Types of Detergents and Sanitizers

The cleaning and sanitizing of dairy plant equipment is essential for providing safe dairy products to the consumers as well as for ensuring the keeping quality of these products. Cleaning operations have been followed ever since processing food has been known. Only the scale of operation and the ingredients used have changed with alterations in the type of food, the quality expected and the kind of equipment used. Earlier, the traditional detergents and their methods of use were aimed at providing only satisfactory cleaning. As the function of specific chemicals and the contribution of other factors were made known along with improvements in plant design, detergent handling and control was also made an important research topic.As a result, detergents and sanitizers gradually improved, leading to the high standards expected and obtainable today.

Cleaning is the process of removing any extraneous matter (including dust, dirt,particles from the food handled) adhering to the equipment surface with the help of a suitable detergent and water. Thus, cleaning agents are materials that help to remove dirt from equipment surfaces and keep them visibly clean.

Sanitization is the process of reducing microorganisms to a level acceptable by public health authorities in terms of destruction of pathogens and minimizing other microflora. Therefore, sanitizers are agents that reduce microbiological contamination to levels conforming to public health requirements.

Distribution of Processed Milk

There are many methods and regulations for the distribution or retailing of milk,depending on the country and type of milk that is being distributed. The dairy industry does not invest in liquid milk retailing in most countries, other than for promotion and demonstration centres and automatic vending machines. The grocers sell and sometimes distribute the milk delivered by the plant. They charge a little extra (the ‘retail margin’) over the milk plant price for their services. In countries where the milk plant delivers milk directly to consumers, the plant absorbs the retail margin. There is no common system for establishing retail margins. They vary considerably between countries and sometimes, even between localities within the same country due to variation in cost of labour, transport and capital.

 

i. Distribution of bulk milk

Distribution in cans: Bulk milk may be distributed to institutions such as catering establishments, university hostels and hospitals in cans. These cans are filled in the dairy directly from the tanks in which milk is stored, either by pumps or by gravity filling. They are then transported to the consumer in appropriate vehicles (tempos,trucks etc.). The consumers empty the cans into their own storage systems and return the cans immediately. Alternatively, the cans may be retained and the previous time’s empty and cleaned can are returned to the vehicle for transport to the dairy.This cycle is maintained. It is important that the cans are cleaned again in the dairy,even if they have been cleaned at the consumer’s location.

Distribution in automatic vending machines: The sale of milk in bulk through automatic vending machines has been introduced successfully in Mexico and India.This system does not involve retail packaging by the milk plant as the consumer provides his own container. Thus, the milk is handled in bulk up to the point of retail sale. This eliminates the cost of packaging and there is substantial saving in running cost. The milk plant sells milk directly to the consumer and so, there is no need for middlemen or a distribution network.

A tanker delivers the refrigerated pasteurized milk to the vending station housed in a simple building. This consists of a refrigerated room in which a milk storage tank of the requisite capacity is placed at a high level. A small service room for the cleaning-in-place (CIP) equipment is essential. Office or storage facilities may also be required. The milk is pumped from the tanker to the storage tank.

 

ii. Distribution of milk packed in multiple-use packages

Countries that use glass bottles for packaging at least some portion of their total packaged pasteurized milk include Bulgaria, India, Japan, Malta, New Zealand,Poland, South Africa and the United Kingdom. The proper working of the distribution system involving re-use of containers - mostly glass bottles - depends entirely on how effectively the empty bottles are retrieved. There are normally three ways in which this is done.

In the first case, the milk may be delivered to the customer’s doorstep and empty bottles recovered at the same time. This system has the advantage that it can work in the customer’s absence. The payment may be collected after a set period of time, for instance, once per week.

Secondly, vehicles carrying the packaged milk may tour the customers’ locality with many stops in each street. They normally announce their presence by blowing the horn or any other audible signal. The customers come to the vehicle bringing empty bottles. The customers need to pay a deposit on empty bottles, normally equal to the cost of the bottle. If they require more full bottles than the number of empties they return, extra deposits must be paid. If they require less a corresponding credit is allowed.

In the third method, milk may be sold only from shops to which the customer must go to purchase milk. A similar system as before, for paying a deposit on the bottle prevails here also.

 

iii. Distribution of milk packed in single-use packages

Single-use packages offer the advantage of doing away with the cleaning-collecting-returning-cleaning routine that is needed for the multiple use containers. Out of several distribution methods, the most commonly used is the stacking of the filled packages in crates. Sometimes they are also wrapped in non-returnable materials.Crating already has been mentioned in the section dealing with pasteurized milk in tetrahedral cartons. For single-use transport packaging, shrink-film is most often used. This wrapping can be applied manually or mechanically. This system is used where short storage times and short distribution distances are involved. This has the disadvantage that the film does not give much mechanical support to the packages.Cartons wrapped in shrink-on film are often susceptible to damage, leakage and contamination, especially when handled manually. Special (perforated) shrink-films may be required in areas with high humidity to prevent condensation.

