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Materials, technologies for shelf-stable dairy products
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Friday, 01 November, 2013, 08 : 00 AM [IST]
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Dr Avneet Rajoria
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fiogf49gjkf0d The primary objective of dairy products packaging is to protect them against spoilage caused by physical damage, chemical changes or microbial growth. Whether processed or raw, foodstuffs may be classified into two main categories with regard to shelf life viz. fresh or perishable products and shelf-stable or semi-perishable products.
Fresh products have a shelf life ranging from a few hours to few days depending upon the processing techniques and storage conditions. Perishable products include pasteurised milk, fermented dairy products and sweets prepared from desiccation and acid coagulation processes. Shelf stable products keep well for up to several weeks or months at ambient temperatures like skim and whole milk powder, ghee, dairy whiteners and ready mixes. In either case, packaging has an important role to play in the product’s shelf life.
Product environment
Besides the temperature of storage and humidity, the product’s environment within the package is often crucial to its shelf life. The latter forms the basis for selecting an active packaging and modified atmospheric packaging (MAP) systems. Gas permeation properties are critical not only to MAP of foodstuffs such as fruits and vegetables, but also to products obtained through in-package thermal processing, or aseptic packaging. Even intermediate moisture products rendered shelf-stable by controlling water activity rely heavily on this principle for achieving extended shelf life.
Types of packaging material
At present, dairy industries are using tin containers, flexible films, laminates, preformed packages (cups, trays, jars), aluminium foil, bag-in-box package, and transparent bottles for different products. Cans made of tin plate or drawn aluminium are generally used for packaging of condensed milk, ghee, gulab jamun, rasogolla and viscous liquids and gravies type products. The tin containers are expensive, bulky and require large storage and shipment spaces.
Heat sterilised
For products like rasogolla and gulab jamun, the empty cans are heat sterilised and filled with products along with sugar syrup and carried on a conveyor tunnel for sterilisation (exhausting) with super heated steam. The can lids are separately sterilised, placed on the cans and hot sealed. The can sanitisation, filling and sealing zones are sterilised with the help of superheated steam and flue gases. Cans were used in for in-package sterilisation of milk products in past for long time. Manufacturers of UHT (Ultra-High Temperature) products now use the continuous process and use of cans identified as an old technology is phasing out.
Different layers of flexible films of different materials viz. paper, polyethylene and aluminium foil are co-extruded to form a laminate. These materials have specific properties viz. low water vapour and gas transmission, high moisture barrier and high burst strength and when co-extruded form an ideal packaging. Such laminates may be 3, 4 or 5 ply and generally used for products like milk, cream, fruit juices, and soups. These laminates are supplied as film rolls and mounted on FFS (form-fill-seal) machines. Alternatively, cartons made of laminates are available as preformed blanks, which are assembled at the top of the machines for filling and sealing.
Low-cost coloured and transparent polyethylene films are co-extruded for packaging of UHT processed milk intended to be stored for 2-3 weeks.
This co-extruded film protects the product against light but not oxygen. The packaging machines also need to operate at not more than 45-50oC filling environment. Co-extruded polyvinylidine chloride (PVDC) or ethylene vinyl alcohol (EVOH) with black or white polyethylene film is also used as a packaging film. Such a combination imparts protection against oxygen as well as light and shelf life of milk can be extended upto 3 months.
In stretch-wrap packaging, food is placed in a tray and film is stretched over the food to cover it. Stretchable PVC (polyvinyl chloride) films, PE (polyethelene) films or PP (polypropylene) films are used as packaging materials. Shrink packaging is a form of packaging in which one or more items are covered with films, which shrinks when heated. The film is shrunk using either dry or moist heat. Sweets like burfi, kalakand, peda and other milk sweets can be packed in trays.
Bag-in-carton or bag-in-box containers have double construction, with an inner and outer liner. The former type is used for several food items including liquids while the latter is used for institutional sale and for bulk shipment. The external package provides mechanical strength, while the inner bag protects the contents against water vapour, gases and volatiles. This can be made of a single substance or multi-layer structure.
The cups used as containers for dahi, yoghurt and ice cream include thermo-formed, air-pressure formed and expanded plastic sheets. Recently, a cup with a barrier layer manufactured using pressurised air with laminated sheet and a composite cup with an inner layer of aluminium foil has been introduced. Paper cups, with PE, PP or PET (polyethylene terephthalate) inserts, thermoformed are also being used.
