The basic features of systems using returnable containers are the collection of empties and washing prior to re-filling. Differences in operation times and capacities of the various machines involved make intermediate storage necessary. Storage of unwashed empties is normally essential and may extend overnight so that washing and filling operations can begin next morning before the day's supply of unwashed empties arrives. Storage of washed cans is permissible as they have lids but storage of washed bottles is extremely bad practice because they are unsealed and therefore liable to contamination. Normally storage must be provided for filled cans and bottles to give flexibility in the distribution arrangements. For pasteurized milk this must be refrigerated. The requirements are shown schematically in Fig. 4.
Fig. 4 Sequence of operations with returnable containers
Bottle washing, filling and capping machines should be of matching capacity, otherwise the labour-intensive operations of decrating and crating, as well as unstacking and stacking, would have to be repeated unnecessarily. This problem does not arise with cans, since they are not crated and may be easily stored empty after cleaning.
The required storage area, both for empties and for product, depends on the operation schedule of the plant which in turn is affected by the relation between required dispatch capacity and the capacity of filling machines in operation. The inter-relations between these factors are shown in Figs 5 to 8. As can be seen, the required storage areas may differ considerably depending on the arrangement of working time, type of packaging and capacity of equipment. Although no attempt is made to present here precise examples of operation schedules, some particulars of the operations are indicated on the diagrams. For instance ‘dispatch’, which represents on the diagram the loading time in the milk plant, should be completed about three hours before the end of the last transport shift, i.e. it can last either five or thirteen hours, otherwise the vehicles would not return to the plant before the end of the shift. Similarly, the collected empties will not arrive at the plant at the beginning of the shift, since time is required for collection. Thus empties reception can last also either five or thirteen hours.
In this model study no consideration is given to situations where more than one product is manufactured and kept in the same store (such as liquid milk in a variety of packaging and sizes, different grades, etc). Very often quantities wanted by a particular retail centre may not represent an integral number of full crates or cans. The additional area required for marshalling such loads is not included in this analysis. In some markets the daily quantities sold may differ at week-ends and holidays or have peaks which the stores have to balance as far as the availability of milk permits such operations. Such facilities also could not be considered in this study.
Pasteurized milk in bottles. Bottles with wide necks (36 to 40 mm), suitable for sealing with aluminium foil caps made in situ from reeled strip, form the most common system for packaging of pasteurized milk in returnable containers. The bottles are placed into crates, formerly made of galvanized steel wires or strips and nowadays usually of plastic. The crates have internal divisions so that the bottles are not in contact with one another to minimize risk of breakage. They are designed to interlock, so that a stable stack can be built. For manual handling crates with filled bottles are stacked five and six high for one-litre and half-litre bottles respectively; with empty bottles two to three crates more are put in one stack. Crates usually hold twenty half-litre and twelve to fifteen one-litre bottles.
The stacks can be palletized (Standard Europallets 800 × 1 200 mm are applicable) and moved by fork lift trucks or manually or by hand trolleys without palletizing. There are no international standards for the dimensions of pasteurized milk bottles and crates. However as an example Fig. 9 gives details of those in use in the Netherlands.
Fig. 5 Estimation of storage area requirements - suitable for low throughput of plants processing liquid milk in returnable containers
Fig. 6 Estimation of storage area requirements - suitable for higher throughput of plants processing liquid milk in cans
Fig. 7 Estimation of storage area requirements - suitable for higher throughput of plants processing pasteurized milk in glass bottles
Fig. 8 Estimation of storage area requirements - suitable for higher throughput of plants processing sterilized milk in glass bottles
Fig. 9 Dimensions of glass bottles, crates and stacks for pasteurized milk
The floor area occupied by one stack of crates is thus about 0.15 m2, equivalent to a milk storage capacity of 400–470 1/m2 depending on bottle capacity and stack height. For crated empty bottles the equivalent storage capacity is about 30% greater. Additional space must be allowed for access and this is discussed further in Chapter 3.
The initial high cost of a glass bottle prevents single-service use for pasteurized milk. The effective cost depends on the number of times the bottle can be re-used (trippage) which in turn is determined primarily by the effectiveness of the bottle recovery system and the ability of the bottle to withstand breakage. In the UK the weight of a 1-pint bottle (568 ml) is about 12 oz (340 g) and costs about 4.25 p (1977). In recent years a new design has been introduced weighing about 8 oz (227 g) costing about 15% less but giving the same, or only slightly less, trippage.