Rectangular cartons filled with milk are placed first on a flat tray blank in groups of twelve. The sides of the blank are then folded up tightly around the cartons.Thus, the pressure-absorbing ability of the filled cartons is fully utilized. The packed corrugated tray is then wrapped in shrink-on film, thus protecting against dust and moisture. It also presses the sides of the tray in towards the cartons so as to improve the pressure-absorbing ability of the corrugated tray. This method is a safe,but more expensive system than the shrink-wrap. The units can easily be handled manually. This system is normally used for UHT milk, which is to be stored for longer periods and distributed over greater distances.

 

iv. Comparison of bulk and retail sale of milk

The merits and demerits of either of these systems depend on the locality and attitude of consumers and the type of milk being sold. Some of the differences in the two systems are enumerated below.

1. Although pasteurized milk can be sold in bulk, sterilized and UHT milk cannot be sold without packaging.

2. Packaged milk is measured accurately.

3. Milk sold in bulk is amenable to adulteration at different levels: transportation as well as vending.

4. Keeping quality of pasteurised milk depends on maintaining the ‘cold chain’ throughout its distribution network. If milk that leaves the plant at temperatures much below 10° C in insulated vehicles and is kept by the retailer under refrigeration, the quality of milk is bound to be much better than the bulk-vended milk, particularly in cans.

5. The sale of milk in automatic vending machines ensures the continuity of the cold chain. However, there may be a need to redesign or reconstruct buildings to accommodate the equipment, which is often difficult in crowded cities. In Mexico and India, where this system is in operation, it is considered a success and has the full support of every group of customers. However, there is feedback regarding the equipment, which comes in capacities of 1000 and 2000 litres. The machines need to be developed further, in order to make them more suitable to varying conditions and requirements.

Storage of Processed Milk

While designing a milk plant, adequate amenities must be provided for storing packaged milk in terms of area as well as facilities such as refrigeration. Space would also be required for storing packaging materials, whether they be empty bottles and crates or rolls of packaging films.

 

 i. Storage of Bulk Milk

Insulated tanks: Bulk milk after processing, is stored in insulated storage tanks in the dairy till transportation. The milk remains in the chilled condition. It is recommended that milk be transported to the place of delivery as soon as possible.However, due to limitations in infrastructure milk is stored for periods ranging from 4 to 12 hours after processing.

Storage in cans: Processed milk for bulk supply is also sometimes stored in cans.Since the beginning of the second half of last century aluminium milk cans have replaced mild steel cans that were used earlier. The most common are cans with lids which do not require rubber gaskets for sealing. An adequate sealing arrangement is achieved with sunken grip or mushroom lids. Lids attached to the cans by chains are no longer used owing to problems in mechanical washing. Lead or other seals can be applied through simple arrangements at the lid ring to avoid tampering and pilferage. Normally floor conveyors are used to transport cans from one place to another in the processing area of the dairy plant.Full cans are stored in one layer, thus permitting about 320 to 360 litres of milk to be stored per square metre excluding movement and working space. Empty cans,after washing, are stacked in layers horizontally, up to a 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 example, about 20 cans each of 40 litres capacity can be stored on trolleys about 1.7 × 0.7 m with a supporting frame made of a 1/2" pipe.

 

ii. Storage of Milk Packed in Multiple-use Packages

Storage in bottles: Bottles with wide necks (36 to 40 mm), suitable for sealing with aluminium foil caps are most commonly used for packaging of pasteurized milk in multi-use containers. The filled bottles are placed in plastic crates. These were earlier made of galvanized steel wires or strips, but plastic is in vogue these days.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. Crates with filled bottles are stacked five and six high for one-litre and half-litre bottles respectively for manual handling. Two to three crates more can be put in one stack for empty bottles. Crates can generally hold twenty half-litre and twelve to fifteen one-litre bottles. The stacks can be moved manually or by hand trolleys.

Bottles used for in-bottle milk sterilization have narrower necks (26 mm) than the pasteurized milk bottles so that a more effective seal can be made. Prefabricated crown seals are used to seal the bottles. The requirements with regard to sturdiness during mechanical and thermal shocks are higher than those of a pasteurized milk bottle. The bottles are packed in crates as is done 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.

 

 iii. Storage of milk packed in single-use packages

The common feature of single-use containers is that they are discarded after emptying. This fact has an important bearing on the milk plant construction,organization and on the economics of the whole business. There is no collection and washing of the milk packages. Only the crates that carry the filled milk packages are collected and washed. Single-service delivery wraps, trays or boxes may also replace these. Intermediate storage of packing material and filled packages has to be organized at the plant.