The ‘pure-pak’ type containers, with their distinctive top and the ‘brick-type’ smaller containers are used for a number of milk products. Paper laminated to PE is used for ordinary milk while for long-life milk; fruit drinks etc, laminated aluminium foils are used.
Aseptic packaging
Beside continuous FFS system, preformed packages of different shapes and sizes are also used for aseptic packaging of value-added dairy products. Blow-moulded plastic bottles of polyethylene or polypropylene are used as cheap substitutes. However, these are transparent and permeable to oxygen. Multilayer materials with high light and oxygen barrier properties have also been developed for varied applications. Preformed plastic cups of PP or polystyrene (PS) are now gaining popularity. Bulk filling bags are made of laminates of 3 or 4 layers of which one will be barrier material such as metalised polyester (polyester with a coating of aluminium particles) or ethyl vinyl alcohol (EVOH). The bag with filling valve is sterilised by defined g-radiation before shipping. Bags remain sealed and sterile until opened. The pre-sterilised bags are filled and sealed under aseptic condition. All product contact surfaces in the filler however need to be sterilised with steam before the filling operation begins.
Most of the aseptic packaging machines are of FFS type and require laminate of polyethylene-paper-polyethylene-Al foil-polyethylene. Packaging film roll is mounted on top of the packaging machine. The film moves continuously downward in the form of a strip and the shaping jaws give it a cylindrical shape. Heat sealing forms an overlapping longitudinal seal. Simultaneously extra polythene strip is heat-bonded alongside the longitudinal seam. This is done to prevent filled product penetrating the paper layer. As this continuous cylinder moves downward, jaws at the bottom make transverse heat seal. The product is filled instantly and another jaw seals the package at the top. Depending on the type of machine, different shapes can be given to the package. The most popular is brick shaped package. Some new innovations now being used for packaging of fruit juices are Fino packs. To cut down on costs, some dairies have introduced pillow packs for packaging of milk. A list of packaging materials used for dairy products is given in Table 1.
Edible films
Edible films and coatings manufactured from proteins, polysaccharides and lipids offer a great potential for increasing the food stability and reducing packaging cost. Edible films used as coatings or placed between food components provide possibilities for improving the quality of heterogeneous foods by limiting the migration of moisture, lipids, flavours and colours among food components. Edible coatings improve the mechanical integrity and handling operations of the food articles.
Edible films can be consumed with packaged product like milk chocolates, milk sweets with enhanced antimicrobial and anti-oxidative properties. They are produced exclusively from renewable edible ingredients. Edible coatings may enhance the nutritional value and organoleptic properties of packaged foods by directly contributing to flavour, colour and sweetness. They may be applied inside heterogeneous foods at the interfaces between different layers of components and tailored to prevent movement of inter-component moisture and solute migration in foods such as pizzas, pies and candies.
Milk proteins, wheat gluten, corn proteins and soy proteins are used for edible film formation. Manufacture of edible films and coatings from whey protein products is an effective method of whey utilisation resulting in alleviating the whey disposal problem. Polysaccharides such as cellulose and cellulose derivatives, starch, some hydrocolloids like carrageenan and pectin are also used for edible film production. Polysaccharide films have poor moisture barrier but good mechanical properties. Lipids like bee waxes, rice bran products, paraffin wax and acetylated mono-glycerides have been used for edible coating. Wax is used for coating of cheddar cheese. Lipids are hydrophobic and, therefore, act as good moisture barriers, however, their mechanical properties are inferior to protein and polysaccharide based films. Composite films consist of two or more components so that characteristics of the films are enhanced by individual contribution from each component, e.g. protein and polysaccharide films by themselves are fairly hydrophilic but have very good mechanical properties.
Biodegradable plastics
Biopolymers or bioplastics are intrinsically biodegradable and their use would reduce the damage inflicted to the environment due to their extended lifetime degradation behaviour. These polymers are utilised by bacteria as carbon and energy source and accumulated by them when other nutrients are depleted from the medium. Plant derived starches have been used to produce biodegradable plastic articles e.g. pharmaceutical capsules by blow moulding process. Other sources of biodegradable materials are poly lactic acid, poly malic acid, or poly E-caprolactones, which are synthesised chemically. In contrast, poly hydroxy alkanoates (PHAs) are produced microbially from renewable, plant-derived feed stocks. It can be processed by traditional techniques used in the plastic industry viz. injection moulding, PHA has the potential to become an important source for biodegradable plastics.
