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Introduction

Dear reader,
    
Welcome to our winter edition of the IFST and IMechE Food Engineering Committee newsletter. We have included a further paper from previous nominations received for the Food Engineering Prize and I hope this is of interest. We are now receiving nominations for the 2008 prize and if you would like to enter the competition please forward us your paper.

In November we held a very successful seminar on the subject of ‘Rheology’, a review of which is provided below.

I trust this news letter is of interest and we would like to hear from you on future topics of interest.

Don Ives, Committee Chairman 
January 2008 


IMechE Food Engineer of the Year Award 2008
Sponsored by the PM Group 

The IMechE Food Engineering Committee is proud to announce the opening of the IMechE Food Engineer of the Year Award 2008, which has been specifically designed to help raise the profile of food engineering and promote excellence within the industry. 

This prestigious award of £1000 will be presented to the author of the best paper relating to the food industry. The competition is open to professional engineers, as well as undergraduates and graduates studying within the field. Papers are welcome from all areas of food engineering. 

If you would like to enter the competition, please submit a synopsis of your proposed paper process@imeche.org. Entries will be put to a panel of judges who will shortlist the submissions for the second stage of the competition. Those candidates short-listed will be asked to provide a full paper by a second deadline, after which the winner will be announced.

Deadline for synopsis is 15 September 2008.

Nominations are now welcome.


2007 Winner 

This years prize winners are Wenfeng Xiao, Maria N Charalambides and J. Gordon Williams for their paper entitled 'Sheeting of Wheat Flour Dough' . 
The prize will be presented at a ceremony in January 2008 at the Worshipful Company of Butchers and full details will be covered in the next issue of the newsletter.


The Application of Rheology in Food Process Engineering
Report by Dr Maria Charalambides, Imperial College London

On 15 November 2007, the Food Engineering Committee organised ‘The Application of Rheology in Food Process Engineering’ which was held at the IMechE headquarters. The event was well attended with thirty seven delegates including four rheological instrument exhibitors.

The day commenced with some opening remarks from the chair Professor Gordon Williams, Imperial College London, who commented that many characterisation methods developed for engineering materials are applicable to the study of the behaviour of foods, if one takes into account first the orders of magnitude change in mechanical properties such as modulus!! Dr Ian Wilson, Cambridge University, followed with his talk on ‘Putting Structure into Structured Products’ which first gave an introduction to rheology of foods and then emphasised the importance of the solid-liquid transition as well as introduced the capabilities of Magnetic Resonance Imaging to image foods and their structures during processing. Dr Lucio Cicerelli, United Biscuits, then talked about ‘Applications of Food Rheology Tests in the Biscuit Industry’. During the talk he demonstrated the importance of such rheological methods in product development and quality and process modelling and control by using several materials as examples.

A coffee break followed where delegates had the first chance to look at the various instrument exhibits and hear about the latest developments in rheological characterisation instruments. Dr Peter Martin, University of Manchester, then gave a talk on ‘The Importance of Wall Slip Effects when Processing Soft Solid Foods’ with the emphasis on helping to preserve microstructure. He also discussed methods for analysing and correcting for wall slip effects.

After lunch, Dr Jan Engmann, Nestlé Research Center, Switzerland, gave his talk on ‘How many Types of Rheological Measurement do we need?’ where he introduced to the audience the wide choice of rheological methods available for industrial applications in the areas of process optimisation, quality control, structure formation/stability and product screening. He also stressed the limitations with some of these methods that the food engineer should be aware of. The next talk was by Dr Massimo Migliori, University of Calabria, and was entitled ‘Modelling support to Food Production: Engineering Approach’. Massimo presented modelling predictions for bubble growth during biscuit baking and hence biscuit height as well as for foam flow and expansion in whipped ice-cream.

My talk on ‘Experimental and Theoretical Investigations of Sheeting and Extrusion of Bread Dough’ followed the afternoon tea. A material model was first introduced, capable of predicting the very complex behaviour of bread dough. This was subsequently used to predict important process parameters such as loads and exit dimensions for sheeting and extrusion. Last but not least, Dr Bogdan Dobraszczyk, School of Food Biosciences, University of Reading, presented his work on ‘Polymer Molecular Modelling of Wheat Gluten and its Relationship to Baking Performance’. In this talk he showed results derived from using the Pom-pom model to predict the wheat gluten’s mechanical behaviour and its associated effect on baking quality.

In summary, I thought (and many others agreed with me!) that the day was successful. The speakers all contributed to an excellent and very informative seminar. The talks effectively demonstrated the relevance of rheology in food engineering and highlighted problems and possible solutions. Having four exhibitors also provided the excellent opportunity to the delegates to see the latest capabilities of rheological instrumentation.

Please see below a paper from a previous entrant of the Food Engineering Prize.

A New Model to describe the Kinetics of Bacterial Inactivation by High Hydrostatic Pressure – Project synopsis
By Bernadette Klotz, Leo Pyle and Bernard Mackey, University of Reading, School of Food Biosciences

In recent years consumers have expressed an increasing preference for fresher and more natural food products. In response to this there has been a trend towards the use of milder processing regimes and reduced amounts of preservatives. Several new, non-thermal, methods for food preservation are being evaluated in order to satisfy the demand for foods that are safe but retain fresh attributes. High hydrostatic pressure (HHP) is regarded as one of the more promising of these emerging technologies since it can inactivate microorganisms even at room temperature and has the potential to provide safe food with extended shelf life.