Sterilized milk in bottles. Bottles used for in-bottle milk sterilization have narrow necks (26 mm) so that a more effective seal can be made. Prefabricated crown seals are used to seal the bottles. These bottles must be able to withstand not only mechanical shocks during handling, but also thermal shocks during sterilization and, even more, during cooling. As the milk in the bottle is heated and expands during heating more than the bottle, the air above the milk becomes compressed and the pressure inside the bottle exceeds the external pressure. The contraction of the milk as it cools below the filling temperature results in a vacuum in the space above the milk. This vacuum may encourage contamination through the seal between the bottle and the cap. It is therefore important that the seals be fully airtight.
As can be seen from the above, the demands for a bottle for this process are particularly high, and it must therefore be heavier and more expensive. The bottles are packed in crates as in the method described for pasteurized milk bottles. The stacking system and the resulting storage area required are also similar. As with pasteurized milk, dimensions differ from country to country but a neck diameter of 26 mm is universal. Fig. 10 gives details of the bottles in use in the Netherlands. Crates and stacks in this case have the same dimensions as for pasteurized milk.
Fig. 10 Dimensions and weight of glass bottles for sterilized milk
Pasteurized milk in cans. The aluminium milk can has proved very satisfactory in service and, since the beginning of the second half of this century, has rapidly replaced the previously-used tinned mild steel can. In recent years high density polyethylene cans began to be introduced in a number of countries, but have not proved popular for various reasons. The most common are cans with lids which do not require rubber gaskets, an adequate seal being achieved with sunken grip or mushroom lids. Because of mechanical washing problems lids attached to the cans by chains are no longer used. Through simple arrangements at the lid ring lead or other seals can be applied to make the contents of the can pilfer-proof. The cans may be palletized, but more often floor conveyors are used. Full cans are stored in one layer, thus allowing about 320 to 360 litres of milk to be stored per square metre excluding access space. Empty cans, after washing, are stacked in layers horizontally, up to the height of 1.5. m. For storing and moving washed empty cans simple trolleys on which the cans can be stored in 4–5 layers are very useful; for instance, about 20 cans each of 40 litres capacity can be stored on trolleys about 1 700 × 700 mm with a supporting frame made of a 1/2" pipe. Some details are given in Fig. 11.
Fig. 11 Dimensions, weights and stacking of empty pasteurized milk cans
The common feature of single-service containers is that after emptying they are discarded. This fact has a significant impact on the milk plant construction, organization, and on the economics of the whole enterprise. There is no collection and washing of the milk packages - only crates are collected and washed, but even these may be replaced by single-service delivery wraps, trays or boxes. Palletization may be applied as in the case of returnable containers. Intermediate storage of packing material and filled packages is required and this must be provided in the plant.
Two basic types of single-service containers are considered in this analysis, i.e. cartons and plastic sachets. Cartons are usually made in one of the shapes illustrated in Fig. 12.
Fig. 12 Basic shapes for milk cartons
The material used in each case is polyethylene-coated paper-board; in aseptic versions an aluminium foil lamina is normally incorporated. Pre-formed cartons or pre-cut, single-piece blanks pre-creased ready for forming into a container must be used for the container with a gable top and these generally are more expensive. The other method of making a carton is to form a tube from a reel of material, seam it longitudinally, fill with milk and then make transverse seals. Alternatively the tube may be cut into lengths which are formed into cartons before filling and sealing.
Plastic sachets are usually pillow-shaped and made of low density polyethylene film. They may be reeled single or double film or lay-flat tube, the latter avoiding the necessity of making the longitudinal seam in the packaging machine. The material should be coloured to reduce light transmission.
The sequence of operations when packaging into single-service containers comprises forming the container, filling and sealing, storage of the packaged product and dispatch to wholesale and retail outlets. As with packaging in glass bottles the storage space requirements must be related to the process schedule (see Figs 5–8) but there are important differences. Only where returnable crates or pallets are used is space for returned empties required and this can be assessed by the method shown, for example, in Fig. 7. Otherwise there is no constraint on the beginning of the day's packaging operations. However, space must be provided for the paper or plastic stock and this is discussed further below.