Storage in pouches: The pillow-shaped sachets of pasteurized milk are collected in tubs after they are cut and separated from the packaging machine. They are placed in rectangular plastic crates holding 20 packages each and the crates are then stacked.Thus with crates stacked 10-high containing 1/2-litre sachets the store capacity without space for entry, movement and working is about 500–600 1/m2.Both 1-litre and 1/2-litre packages have the same width and thickness (0.09 m).Varying the distance between transverse seals facilitates changing the capacity. It is important that the film is free from pinholes so as to avoid leakage, particularly when subjected to pressure from the adjoining pouches in the crates. Below Table illustrates the general details and space requirement for storage of film rolls used for pouch filling.

Space requirement for storing film rolls for milk sachets*

Storage in cartons: In several countries, pasteurized milk is packaged in cartons on similar lines as UHT-milk.Rectangular cartons are normally used for UHT milk, though this is not a strict regulation. The tetrahedral packages, after filling, are placed in hexagonal plastic crates holding 18 cartons each. The crates are then stacked normally 6-high as illustrated in Fig. 11.6. Thus with a 6-high stack of crates containing 1⁄2- litre cartons, the storage capacity excluding the area required for entry and working is about 500 1/m2.Empty crates for tetrahedral cartons are normally stacked 15-high. The crates occupy less space when empty, as they are tapered. Rectangular crates occupy the same space empty or full. below Tables give the space requirements for storage of packaging material for tetrahedral and rectangular cartons, respectively.The quantity of raw stock to be stored depends on the location of the milk plant and the paper supplier and also the purchasing power of the producer. It is generally necessary to carry two to three months’ stock.

 Storage space requirement for paper rolls for tetrahedral
milk cartons*

Storage space requirement for paper rolls for
rectangular milk cartons*

Packaging Long Life Milk

Ultra-High-Temperature (UHT) processing and packaging of milk are employed to increase its shelf life. This process needs special equipment for aseptic packaging of UHT Milk.

Packaging in single Service Pouches

The sequence of operations when packaging into single-service pouches includes forming the container, filling and sealing, storage of the packaged product and dispatch to wholesale and retail outlets. The parts of the packaging machine are enclosed in a stainless steel (SS) cabinet. All parts are made of SS or treated aluminium protected by a coat of weatherproof paint.

Plastic sachets are pillow-shaped and made of low-density polyethylene film (LDPE).The packaging material may come in a roll of single or double film or as a flat tube.The material should be coloured to reduce light transmission. Heat-sealable film rolls are mounted inside the compartment at the rear bottom. The film is sterilized by exposing to ultra violet rays just before use. The film is shaped and heat-sealed into a tube by a vertical electrode. The flat tube type comes pre-shaped as a tube and so there will be no longitudinal seam.

After vertical sealing, the film is then moved downwards by a set of rubber rollers.It then comes to contact with the horizontal electrode. A simultaneous seal is made at the bottom of each packet and across the top of the preceding packet. The transverse seals are thus made above milk level. This also separates one pouch from the other. Injection of milk into the pouch takes place between the strokes of the horizontal electrode. The milk is kept in a small balance tank above the packaging machine, 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 heat generated by both the sealing valves is removed by circulating soft water.After the packaging operation, the machine should be washed and cleaned. A general set of instructions for washing is given below.

•  Bring the machine to manual operation.
•  Switch off the horizontal seal and liquid injection systems.
•  Empty the product in the balance tank into a can.
•  Flush the balance tank with water and drain.
•  Clean by circulating 1% caustic soda at 80° C.
•  Drain alkali and flush with sufficient quantity of water.
•  Cool machine and switch off.

The following is the daily maintenance schedule of a pouch-filling machine.

•  Clean parts of the machine with a soft brush and warm liquid soap solution.
•  Wash with clean water.
•  Dry with air blower.
•  Lubricate parts that need it.
•  Check horizontal and vertical electrodes for deposition of milk solids. Do not use sharp materials for removing these.
•  Elements should be replaced in case of burns or physical damages.

Packaging in Multi Use Containers

The basic aspect of packaging systems using multi-use containers is collection of empty containers and washing these prior to refilling. There may be need for intermediate storage of unwashed as well as washed empties, depending on capacities of machines and operating times. Storage of unwashed containers is normally essential and may extend overnight so that washing and filling operations begin next morning before the day’s load of unwashed empties arrives. Storage of washed cans is permissible as cans have lids, but storage of washed bottles is a bad practice as they do not have lids and are exposed to contamination. Filled cans and bottles normally need refrigerated storage to synchronise with the distribution timings and arrangements. The storage and operational requirements are depicted schematically in the following figure.

