimental data. Further simulations were made on
the effect of initial sample temperature and initial water content on the self-heating
behaviour.
In addition, the measurement techniques and activation energy of various composition of
milk powder have been reported previously but the underlying reactions have still been
less explained. Fat oxidation of milk powder has been claimed to be a major reaction
triggering ignition. For this reason, a set of experiments was carried out to study the
effect of fat content on self-ignition propensity. It was found that a large amount of fat
can inhibit thermal runaway reaction. However, the presence of small fat component
could also enhance the exothermicity. Therefore, a critical fat content in which self
ignition may be more easily initiated has been suggested.