For pasteurized milk the percentage of the day's output which will require storage can also be determined by the method shown in Fig. 7. However, this does not apply to UHT milk which must be stored at the milk plant for a minimum of 4 days after packaging while sterility tests on samples from the day's production are carried out. Thus, to allow for contingencies a product storage area equivalent to about 5 days production is essential. It follows that the times at which packaging begins and ends each day are not affected by storage considerations.
The distribution of food is changing rapidly in the industrialized part of the world. Traffic routes from processor to retailer are becoming increasingly crowded, sales through supermarkets are demanding changes in the systems of packaging and developments aimed at making this distribution feasible and manageable in large concentrations of consumers are taking place constantly. There is no doubt that single-service containers respond better than returnable ones to the demands of this modern trend. Milk distributed in single-service containers in returnable crates relieves the retailer from the necessity of collecting empties from the customer but the need for returning crates to the processor still exists. This may not create too many problems since empty crates may be collected by the processor at the time of milk delivery, provided two sets of crates are put in operation. A completely one-way packaging system requires that the single-service containers are wrapped in non-returnable material, thus forming transportable units. In this study, transporting of single-service containers in returnable crates has been analysed for pasteurized milk and in non-returnable wraps for UHT milk.
Pasteurized milk in cartons. Tetrahedral cartons made from polyethylene laminated paper board and packed in hexagonal plastic crates have been chosen as the model for analysis of a system of pasteurized milk packaging.
The cartons are produced continuously from a roll of plastic-coated paper which is shaped and sealed into a tube. The tube is filled continuously with pasteurized milk. Transverse seals are made alternately at right angles below milk level so that there is no headspace and the shape of the package is a tetrahedron. The packages are separated by guillotine and placed in hexagonal plastic crates holding 18 cartons each. The crates are stacked on pallets. The crates (and eventually also the pallets) have to be transported from the retail centres back to the milk processing plant. Table 2 gives data on the rolls of laminated paper as used for the system:
Table 2. Paper stock for tetrahedral cartons for pasteurized milk
| Rolls for one million cartons | ||||||
|---|---|---|---|---|---|---|
| width mm | weight of one roll kg | cartons per roll | No. | weight t | net space requirement m2 | |
| for 1/2 1 packages | 287 | 53 | 5 000 | 200 | 10.6 | 16 |
| for 1/1 1 packages | 362 | 72 | 4 000 | 250 | 18.0 | 24 |
The space requirements shown in Table 2 for the material required to make 1 million cartons must be considered in conjunction with the daily throughput and the material supply arrangements. For a plant packaging 10 000 1/day in 1-litre cartons, 1 million packages represents 100 days production. For a 250 000 1/day plant packaging in 1/2-litre cartons it represents only 2 days production. Depending on the location of the milk plant and the paper supplier and the most economic purchasing quantities it may be necessary to carry two to three months stock. Figs 27 to 33 give storage areas for packaging material equivalent to about 60 days stock for the 250 000 litre plants and 70 to 100 days for the smaller capacities.
Fig. 13 gives details of the dimensions of the cartons, crates and crate stacks chosen for this example. As these particular crates are tapered they will nest when empty so economising in space. (Rectangular crates occupy the same space empty or full).
Fig. 13 Dimensions (in mm) of cartons, crates and stacks for pasteurized milk
Thus with a 6-high stack of crates containing ½- litre cartons, store capacity without access is about 500 1/m2.
UHT milk in cartons. Rectangular cartons made from polyethylene laminated paper board in shrink-on wraps strengthened by corrugated cardboard trays have been chosen as the model for analysis of a system for aseptic packaging of UHT treated milk. The system represents a completely single-service milk distribution method, with no return transport to the milk processing plant, except for pallets if they are used for transport outside the milk plant.
The cartons are produced continuously from a roll of plastic-coated paper which is chemically and thermally sterilized before being shaped and sealed into a tube. The tube is filled continuously with UHT processed milk, after which the cartons are sealed below fluid level and formed into a rectangular shape. The cartons are filled completely and can be stacked.