Sequence of operations with multi-use containers

Thus, packaging in bottles may be explained in two phases: first, the bottle filling operation along with the bottle-capping process and second, the bottle washing operation. Bottles with wide necks (36 to 40 mm), suitable for sealing with aluminium foil caps made in place from reeled strip, forms the most common system for packaging of pasteurized milk in multiuse containers. Bottle washing,filling and capping machines should be of matching capacity, otherwise the labour-intensive operations of filling and emptying crates, 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.

 

i. Bottle Fillers

There are two types of bottle fillers in operation, the gravity fillers and the vacuum fillers. In both the types, the filling force is the force of gravity. While one filling tank is maintained under atmospheric pressure, the unit using vacuum filling is under negative pressure. The head of milk over the bottle determines the speed of filling. The filling nozzles in both types are arranged in a circle. Bottles are fed from the bottle washer into the filler in a single row. On this single lane conveyor, the bottle in-feed to the filler may be stopped by a bottle stop. This bottle stop is free to rotate under normal circumstances. But if the operator wishes to stop the machine for any reason, the clutch-operated bottle stop would prevent further movement of bottles to the filling machine.The main components of the filling machine are the float chamber, the filling nozzles and the supports. The assembly of supports revolves the bottles, which are automatically fed into the bottle supports or lifts. Each support raises the bottle in it so that its mouth is pressed and sealed against the rubber gasket of the filling nozzle. The efficiency of this seal depends on the bottleneck finish, hardness and state of the rubber gasket and the sealing pressure. The bottle is now in the position to be filled, being arranged just below the filling valve and the product to be filled in the filling bowl.

The milk flows into the bottle while it still moves along the assembly. The bottle filling valves are opened and closed by a lever from cams fitted to the stationary filling frame. The valves are triggered as the filling bowl and consequently, the valve-activating lever pass the cam. In case no bottle is in position, the centering bell does not lift and therefore, the situation can be sensed. This is a mechanical system to ensure that the filling valve is not activated. Therefore, no filling takes place when no bottle is in place, thus avoiding spillage and losses. After the bottle is filled, just prior to completing a full revolution, the bottle is automatically brought down and an empty bottle takes its place.

Gravity fillers: The bottle-filling system works on the same principle as filling a bottle under a tap from an overhead tank. However, certain conditions are to be fulfilled for the efficient and automatic working of the system. Firstly, the filler bowl should have enough liquid (milk). This is easily achieved by installing a float, which closes the connection to the main tank when the bowl is filled and opens up when the milk level falls.The next step is to ensure that the bottle mouth is adequately sealed to the filler bowl. To achieve this, the bottle is connected to a filling tube attached to the filling bowl. In the simplest design, this is achieved by placing the bottle on a stirrup that could be lifted up to seal against the gasket on the filler bowl. Therefore, the neck design and quality of the bottle should be capable of creating this seal. Once the bottle is sealed to the bowl, the flow of milk can start. To control the flow, the filling valve in the filling tube opens to start the flow and closes when the bottle is filled.The lifting stirrup is then lowered and the bottle is removed from the filler and the next empty bottle is introduced into the line.The pressure head from the top of the filling bowl to the outlet of the filling valve provides the ‘driving force’ or the ‘gravitational force’ to fill the container.

Vacuum fillers: In this case, the filling bowl is closed and as opposed to the gravity fillers, which work under atmospheric pressure, a negative pressure of about 0.05 bar is applied to it. The bottle is placed in position such that an airtight seal is created between the filling bowl and the bottle to be filled. However, this type of filler is not suitable for liquids of low viscosity that tend to foam. This is because the liquid in the balance tube is sucked back into the filling bowl owing to the vacuum that exists, as the bottle is lowered from the seal on the filling bowl.Therefore, vacuum fillers are normally used for liquids of higher viscosity.

 

ii. Bottle Cappers

Bottle-filling and capping are related activities and have to be considered in combination with one another. The filled bottles move in a smooth continuous fashion to the capping section. A movable support raises the bottle against the capping machine.The rising bottle activates a mechanism that slides a cap over the bottle and then brings the capped bottle against a plunger. The plunger is backed by a spring so that the cap is forced onto the bottle under regulated pressure.The most commonly used closures for bottles are aluminium caps and crown corks.The former has a thickness ranging from 0.04 to 0.06 microns. The cap should cover the bottle mouth entirely and should be sterilized before use. Crown corks are the oldest types of closures and are sill used widely in the beverage industry.They are made from precoated tin plate or tin-free steel sheets. These sheets pass through a press tool machine, which produces discs and also shapes them into crowns with corrugated edges. A liner is inserted in the crown. This compounds the crown and also acts as the main seal. When closing the bottle, a vertical load is applied to the bottle to ensure that the liner is pressed on to the neck finish. At the same time, a specially designed circular disk is forced onto the crown edges,forcing them inwards and making a lock around the edge of the bottle mouth.Crown corks are normally used for sterilized milk bottles.

