B Y R. W I L E S

Development Technologist, Milk Marketing Board, Thames Ditton

The dairy industry has been instrumental in developing UHT processes and aseptic packaging. However, different standards may be necessary for foodstuffs other than milk. Products, their development and preparation prior to processing using existing technology and equipment are outlined. Heat treatment, commercial sterility and enzyme survival are discussed. The value and cost of proving aseptic systems are discussed together with suggestions for aseptically linking process and packing plants using aseptic tanks or otherwise. The accurate evaluation of equip- ment prior to purchase is stressed. A conclusion is drawn that the exploitation of UHT products is dependent on the development of truly aseptic packing machines.

This paper will concentrate on areas which ex- perience has shown must be given special attention during the development of UHT pro- ducts and the design and commissioning of aseptic handling equipment.

UHT and similar continuous heat treatments have firm roots in the dairy industry largely be- cause the UHT process is a natural extension of high temperature short time pasteurization and because of the comparative lack of flavour change in traditional dairy products while bestowing long-life properties.

The food industry in general is adopting more and more the UHT process with subsequent aseptic packaging because of the benefits of continuous flow production, more precise heat treatment and the increased choice of packaging. The UHT process is now largely synonymous with plastics or laminated packaging materials, which, particularly when thermo-formed give a wide range of pack choice thus giving a new dimension to product development at minimum cost.

The dairy industry has in many respects set the pace in aseptic processing and packaging. There has been a willingness to accept new technology while maintaining very high standards of product and pack quality. High standards have to be set when UHT processing and aseptically packaging products of neutral pH.

Milk has a natural resistance to the growth of some food poisoning organisms or the production of their toxins and the dairy industry has not been troubled by several food poisoning organisms that are of grave concern to the food processors, and I know of no authenticated case where long- life milk or cream has been the proven cause of food poisoning.

When products are considered which have

lower and lower levels of dairy ingredients the protection from natural inhibitors in the milk is reduced and we enter a field of food processing where the traditional practices of the dairy in- dustry may no longer be adequate. Different criteria may need to be considered if a range of long-life products are to be produced which are commercially sterile and commercially safe.

Products Milk has already been dealt with in the course of this symposium so I have taken products to refer to the following:

(I have classified products into four generations.)

First generation Cream - In its various categories. Single phase homogeneous products - custards, flavoured milks.

Second generation Here there are two phase products of two types: Tuio phase products - Having a homogeneous matrix with suspended small particulates such as fruit puree, or deseminated vegetables. Tnio phase products - Having a homogeneous matrix with large particles such as soups with meat pieces and vegetables or desserts with fruit segments.

Third generation Products of the first or second generation but including

entrained gas -Mousse Whipped cream Fruit fool

Journal of the Society of Dairy Technology, b'ol. 30, No. 3, July, I977 1.51

Fourth generation Any number of products from generations 1-3 combined by in-line mixing. A multi-throw fill will give a layered effect and dual filling will give ripple or similar effects in the final container.

Product preparation Once a product concept exists that is approved by marketing, recipes should be prepared and processed on commercial scale plant as a priority. The development of new products to high levels of sophistication in the laboratory or experi- mental kitchen is counter-productive. I t is ex- tremely unlikely using the same recipe that the product emerging from a commercial process and packing plant will resemble the handmade or pilot scale product, and the effort put into finally tuning a product for viscosity and texture or even colour and flavour will have to be re- peated to re-establish optima when commercial plant is used.

The changes which can take place on com- mercial plant are not entirely due to UHT heat- ing although this is often the cause of texture and flavour changes. The effect of large-scale pumps, passage down pipework, differential rates of heat exchange and change in temperature profiles all have their effect. Even the type and size of mixer and mixing vats for the raw materials can have a very marked effect on the final product. I t is often found that different levels or even different types of ingredients have to be used if the commercially produced product is to match the target product. Furthermore it is extremely difficult to simulate a UHT process in the laboratory and this again indicates the use of commercial plant at a very early stage of development.