High pressure has only minor effects on quality related factors such as vitamin content, colour, and flavour, which are thus preserved in pressure-treated foods. Pressure does affect the conformation of large molecules including proteins and polysaccharides in a way that is different from the effect of heat, so the functional properties of food components can be changed by pressure leading to novel types of food (San Martin et al. 2002; Ledward and Mackey 2002).

HHP processing is currently a technology with high capital but with low running costs. The technology is basically very simple consisting of (1) a pressure vessel of cylindrical design (2) end closures with (3) a means to hold them in place under pressure (e.g. yoke, threads or pin), (4) a low pressure liquid pump which generates high pressures via a pressure intensifier and (6) system controls and instrumentation (Farkas and Hoover 2000). Pressure vessels capable of withstanding 400 MPa can be constructed from two concentric cylinders of high tensile strength steel the outer member of which compresses the inner so that the system is always under slight residual compression when under operation. Wire-wound outer cylinders allow operating pressures up to 700 MPa over 100 000 cycles. For safety reasons pressure vessels are designed so that the inner cylinders crack and allow leakage to relieve pressure before catastrophic failure occurs. A 75 kW pump can raise the pressure of a 50 litre vessel to 700 MPa in about 3 to 4 min. Most commercial processes operate at dwell times of 5 min or less.

Commercial scale processing rigs operating up to 700 MPa typically cost between £280 000 and £1.5 million depending on capacity and the extent of automation. A 215 litre batch system has the capacity to produce about 4.5 million kg of food per year and factories with multiple rigs are now achieving production rates of 18 million kg per year. Semi-continuous systems are also available for processing liquid products such as juices. Production costs are currently 4 – 12 pence /kg more than for heat-treated products.

Heat-sensitive products are ideal candidates for HHP processing as exemplified by the highly successful marketing of pressure-treated guacamole in the US. Pressure treatment was initially investigated as a means of inactivating pathogenic vibrios that are a risk factor in fresh oysters, but had the unexpected additional benefit of opening or `shucking' the oysters, normally a labour intensive and unpleasant job. These combined benefits have led to a very successful product. Acid foods, which do not permit growth of Clostridium botulinum, are also good candidates for pressure preservation and, in Europe and elsewhere, pressure-treated fruit juices and smoothies are on sale that have extended shelf lives but retain the freshly squeezed fruit flavour. Pressure-treated foods stored under refrigeration with extended shelf life can be considered as second-generation products. These include delicatessen cured meat products and tapas. Possible candidates for a third generation products will be shelf-stable low acid foods where pressure-induced adiabatic heat is used to deliver a more precise thermal treatment. Process temperatures of 90C-110C in conjunction with pressures of 500 to 700 MPa are used to inactivate spore-forming bacteria, providing conditions to achieve commercial sterility.

In the future, as equipment costs are reduced and with better understanding of the effects of pressure on food components and microorganisms, this emergent technology has the potential to develop as an alternative to conventional processes and to provide opportunities for the development of novel foods. However deciding on the processing requirements needed to inactivate microbial pathogens foods with an adequate margin of safety, remains a largely empirical process. Commercialisation of high pressure technology would be helped if mathematical models were available that could predict microbial inactivation at different pressures and allow process criteria and resistance parameters to be defined. Unfortunately the inactivation kinetics of pressure-treated microbes seldom conform to the classical first-order type process seen in inactivation by heat and radiation, and survivor curves typically show a decrease in inactivation rate with time leading to pronounced `tails' (Farkas and Hoover 2000). Modelling such curves is problematic and interpretation of the underlying basis of the curve shape is controversial. The conventional view is that differences in survival times within bacterial populations are stochastic and related to the probability of a bacterial cell receiving a lethal `hit', whereas the currently popular `vitalistic' interpretation of such curves assumes that differences in survival times are the result of differences in pressure resistance among cells of the population. The latter approach has been applied using a range of probability distributions (e.g. Weibull) but the model parameters often do not vary in a regular way with pressure.

In this work we obtained inactivation curves for Escherichia coli NCTC 8164 at pressures of 250, 300, 400, 450, 500, 600 and 700 MPa at room temperature. The curves all showed an initial rapid rate of inactivation followed by a phase of decreasing death rate. A new model was constructed, based on a thermodynamic first order kinetic approach that could nevertheless describe such non log-linear inactivation kinetics. The model assumes a first-order process in which the specific inactivation rate changes as a function of the square root of time. The model would be consistent with a mechanism of inactivation dependent on a diffusion-limited process. The model gave reasonable fits to experimental data over six to seven orders of magnitude. It was also tested on 140 published data sets and provided good fits in about 70% of cases in which the shape of the curve followed the typical convex upward form. In the remainder of published examples, curves contained additional shoulder regions or very extended tail regions. These more complex shaped curves were not fitted by the model but curves with shoulders could be accommodated by including an additional time delay parameter.

The model parameters varied regularly with pressure which may reflect a genuine mechanistic basis for the model. This property also allowed the calculation of (a) parameters analogous to D and z used in thermal processing, and (b) the apparent thermodynamic volumes of activation associated with the lethal events.

The data used to test the new model came from a range of different microorganisms and suspending media (including foods), and the model test must therefore be regarded as robust. Most of these data were generated using small-scale laboratory rigs but scale-up is less problematic than with heat treatments because of the isostatic rule that states that all parts of the vessel will experience the same pressure, unlike the situation with heating, where temperature gradients are generated. Most published inactivation models are based on empirical correlations whereas this one has a more mechanistic basis. It is the first such approach and will engender greater confidence in the design and operation of pressure-driven processes.


Food Engineering Committee
The Food Engineering Committee is one of the 5 technical committees which report to the Process Industries Division Board.


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