Establishing Aseptic Processing Conditions for Liquid Foods Containing Particulates

Neil Heppell Oxford Brookes University Oxford, UK

INTRODUCTION

As a food preservation method, ultra-high temperature (UHT) processing is a well established alternative to in-container sterilisation (canning): the first processing equipment for milk was developed around 1908 and followed by development of aseptic filling systems in 1921. The process is relatively simple in that the foodstuff is pumped through a series of heat exchangers up to a relatively high temperature (around 140°C) and held in a holding tube for a few seconds until sterile, then cooled and packaged under aseptic conditions.

Since the 1950s, UHT processing has been frequently used for liquid foods and liquids containing fine comminuted solids, such as milk, fruit juices, teas and soy milk and, more recently, a slowly expanding range of more viscous foods, such as evaporated milk, ice cream mix, custard, tomato products, soups etc. Recently, though, there has been an increasing interest in processing liquid foods which contain solid particulates e.g. soups, stews, cook-in sauces, meat sauces (e.g. Bolognese, Chilli con Carne sauces) and even whole cooked spaghetti. Terminology is a problem; the term High Temperature Short Time (HTST) processing is usually used for pasteurisation processes, UHT is usually applied to sterilisation of liquid products but when applied to liquids containing solids, aseptic processing is more common, especially in the USA. One term covering all processes is continuous-flow thermal processing, whether for sterilisation or for lower levels of heat treatment.

ASEPTIC PROCESSING

The principle of UHT processing is well known in that, due to the difference in temperature dependency between sterilisation (microorganism death kinetics, with a z-value of around 10 °C) and chemical degradation kinetics (z-values around 30°C), the higher the process temperature for the same level of sterilisation (Fo value), the lower the chemical change in the product, i.e. the lower the loss of vitamins, degradation of organoleptic quality and loss of texture. The thermolabile vitamins are generally taken to be A, B1, B2, B12, C, D, E, nicotinic acid, pantothenic acid, thiamin, folate and Biotin, but there are many complicating factors and vitamin-vitamin interactions which complicate the situation. In addition, cooking kinetics also change less quickly with temperature, (the z value is about 33 °C), and so high processing temperatures make it possible to produce a product that is sterile but undercooked, which allows for a degree of cooking by the consumer as they reheat the product. Natural enzymes in the foodstuffs may, however, survive the process and a blanching operation may be required.

The majority of advantages of UHT or aseptic processing over in-container processing lie in the improved organoleptic quality in all aspects (table 1). Disadvantages, however, include the higher cost of processing, mainly due to the increased cost and relatively low throughput of aseptic filling systems. Previous experience has shown consumers to be uninterested in buying products manufactured by new, 'exciting' technology (and may even put them off the product) but will only be attracted by the advantages they themselves can detect and are prepared to pay for. It is very important, therefore, that the maximum advantages of the technology listed above are obtained, otherwise no-one will be prepared to pay the extra cost and the technology will become obsolete.

The aseptic processing of liquid foods containing solid particulates adds extra technical complications over a UHT process for liquid foods. In all conventional aseptic processes, the liquid phase is heated first and the heat must then transfer to the solid particulates as they are transported by the liquid. The heat transfers from the liquid, through the boundary layer around the solid particulate (a layer of static fluid present at any liquid - solid interface) to its surface, then through the solid to its centre, which is the point with the least heat treatment and around which the process should be designed. The size of the boundary layer depends on turbulence in the liquid and the rate of flow of liquid past the particulate, transporting it, as well as any other factors which may disturb the liquid flow, e.g. rotation of the particulate, pipebends or other hydrodynamic factors.

The two major problems to be resolved are:

• residence time distribution of particulates in different parts of the food processing equipment and
• the heat transfer rate between the heated liquid and the solid particulates as they are transported by the fluid.

Other problems include the pumping of these foods to about 4 bar and releasing this pressure after cooling without particulate damage and in the design of aseptic filling systems to cope with these foods.

There has been considerable research work in residence time distribution and liquid-particle heat transfer over the last twenty years.

RESIDENCE TIME DISTRIBUTION

The residence time of solid particulates in the different sections of the processing plant is very important, especially when compared to the liquid phase, in terms of the average residence time, and also the minimum residence time. The minimum residence time is of importance in ensuring microbial safety of the product, while the average residence time (and the spread of residence times) are important in determining the quality of the food produced. There are many factors affecting both residence times, especially the liquid viscosity, velocity, flow regime, fluid/solid relative density, particulate shape and size, pipe diameter and the particulate-liquid ratio. Methods used to measure residence time distribution include direct observation with dyed particulates, time-of-flight, magnetic particulate sensors and Hall-effect sensors¹. Residence time distribution of the liquid phase alone can be measured by chemical or dye tracer2. It is important that any tracer used in the particulates does not affect the particulate characteristics, especially the density in the case of magnets.