Unfortunately it is becoming more and more essential when formulating products to make them compatible with existing and proven process- ing equipment. Experience shows this is necessary not only when old plant is being re-used to pro- duce a new generation of products but also in new installations. Engineers and food techno- logists live in separate camps and more should b: done to integrate these disciplines.

There are two concepts for the production of long-life products by UHT treatment.

Either: Batches of product are prepared, mixed,

partially or wholly cooked all using con- ventional techniques and equipment. Finally the product is continuously heat treated to give it Iong-Iife properties before packaging aseptically. Or:

A wholly continuous operation in which the

UHT or heat treatment plant is part of a more fully automatic system, here there will be in-line metering of ingredients and in-line mixing. The product will be cooked and sterilized in a con- tinuous operation by adjusting the time and temperature profile of the heat treatment plant. The product is then under aseptic con- ditions textured, whipped, homogenized, etc. then packed aseptically. The latter system is more difficult only in that

it requires close co-operation of several disciplines including engineers and food technologists. The ultimate result can be a more satisfying, more effective, more economical, complete processing unit. This is even more worth while if the plant is constructed largely from basic units. It can then be re-arranged when the market life of a product is over to give a new plant and a new pro- duct. This overcomes the problem of what to do with equipment with a 10 to 15 year life that has outlived a product life of 3 to 5 years. Flexibility of operation should be the keynote of plant designed to handle products for UHT treatment and aseptic packaging.

Whichever processing concept is being used the most important aspect of product preparation, next to actually measuring correctly all the ingredients, is the mixing. Mixing must be at the optimum temperature or temperatures and be sufficiently vigorous to ensure thorough incor- poration of trace ingredients such as stabilizers, emulsifiers, and dyes, yet must not cause undue damage to any particulates or sheer damage to components such as starches and gums. In many cases the incorporation of air bubbles during mixing must be prevented otherwise rapid burn on of product or even air locks can form in the UHT plant. Incorporated air also gives rise to flavour and shelf life problems in the final pro- duct. The most versatile of our mixers and there- fore the most useful is one which will operate on 15-150 gal of product. It is fitted with a central stator and an anchor stirrer which has scraper blades to sweep the product from the sides of the bowl. The stirrer can be run at variable speeds.

There is a high sheer or emulsifying head fitted into the base of the vessel to give extremely vigorous mixing. This vessel finds many additional uses but we find i t invaluable for product de- velopment. It has been used to simulate experi- mentally many processing techniques. It can be steam sterilized and used as an aseptic storage or buffer tank. I t can be used to batch sterilize pro- ducts by steam injection and vacuum cooling. Products can also be sterilized by the use of the steam heated jacket. This jacket is also connected to a chilled water supply so that indirect cooling can take place. Vacuum facilities make possible the degassing of products.

152 Journal of the Society of Dairy Technology, Vol. 30, N o . 3, July, 1977

Product processing The following is a list of processing methods which we have used experimentally, some of which are in wide use commercially:

(a) continuous flow over plates with indirect

(b) steam injection with vacuum cooling. (c) scrape surface heat exchange. (d) steam injection without vacuum cooling -

here the condensed steam stays in the product as one of the ingredients. This operation requires the use of culinary steam.

(i) one (minor) part of the product is UHT treated and held aseptically. Then in- line mixed with the bulk of the product which is continuously UHT processed prior to packaging.

(ii) one part of the product is batch or vat sterilized and then metered into the continuous flow of the remainder of the product from a UHT plant.

( f ) Steam injection or infusion - here the pro- duct is treated in bulk in an atmosphere of pressurized steam. Liquid products can also be treated in thin films in a steam filled vessel.

(g) Hybrid systems - here any combination of (a) to (f) can be used such as batch cook- ing, plate pre-heating, steam injection or infusion and scrape surface cooling i f the products become viscous.

All proposed installations should be carefully thought through by production and marketing personnel as well as by designers and engineers. The final plant must be as simple as possible yet capable of easy operation and adequate to pro- duce a final product that the marketing people can sell.