Table 3 gives data on the rolls of laminated paper as used for the system:
Table 3. Paper stock for rectangular cartons for UHT milk
| Rolls for one million cartons | ||||||
|---|---|---|---|---|---|---|
| width mm | weight of one roll kg | cartons per roll | No. | weight t | net space requirement m2 | |
| for 1/2 1 packages | 322 | 60 | 3 500 | 285 | 17.1 | 24 |
| for 1/1 1 packages | 322 | 63 | 2 500 | 400 | 25.2 | 34 |
The storage space required for the paper stock is subject to the same considerations discussed above for pasteurized milk cartons.
Fig. 14 gives the dimensions of the cartons, individually and when shrink-wrapped in packs of 12, and of carton stacks suitable for Europallets. The storage space to accept 4 or 5 days production is not subject to critical requirements. It should be within the milk plant, though not necessarily in the same building as packaging, and of such construction that temperature, humidity and cleanliness are maintained within reasonable limits.
Fig. 14 Dimensions (in mm) of cartons, packs and stacks for UHT milk
Out of several distribution alternatives for the method by which the single-service containers are collated in transportable units, the most often applied are: (a) crating and (b) wrapping in non-returnable materials. Crating already has been analysed in relation to pasteurized milk in tetrahedral cartons. For one-way (non-returnable) transport packaging shrink-film is most often used. This wrapping can be applied manually or mechanically. When machines are used, the cartons are fed to the marshalling station and then wrapped in shrink-on film. After having passed through the shrinking tunnel the packages are loaded on to pallets and are then ready for distribution. This system is used where short storage times and short distribution distances are involved. However, the film does not give much mechanical support to the packages and cartons wrapped in shrink-on film are often subject to damage, leakage and contamination, particularly when handled manually without the use of pallets and other auxiliary equipment. Special (perforated) shrink-films may be required in areas with high humidity to prevent condensation. A much safer, although more expensive, system is that by which a group of twelve rectangular cartons filled with milk is placed first on a flat tray blank. The sides of the blank are then folded up tightly around the cartons so utilizing to the full the pressure-absorbing ability of the filled cartons. The packed corrugated tray is then wrapped in shrink-on film which not only affords protection against dust and moisture but also presses the sides of the tray in towards the cartons. This brings about a substantial improvement in the pressure-absorbing ability of the corrugated tray, at the same time giving a compact transport packing capable of resisting the strains and stresses encountered in the course of distribution. The units can easily be handled manually.
This system of packaging the cartons in one-way transport units by wrapping them in shrink-on film after having strengthened the unit of 12 cartons by a tray of corrugated cardboard is used for UHT milk basically meant for longer storage and longer distances of transport (such as once-a-week delivery to a consumption centre from a distant milk plant).
It seemed to be appropriate to take into consideration and present data for this method of wrapping the cartons into one-way strengthened transportable units so as to arrive at indications for the costs - by today's standards - of the most modern liquid milk processing and distribution system. Up to about 600–700 litres of milk per square metre can be stored when pallets are used (without access).
Pasteurized milk in sachets. Pillow-shaped sachets with a longitudinal seam made from reeled low-density polyethylene film have been chosen as the model for analysis of the system. The film is shaped and welded into a tube. The tube is filled with pasteurized milk from a small balance tank, where the level is kept constant by means of a float. A timer-controlled pneumatically-operated valve is used to dispense constant quantities of milk. The transverse seals are generally made above milk level. The packages are separated by guillotine and placed in rectangular plastic crates holding 20 packages each and the crates are then palletized. The same width and thickness (90 mm) is used for both 1-litre and 1/2-litre packages; capacity is varied by varying the distance between transverse seals. It is important that the film is free from pinholes or micropores.
Table 4 gives data on the rolls of polyethylene film as used for the system.
Table 4. Film stock for pillow sachets for pasteurized milk
| Rolls for one million cartons | |||||||
|---|---|---|---|---|---|---|---|
| width mm | weight of one roll kg | cartons per roll | No. | weight t | net space requirement m2 | ||
| for 1/2 | packages | 300 | appr.25 | appr.5 500 | 180 | 4.5 | 7 |
| for 1/1 | packages | 300 | appr. 25 | appr. 4 000 | 250 | 6.25 | 10 |
Fig. 15 gives details of the dimensions of sachets, crates and crate stacks. These crates also are usually tapered so that they will nest when empty.
Fig. 15 Dimensions (in mm) of pillow sachets, crates and stacks for pasteurized milk
Thus with crates stacked 10-high containing 1/2-litre sachets the store capacity without access space is about 500–600 1/m2.