For flow in holding tubes, there is a minimum holding time that can be calculated from fluid dynamics, depending on the viscosity behaviour of the liquid. The particulate is unlikely to have a shorter residence time than the shortest liquid residence time, except where there is a sudden change in velocity of the fluid. It is possible to design the system around this minimum residence time, as a worst-case, and the food product will be at least safe. If the actual residence time distribution were known, the process could be designed with more accuracy and an improved organoleptic and nutritional value would be obtained².

LIQUID-PARTICLE HEAT TRANSFER COEFFICIENT

The measurement of the liquid-particle heat transfer coefficient poses an interesting question viz. how to measure the temperature within a solid particulate being transported by a fluid. It is not very representative to hold a particulate statically and pump liquid past it, nor to influence the movements of the particulate by inserting a thermocouple into it or other logging device which will alter the density and hydrodynamic behaviour of the solid, though these techniques will give some idea of the heat transfer. These and other techniques which have been used are reviewed by Sastry and Cornelius1 and include the immobilization of a thermolabile marker, e.g. spores or enzyme, Magnetic Resonance Imaging or the use of thermochromic paint, which changes colour depending on the temperature and can be tracked in a transparent system by video camera.

As with residence time distribution, there is a 'worst-case' situation, one in which the cold particulate is immersed in a hot static liquid and this minimum heat transfer rate can be calculated. In the real situation, however, the liquid (together with the turbulence or shear in it) flows past the particulate as it is transported along, and the particulate itself may rotate, all of which would improve the heat transfer substantially.

PROCESS VALIDATION

One area of special interest is the development of time-temperature integrator (TTI) particulates, which can be used to validate the heat process given to the particulate in the food in the absence of any other method of direct measurement of temperature within the particulate. The aim is to find a suitable marker, microbial spores, a first-order chemical reaction, or other reaction which has approximately the same kinetics as microbial sterilisation, and to entrap this in a solid particulate which can then be passed through the sterilisation system within the foodstuff. The particulate is then recovered and the change in marker measured, which can then be converted to a Fo value, or other sterilisation measure. Their use has been reviewed by Van Loey et al³.

Early techniques used microorganism suspensions sealed in glass spheres but now the use of the calcium alginate technique is common, in which microbial spores are mixed with sodium alginate and set to a solid particulate of suitable shape using calcium chloride. By incorporating other comminuted foodstuffs with the spores, e.g. chicken, pea flour etc, a simulated food particulate can be made. The particulate is robust enough to survive passage through heat exchangers and can also be dyed to aid identification in the bulk of product. The particulate can be dissolved after processing using sodium citrate and the surviving spores plated and enumerated. Enzyme or other chemical markers may also be used.

CONCLUSIONS

It is certainly feasible to produce a microbiologically-safe aseptically-processed food containing particulates using the minimum residence time distribution and heat transfer coefficient values obtained by calculation, and it is also possible to verify the product's safety using the calcium alginate technique. However, the quality of products produced under these circumstances will almost certainly be poor and over-processing will be relatively severe. Even small improvements in the liquid-particle heat transfer coefficient will dramatically reduce the residence time required for sterilisation and improve the organoleptic quality. The work on residence time distribution and liquid-particle heat transfer coefficient are very important in obtaining the very best quality product possible and maximizing the appeal to the consumer. Strangely, there appears to be little published research work on the sensory aspects of aseptically-processed products as compared to in-container sterilisation, or in quantifying the improvement in organoleptic or nutritional qualities. Little data is available on the thermal degradation kinetics of micronutrients at elevated temperatures.

REFERENCES

1. Sastry, S.K. & Cornelius, W.D. (2002), Aseptic Processing of Foods Containing Solid Particulates. Wiley Interscience, New York, USA.
2. Lewis, M.J., & Heppell, N.J. (2000)., Continuous Thermal Processing of Foods. Aspen Publishers, Inc., Gaithersberg, Maryland, USA.
3. van Loey, A. Hendrickx, M., DeCordt, S., Haentjens, T. & Tobback, P.(1996). Quantitative evaluation of thermal processes using time- temperature integrators. Trends in Food Science & Technology 7 (1) 16-26.

BIOGRAPHY

Neil Heppell is Senior Lecturer in food engineering at Oxford Brookes University, and has been working on UHT and aseptic processing since 1979, at the National Institute for Research in Dairying, Reading and Leatherhead Food RA. His current research interests are in the nutritional implications of food processing and in micronutrients.