Caution is again advised that the plant is not too specific and therefore re-usable if the product range is to be extended.

heat exchange.

(e) Two stream UHT process

Heat treatment Whichever installation is finally chosen the most important technical aspect is that of obtaining a commercially sterile product. Despite criticism of the term ‘commercially sterile’ I have used it here because it is an apt description of what is to be achieved.

We in the Development Department of the MMB heat treat to give an Fo value of 3.0 in the UHT section of a UHT plant when treating milk or cream. Experience has shown, however, that more sophisticated products, especially those containing particulates, can achieve optimum flavour, texture, colour, etc. at heat treatments

Journal of the Society of Dairy Technology, Vol. -?O, No. 3,

other than those that give a botulinum cook. The technique used in the MMB to determine

the necessary heat treatment to give a botulinum cook is by extrapolating conventional canning data. This technique can be criticized but we have used it for many years and experience has shown that it can be a very good starting point for determining heat treatments.

Acidic products with a pH of 4 5 or less do not require the same heat treatment as neutral pH products to obtain an equivalent micro- biological shelf life. Similarly some products, for example, cream; have been shown to have natural inhibitors against certain spoilage organisms. In- creasingly it is being reported that enzymes pre- sent in raw milk are surviving some UHT processes. These enzymes can persist in cream or other products utilizing milk and it is reported that some of these enzymes which are proteolytic require seven times the heat treatment to destroy them as is required to give a commercially sterile product. In addition to the enzymes present in raw milk there will be enzymes or precursors present in particulates such as fruit that will sooner or later during the shelf life of the product adversely affect dairy ingredients unless rendered inactive. These examples indicate that a particular product will only have the optimum organoleptic and physiochemical stability over a reasonable shelf life without showing signs of microbiological spoilage if the heat treatment is carefully selected and specific for a particular product and a particular plant.

With a composite or two phase product it may be desirable to give differential heat treatments. A fruit/syrup phase can be high heat treated to sterilize it and destroy enzymes or give a cooked flavour while the dairy fraction of the product is given minimal heat treatment to sterilize it with- out impairing its Aavour properties. The two differently heated phases of the final product can then be in-line mixed or filled using a multi-throw technique under aseptic conditions.

Packaging Now let us consider aseptic packaging, in parti- cular the principles of an aseptic packaging system so that we can understand what is needed in a practical or commercial situation.

Primarily we need an enclosure, the inner surfaces of which together with any components such as the filler and mechanical supports must be capable of sterilization. Thereafter anything entering the box must pass an effective micro- biological barrier. This includes the product, packing materials, air, etc.

Any number of operations, material introduc- tions or extractions can be made provided this simple rule is followed.

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Commercial aseptic packing machines Two very critical areas of a commercial aseptic packaging installation are the filling and sealing.

(a) Often fillers are intermittent in operation but the packing machine must be capable of dealing with the continuous flow from the UHT plant. An aseptic buffer tank may be the only answer.

(b) The filler must be sterilizable and steam is the first choice for sterilization. Chemical sterilization of process equipment is second best. For steam sterilization the materials of construction of the filler must be capable of heating to 15OOC for 30 minutes or so and yet capable of operating at a product filling temperature of 10 to 5OoC.

(c) The filler must be capable of being cleaned in place. Indeed all parts of a sterile/aseptic packaging system must be capable of thorough and effective CIP. Hand washing is not acceptable as it lays too heavy a burden on the operator to ensure a satis- factory final product. I t must be perfectly understood that only clean surfaces can be sterilized effectively.

(d) Fillers must be capable of giving a desired fill at the correct rate to match the speed of the process and packing machines. Long textured or very viscous products are often handled more slowly by a particular filler than short textured or less viscous pro- ducts. This difference in performance of the filler can be evident when handling milk compared to handling cream. At the same temperature and settings a specific filler will perform quite differently with these two, on the face of it, very similar products. Malfunctions of a filler often result in drips and spillage. The integrity of heat sealed containers can be seriously affected if there is product between the surfaces prior to heat sealing and there- fore product on the seal area must be prevented.