 

iii. Bottle Washers

Bottle washing machines work on any of the following principles: 1) Spray or hydro-rotary, 2) straight-through soaker hydro type and 3) soaker type.The jet or spray type washer sprays the inside and outside of the bottle with jets of washing solutions. This design is simple and compact. It is, however, not as efficient as the soaker type. Its initial cost, however, is less than the soaker type machines. It is more suited for smaller plants.

The come-back-soaker-hydro machine is the most common in large dairies. Bottles are kept soaked in a strong alkali solution for some time (~ 5-10 min). An endless(round) conveyor carries the bottle carriers through the washer.The typical washing sequence (Fig. 10.4), in which the carrier conveyor chains dip in and out of the soak tanks allowing sufficient type for soaking consists of the following steps:

•  Pre-rinse with tap water, warm clear water or the overflow from the final rinse tank. The bottles are either jetted with water or soaked. Water temperature: ~35-40° C.
•  Empty the residue in bottles.
•  Prerinse using warm water. Water temperature: ~ 55° C.
•  Soak in hot solution of strong alkali. Solution strength depends on its temperature and soaking time. (usually ~ 3% alkali, 55° C, 5 min soaking). Temperature ranges unto 80° C with alkali solutions of 1-3% strength.
•  Scrub with solution. Bottles are exposed to hot solution jets at high pressure from inside and outside to dislodge stubborn particles that stick to surfaces.
•  Rinse bottles with warm water. Water temperature: ~ 60° C.
•  Repeat the above two steps at ~ 80° C.
•  Invert the bottles to empty contents.
•  Pressure rinse with warm water inside and outside. Water temperature: 35-40° C or higher. This water is not circulated, but is normally routed to the pre-rinse tank. Empty the bottles.
•  First rinse with fresh water. Jets from inside and outside. Empty the bottles.
•  Final rinse with fresh water. This water may be chlorinated to ensure sanitisation.
•    Sodium hypochlorite of strength 25-50 ppm of available chlorine. Empty the bottles.

Whatever the principle on which the bottle washer works, the following basic conditions should be fulfilled by the machine.

•  The cleaning and sterilizing solutions should reach all corners inside and outside of the bottles being washed.
•  Sufficient contact time should be allowed for efficient washing and cleaning.
•  Different solutions should be in individual chambers and should not be allowed to mix.
•  There should be of predetermined concentration and should not be diluted with rinse water or steam injections.
•  The temperature and strength of detergent solutions should be maintained.
•  Soft water should be used.
•  Cleaning solutions should be changed at least once a week and all tanks are cleaned.
•  If hard water is used sodium gluconate and/or tetrasodium pyrophosphate should be added to the washing solutions.
•  A 15% solution of muriatic acid along with an inhibitor may be used as a descaling agent in bottle washers.

After bottle washing, inspection should be done for dirty bottles, chipped bottles,residual liquid and foreign objects in bottles.

Operational Details for Common Packaging Systems for Fluid Milk

The pasteurized and cooled milk is immediately packaged in suitable containers or bottles so as to protect the product against contamination, loss, spillage, pilferage,evaporation and/or damage due to microbial attack, insect, exposure to light, heat,moisture or oxygen. It also helps in marketing the product. The several processing operations that milk is subjected to provide the producer a choice of the packaging system, depending on the availability of the raw materials and the consumers’preference. These different packaging systems have different operating procedures,which are briefly described below.

Machinery Involved in Packaging Fluid Milk

i. Machinery for single-use packaging

The machinery for single-use packaging involves a pouch-filling machine besides the normal milk handling equipments that are necessary. The available machines normally pack one litre, 500 ml and 200 ml capacity pouches. The packaging material coming out of a roll is suitably sterilized before it is shaped into a chute and sealed vertically to form a pipe. It is then sealed horizontally and a preset quantity of milk is filled from an overhead tank. The pouch is then again sealed horizontally to contain the milk before being cut and stacked manually into crates.The crates are stored in the cold room till marketing.

 

ii. Machinery for Multiple-use Packaging

Bottles, being reusable are returned to the dairy every morning and evening after the market supply and are washed and sterilized in bottle-washers before filling.The most common among bottles being glass, we shall be discussing here, the processes dealing with glass bottle-washing and filling only. The bottle washing operation takes care of pre-rinsing, washing, rinsing and sanitizing the bottles. The clean bottles are then passed onto the filling machine where they are filled to preset volumes before capping, crating, cold storage and distribution.

 

iii. Machinery for Bulk Supply

Bulk supply to hotels, cafes, hospitals and other institutions is generally done in cans in India. These are filled manually and therefore, the process does not require the elaborate machinery that is needed for retail packaging of milk. They can be filled directly from the outlet point of the pasteuriser or from a holding tank provided after the pasteurising equipment.

In the case of bag-in-box type of packages that are used in the western countries,the purchase of a filling machine depends on several factors. These include (a)whether the packing is to be retailed as per volume or weight, (b) the product needs to be aseptically packaged or not, (c) the process needs to be fully automatic or not, among others.

 

iv. Machinery for Long-life Milk

The machinery needed for filling and packaging long-life milk is very expensive.Ultra-High-Temperature (UHT) processing and packaging of milk are employed to increase its shelf life.

Processes for Packaging Fluid Milk

i. Processes for single-use packaging

The pouch-filling machine is an integral part of a modern fluid milk plant. Several types of plastic materials are used in forming the pouches. The material generally comes in the form of rolls, which are loaded onto the machine. The layer of plastic is folded vertically and sealed. A horizontal sealer seals the bottom of the pouch.Simultaneously, milk from an overhead float tank is siphoned into the formed cylinder.It is then again horizontally sealed at the top and cut off to form a pouch filled with milk. These machines come with adjustable filling heads capable of filling several volumes of milk as the need be.

 

ii. Processes for Multiple-use Packaging

The filling of milk in glass bottles was an important operation in the fluid milk factory till recent times when the sachet-packing systems gained popularity for several reasons. However, inspite of the weight of the bottle and the problems regarding the return and cleaning of bottles before refilling, it has several advantages such as ease of cleaning and ease of visual detection of spoilage and impurities.

The bottle-filling process is a cycle of events that follow one another. Clean bottles are filled in the bottle-filling machine and capped (generally with aluminium foil caps). The filled bottles are stacked in crates and sent for cold storage/refrigeration.The crates of empty bottles, after selling the product are returned to the factory.The crates are emptied and cleaned separately. The bottles go to the bottle washer,where the broken/chipped bottles are discarded and the clean, disinfected bottles are returned to the bottle-filling machine.

Bottles require the following properties: The shape of the bottle should be such that it facilitates easy cleaning and allows brushes and jets (of water and detergents) to act on the entire inner surface. The neck of the bottle should be joined to the body smoothly with no sharp and protruding angle. The base should be concave so that the sediments and residues collect in the center of the bottle rather than on the periphery, thus making cleaning easier.

The resistance of the bottle to shocks is influenced by shape, consistency of quality and the thickness of the glass. The bottles should also be able to resist the high internal pressures and temperatures created during in-bottle sterilization.Optical defects such as irregularities in the composition of glass, presence of air bubbles, deformations and coarseness in the surface or extraneous matter in the glass also decrease the resistance to mechanical and thermal shocks.The capacity of the bottle should be constant and consistent and should match with the capacity of the filling machine. The most common volumes in the market are one litre, 500 ml and 200 ml.

 

iii. Processes for Bulk Supply

The milk for bulk supply is generally processed and filled in containers manually.These may be cans of PET containers. Bulk milk is also sometimes supplied in small tankers or vans fitted with SS tanks.In the modern dairying countries where the bag-in-box-type of containers is used,the bags, which have a separate bung with a flexible tube attached, are supplied to the producers. The dairy owners use semi-automatic filling machines to fill directly into the box. When delivered to the caterers the filled box fits into a custom-built refrigeration unit with a prefitted, simple on-off tap. The tube on the bag is fed around the tap and cut off to open the bag and permit the product to be dispensed.

 

iv. Processes for Long-life Milk

Long-life milk is that milk from which most of the spoilage bacteria are removed so that the milk could be stored for a longer period than normal milk. Specific processes such as bactofugation, microfiltration, ultra-pasteurisation and ultra-high-temperature (UHT) processing help to increase the shelf life of milk.

Bactofugation is a method by which the bacteria in the milk are removed by centrifugation. This uses the theory that bacteria have higher density than milk and thus will separate out into the outer orbit during centrifugation and can be removed after the process. The bacteria-rich portion is separately sterilized and added to the centrifugally sterilized bulk of the milk. Any packaging systems for fluid milk packaging may be employed to pack and market the product. However, the packaging system has to be clean and the seals preferably hermetic, in order to avoid recontamination during packaging and storage.

Microfiltration is a non-thermal, pre-pasteurization step that can extend the shelf life of milk up to 45 days. The microfiltration process uses a ceramic filter membrane to remove spoilage bacteria from milk, thus extending shelf life. Because it is a non-thermal process, milk is less susceptible to heat-related sensory defects with a 90-day shelf life. Packaging systems that prevail for fluid milk packaging may be used to pack and sell the product.

Aseptic packaging is a technology wherein the product and package are separately sterilized, and the product is then filled into the package and the package sealed in a sterile environment. The product is commercially sterile (meaning that any pathogenic or other spoilage microorganisms have been destroyed) and shelf stable(does not require refrigeration or freezing). Containers for aseptic filling have traditionally been aluminum cans, high-barrier pouches and multi-layer, foil barrier boxes. Aseptic packaging using flexible materials is also employed where extended shelf life is required. Many aseptic packaging systems are based on form-fill-seal technologies that eliminate the need to ship preformed containers to the processor.That is, the processor procures the packaging material in the form of rolls or stacks and they are formed (shaped) during the filling process.