(e) There should be fillers or change parts available for a particular machine which are capable of dealing with particulates. Progression can be expected through generations of products similar to those which were outlined at the start of my paper and installations of a specific system that will handle simple products very well but will in no way cope with sophisticated particulate products, could well be a step in the wrong direction and reduce the ulti- mate flexibility of a particular plant. Again contact between food technologists, sales or marketing personnel and engineers is vital if the correct plant is to be purchased

154 Journal

and installed and the correct final product range produced.

(f) Most fillers on aseptic or claimed aseptic machines are of a volumetric type often but not always piston operated. Whilst it is possible to use a volumetric filler for homo- geneous liquid products, a volumetric fill will not give reliable pack weights if there are two phases in the product such as liquidlair or liquidlsolids which have dif- ferent specific gravities. There could there- fore in the near future be a need for an aseptic filler that gives an accurate weight fill.

(g) Fillers in general and packing machines in particular should be capable of interrupted operation without losing sterility.

When considering seals I think we must all accept that the double seam on the conventional can has proved ideal as a hermetic seal. Never- theless such containers are difficult to open, there is restriction on the shape of conventional cans and by using a can one is immediately giving the impression of a conventional product produced in a conventional way. There is a need therefore, I believe, to move away from this type of packag- ing if the true benefits of UHT treated and aseptically packed products are to be fully ex- ploited. The easily opened or peelable seals that one associates with a rigid plastics container have many attractions and this form of packag- ing is bound to come to the forefront. Peelable and easily opened heat seals can be as effective as double seams in preventing product contamina- tion. Indeed they have already been proven satisfactory on some long-life aseptic products.

Linking the processing and packing plants Technically the most important feature is that the link should be capable of effective steriliza- tion. Process plants are usually sterilized by pressurized hot water on a return loop to the feed vessel or balance tank. Often there is in- sufficient heat available from this source to sterilize the packing machine especially if the filler has a large thermal capacity. Therefore a shut off valve usually of the diaphragm type is incorporated in to the line and a T runs from this loop to the packing machine; steam introduced into this T is used to sterilize the filler, the link line and the aseptic tanks if these are included. The process and packing machines are sterilized simultaneously and one relies on heat transfer across the shut-off valve SO that the temperature of this is raised to the sterilization temperature of the system, and sterilization is therefore effected. After sterilization the system is cooled by sterile water from the UHT plant prior to in- troducing the product. This principle can also be

I of the Society of Dairy Technology, Vol. 30, No. 3, July, 1977

Fig. 1. Mobile aseptic tank

used when linking two process plants for dual filling or aseptic in-line mixing.

During the commissioning of new installations and probably during commercial production an aseptic storage or buffer tank may be required to link or indeed separate process and packing plants.

Ideally both for sterility and simplicity of operation the best policy is to match the rate of the process plant exactly to the rate of the pack- ing plant thus dispensing with aseptic tanks. This can be easily achieved if the packing or process- ing plant or both have a variable through-put. The earIy technique of over production and bleed off of excess product which the filler cannot handle has been superseded by this technique with resulting reduced product wastage. Some products cannot be recirculated through the pro- cess plant, other products cannot be held in storage tanks after processing because they gel or deteriorate or do not cool fast enough and suffer the equivalent of stack burn in cans.

With products which are a little more accom- modating aseptic storage tanks may be indicated. Once a tank has been proved to operate aseptic- ally it can be used to isolate a process plant from its packing machine thus enabling the identifica- tion and source of any contamination which may occur. Incubating product in bulk between pro- cess and packing is the only reliable way of aseptically isolating process and packing plants

and thereby assessing their individual microbio- logical merits. A piece of equipment used in the Development Department of the MMB is a 200 gal aseptic tank which, being mounted on a road trailer, is mobile (Fig. 1).