Plastic materials used in aseptic packaging of milk products are polyethylene,polypropylene, polystyrene as tubes, bottles or plastic film laminates with paperboard or aluminum in the form of cartons. High-pressure steam is used to sterilize product lines and hydrogen peroxide with heat of UV radiation for container materials. The popular commercial systems available for aseptic packaging of milk are Tetra Pak,Tetra Brick, Brick Pack, Combi Block, Pure Pak, Hind Pak, etc. Tetra Pak/ Tetra Brick packs are used to pack UHT-treated milk into pre-sterilized package in aseptic conditions. The first aseptic carton was the ‘Tetra Classic’. It was made from a roll of packaging material that had been sterilized in hydrogen peroxide,formed into a tube, filled with liquid, sealed transversely and cut into tetrahedron-shaped containers. Tetra Pak uses paperboard laminated with 10m LDPE from outside and 70-75m LDPE from inside. The Tetra Brick uses aluminum foil of 7-9m in addition to above laminates. The machinery needed for this system is very expensive.

Ultra-pasteurized products are produced under slightly less extreme conditions than aseptic processing. However, heat processing and clean packaging still play important roles. Ultra-pasteurized milk beverages are usually packaged in barrier-coated paperboard cartons, or HDPE or PET bottles. To prevent light degradation, PET bottles, which are clear as compared to HDPE bottles that are opaque, can be tinted and/or covered with full-body labels. A window in the label allows consumers to see the product.

Ultra-pasteurized bottles include a hermetic foil seal on the bottle mouth. This prevents contamination and enables the product to achieve a 90-day refrigerated shelf life. Some processors who want to obtain a slightly longer shelf life than the standard 14 days, but do not want to ultra-pasteurize the milk, may opt for an intermediate thermal process referred to as higher-heat-shorter-time. The shorter heat exposure can leave the milk free from the sensory defects associated with aseptic and ultra-pasteurization temperatures. The refrigerated shelf life for such products is about 30 days.

Packaging Materials Used for Fluid Milk

Milk has been packaged in several types of containers across the globe. The general requirements of a milk-packaging container are as follows:

•  It must be tamper proof, so that it is protected against fraudulent practices between the time of packaging and consumption.
•  It must be of such a size that the capacity always relates to the keeping quality of the product and that its contents correspond to the consumer’s daily requirements.
•  It must be clean and attractive.

A container of good quality must prevent post-processing contamination from bacterial sources. It should be such that all indirect or direct chemical change in the milk should be prevented. The package should be of such materials that their constituents do not get transmitted into milk. It should also be resistant to the cleaning materials used in the dairy.It may either be used once (single-use) or may be returned to the processing factory for reuse.

 

i. Single-use Packaging Materials

These are also classified as flexible packaging. They, as the name suggests, are used to package milk once and are discarded after removing the product. The consumer is responsible for disposing off the packages. They have been in the news in recent times due to the difficulty in their disposal and their role in environmental pollution.

Plastic pouches: Flexible plastic material can be made into pouch packs. As they are not self-supporting, they generally need some secondary packaging. The advantages offered by the plastic packages are:

•  good barrier properties
•  visibility of the contents
•  light weight (thus reducing the cost of transportation)
•  single-service, thus eliminating the need for return, washing and sanitation
•  easy to carry
•  economical and
•  can be made more attractive.

The most commonly used plastic material for pouches is polyethylene (PE). There are several types of polyethylene available and are characterized by their density.These are low density (LDPE – 0.91-0.925 g/cm3), medium density (MDPE –0.926-0.941 g/cm3) and high density (HDPE – 0.942-0.970 g/cm3). The material has good moisture barrier properties, but poor oxygen barrier characters. HDPE is the least permeable to moisture. PE is suitable both for rigid and flexible packaging,though for some applications, it must be combined with an oxygen barrier.

Flexible pouches are also made from a derivative of polyvinyl chloride (PVC) in the plasticised form (PPVC). It contains plasticisers to render it flexible and stabilizers to prevent thermal and UV light degradation of the product. However, there has been some controversy regarding its toxicity owing to the high chloride content and additives present.

The use of plastic pouches eliminates noise in the milk bottling plants and during delivery, and also reduces water pollution caused by milk residues and detergents used in the bottle washing process. However, they have the disadvantage of poor recyclability and hence, their disposal is a problem.

Carton: The history of the beverage carton is closely associated to the development of milk distribution. The first carton was patented in 1915 in the USA as a ‘folded blank box’. Cartons generally have a square or rectangular cross-section. Tetrahydral and other shapes are also seen, though rarely. These are manufactured by well-developed systems. They are high-technology packaging formats and need well-trained operators to ensure efficient filling of the product. The serving size is also important while designing the package. A 200- or 250 ml single serve packet may be marketed with a straw and if a multiserve (say, one litre), some sort of reclosable feature should be provided. These kinds of packaging materials are normally used for long-life products that need not be refrigerated during storage.

The form-and-fill types of cartons are normally used for liquid milk. They are normally received as rolls of heat-sealable packaging material. The material is laminated paper, consisting of duplex kraft paper coated with PE inside and wax outside. A sandwiched layer of thin aluminium foil between an outer kraft paper and inner PE coating is also prevalent.

The advantages of carton packaging are as follow:

•  Lightweight and compact shape, thus increasing distribution efficiency
•  Made from renewable raw material (wooden fibre)
•  Hygienic
•  One way containers, so no need for cleaning before or after filling operation,hence decreasing water and detergent usage
•  Recyclable
•  Easy to dispose

The distinct disadvantage also is the felling of trees for making the raw material.However, it is a renewable source and can be regenerated with the help of strict laws and effective management. In the forests of Scandinavia and North America,from where the raw material is procured, for every tree cut, three new trees have to be planted.

Paper and paper-based Materials: Several kinds of paper-based materials are used in the form of wrappers, cartons, boxes, bags and cups, while boards are used as cartons and boxes. These materials are made of bleached or unbleached kraft paper, grease-proof paper, vegetable parchment paper, glassine paper, wax-coated paper, plastic-coated paper, straight or corrugated paper boards, solid fibre boards,liner boards and box boards. While boxes and cartons are used generally for packaging products other than fluid milk, they may sometimes be useful in constructing the outer casing material (secondary packaging) for long distance transportation of flexible pouches.

 

ii. Multiple-use Packaging Materials

Of the possible pack types, bottles are the most commonly used multiuse packages.They can be made of glass and plastic. Metal bottles have also recently emerged for specialist markets, but they are rare and expensive.

Glass: Glass containers come in the form of bottles, jars, jugs and tumblers. They may be plain and transparent or coloured and opaque. While the plain glass bottle provides the advantage of direct viewing of the product contained in it, it has the disadvantage of exposing the milk to ultra violet rays that deteriorate it. Glass has many virtues, but also has several disadvantages that limit its use as a packaging material for milk.

Advantages of glass:

•  Strong, inert material
•  Good closure and decorative options
•  Raw materials easily available
•  Recycling possible
•  Excellent gas and water barrier properties
•  Quality image
•  Product compatibility
•  Good internal pressure resistance
•  Reuse opportunity

Disadvantages of glass:

•  Heavy
•  Brittle

Despite its disadvantages, the older generation of consumers still thinks of glass as the best option for packaging milk.

Plastic: Plastic bottles for milk generally are made with ribs to add strength. This additional tenacity is needed for withstanding the vacuum on filling machines, for resisting the lateral forces that act within the pack and during transit. Plastic bottles for milk are common in the developed countries and are yet to catch up in India.Plastic has the distinct disadvantage of making recycling difficult.Plastic bottles may be manufactured from PVC, HDPE or polyethylene terephthalate(PET). Of these, the first two are generally considered together because of the type of moulding machines they are made on and the type of bottle that can be made. They have the following disadvantages:

•  Poor oxygen barrier properties in polyethylene
•  Less clarity than PET, though not very apparent
•  PVC is brittle (specially at low temperatures) and breaks on impact

PET occurs in three physical forms (amorphous, orientated and crystalline), which are exploited to manufacture a wide range of packaging materials. PET bottles are made in two steps. The first is to make a ‘perform’, which is in the shape of a test tube with the actual neck of the bottle at the end that is open. The second step involves the stretching and moulding of this into the finished shape of the bottle.

The advantages are:

•  Light, strong and impact resistant
•  Can be made temperature-stable by suitable heat treatment

  iii. Materials used for bulk supply

Milk is packaged for bulk and institutional supplies in cans made of stainless steel or aluminium and of capacity of 20 L or 40 L. These are robust and offer excellent product protection. However, there is little opportunity to differentiate one product from the other and branding/labelling is the only alternative.PET containers are also used in the Western countries, but they require some form
of separate strap or handle. Milk is also sold to cafes and caterers in the western countries in large 22 or 40 litre packs in the form of bag-in-box. The short life of fluid milk allows the bag to be produced from PE in two layers, each of 50 micrometers thickness.

Packaging Materials Process and Machinery

Packaging is a tool that is used to contain the product, protect the product from environmental hazards and help in marketing the product. Most of the containers in the market today are used to protect a specific quantity of product during procurement, storage, distribution and retail sales, although several are also designed for bulk supply. The quality of the individual container depends on the nature,characteristics and value of the product besides the prevailing social practices and legislation.