This has now been proved over some 4-5 years and many successful operations. It can be used to link effectively process and packing systems which are on separate sites many miles apart. To do this the tank is sterilized and aseptically filled with product or microbial growth medium (or skimmed milk) from a UHT plant. The product is then incubated for several days and after sampling for sterility is discharged under a pressure of sterile air to an aseptic packing machine on the same site or elsewhere.

Use of equipment such as this also means that the commissioning or proving of new plant can be taken out of the dairy and put back into the factory of the manufacturers where it belongs. Real determination can effectively overcome most problems there may be in proving machines prior to purchase and most companies (particu- larly British) co-operate readily in such proving trials.

Machine evaluation It is strongly recommended that any machine or machines are proved to do the duty which is required prior to purchase.

If exaggerated performance claims have been taken at face value a processor could eventually be faced with the choice of abandoning a project representing El ,000,000 or more or alternatively attempting to market a product which is of a much lower standard than initially envisaged.

This could lead to a situation where poor or even dangerous products get onto the market. Were this to happen there could be a setback in the development of UHT products and aseptic packaging for many years to come.

Detailed and thorough investigations of machines are difficult but one must insist that decisions are based on accurate and relevant information. Relying on experimental data can be hazardous, such information whilst made in good faith and accurately recorded may have been obtained on pilot scale or laboratory equip- ment and may not be repeatable in a commercial installation. It may not therefore be relevant to the conditions in the UK market to your factory or your distribution system.

The results of trials must be accepted, there can be no mitigating circumstances for poor results. A machine that cannot be demonstrated at will to give an adequate performance in the manufac- turer’s workshop or in a factory where it is currently operating cannot be expected to give an adequate performance in your own factory.

Journal of the Society of Dairy Technology, VoI. 30, N o . 3, July, 1977 155

It is easy to see that the potential user of aseptic processing or packaging systems may be in a dilemma, the evaluation of a particular machine may take 12 months and cost E30,OOO or more. The final report may show that the system is not suitable for a particular applica- tion and cannot be modified to make it so. In such cases the only return on expenditure is the knowledge that a particular process or packing machine is not appropriate to a particular applica- tion. Is the cost therefore of carrying out such investigations justified? I believe it is if a particular project or operation is not to become a white elephant or bring disrepute onto the dairy industry.

In the final analysis the success of an aseptic packaging system is measured by the number of customer complaints. It is known from good com- mercial canning practice that of the order of 1 can in 10,000 may be non-sterile and this has been used as a standard for the aseptic packaging of UHT treated products. A quick calculation will show that if 1 in 10,000 containers is non- sterile then for a particular installation where say 40,000,000 units are produced annually one can expect to get 4,000 non-sterile containers or 77 dissatisfied customers per week from this source alone.

Is 1 in 10,000 therefore good enough?

Quality control I wish to mention quality control here although the subject is being covered elsewhere in the sym- posium. Once a product has been processed and packaged it is too late to carry out any quality

control, particularly on asepsis. Knowing 10 to 14 days after the product was manufactured and packed that it was not sterile is of no use whatso- ever. In addition the storage of 14 days produc- tion is prohibitive in space and cost. A processor of aseptic, long-life or UHT products must ulti- mately rely as do the conventional canning people on strict and accurate control of the plant, the process and the product preparation condi- tions. Product once packed is then fed directly into the distribution chain without holding and without testing for sterility.

I have dealt at length with the evaluation of aseptic packaging systems during this talk be- cause this is currently the critical path in the exploitation of UHT treated products. It may be that UHT treated and aseptically packaged pro- ducts will not initially appear on the market in their final form. Initially short-life products may be processed and packed using aseptic tech- niques. Then, with increasing confidence and improved technology such products will graduate from chilled to ambient distribution. Again with increased experience and growing confidence the shelf life can be progressively extended to give finally i?. truly long-life product with a shelf life of 12 months or more.

For many years there has been a range of ac- ceptable UHT products but their exploitation has been curtailed by the lack of suitable aseptic packaging equipment and this is the area where I believe development must be concentrated if UHT products are to be fully exploited.

156 Journal of the Society of Dairy Technology, ?701. 30, No. 3, July , 1977