Microwavable packages
The advent of modern urban dynamic lifestyles has created a demand for ready-to-eat foods. The hurried urban middle and upper-middle class consumers have little or no time to cook foods in conventional manner. A number of microwavable products already command substantial shelf-space in retail stores and supermarkets. Microwave oven has now been accepted as a modern domestic appliance in Indian kitchen. The present day market needs innovative food packages, which can be microwaved. Packaging of ready-to-eat dairy products and composite foods in packages made from a special aluminium laminate that can withstand microwave processing should offer a promising milk product marketing opportunities. Microwavable packaging material has excellent barrier properties and available in reel form and as pre-formed containers in various sizes and shapes. For the sake of economy, deep drawn, lightweight containers are generally preferred. Special foils are also available as lid material with easy peel-ability for these containers.
Retortable flexible packages
Basic operations connected with the conventional canning/retorting process includes preparation of the food, filling of the container, exhausting, sealing of container, thermal processing/sterilisation and cooling of the cans and its contents. When foods are preserved by heat, the heating process serves to reduce the population of microorganisms. It may also inactivate enzymes present. pH of the food strongly influences the nature of the heat process required to produce an acceptable product.
The retortable flexible packages are characterised by their heat-resistant plastic layers conjugated with or without aluminium foil and their ability to be thermo-processed to ensure long shelf-stable products. The cost of retort pouches is comparatively lower than metal cans. In many developed countries, most ready-to-eat foods are packaged in retort pouches. The materials used in making retort pouches should possess toughness and puncture resistance normally required in flexible packaging along with good barrier properties and heat sealability over a wide temperature range along with the ability to withstand processing temperatures of 110-140oC. The outer film of the composite structure provides strength and resistance to heating temperatures. The outer layer is printable and withstands temperatures without bursting, shrinking and de-lamination. The most common material used is PET. It has the added advantage of being reverse printed so that ink is embossed between the outer layer and the next inner layer. In order to achieve a shelf life of one year or more, aluminium foil layer is added as one of the inner layers for barrier properties. The thickness range of aluminium foil varies from 9 to 25 ?m, though a thickness of 9-10 ?m is most common. Retort pouches without aluminium foil layer are also used for some products with shelf life of 3-6 months. Nylon is another material used as a barrier film in place of aluminium foil because of its low gas transmission rate and toughness. However, being transparent, nylon-based laminates cannot provide protection from light unless covered with a carton or wrapped. The current material most commonly used as innermost sealant layer is cast polypropylene, though high density polyethylene modified with isobutylene rubber has also been used.
Pouch dimensions for a particular volume are arrived at on the basis of the thickness desired which normally ranges from 1 cm for a 200 gm pouch to 3 cm for a 1 kg pouch. A seal width of 6 to 10 mm is required for good strength. Both direct printing on the pouch or on an outer carton is practiced. Prefabricated pouches or pouches made from roll stock in line with filling and sealing are used.
Retort pouches being flexible, require special racks or pouch carriers to prevent overlapping and maintain pouch geometry. Indiscriminate piling in a retort basket is hazardous. Slotted racks of rectangular sheet-metal plates separated by rod-mounted spacers for horizontal loading or circular retort baskets with divider plates or specially designed individual pouch carriers for vertical loading are used.
Active packaging
The objectives of active packaging are to maintain sensory quality, extend shelf life, maintain nutritional quality and ensure microbial safety. Oxygen scavengers are by far the most commercially important subcategory of active packaging and the market has been growing steadily for the last 10 years. Other active packaging technologies predicted to be used in the future are carbon dioxide scavengers and emitters, moisture absorbers and temperature control packaging. The use of active packaging is becoming increasingly popular. Many new opportunities in the preservation of milk sweets emerging to prevent oxidative and microbial changes observed during storage.
Modified Atmosphere Packaging
MAP replaces the air headspace of package with a food-grade gas or mixture of gases to extend the product’s shelf life and to maintain the product’s initial quality for a longer period. It helps in retarding the growth of moulds and bacteria besides reducing the oxidative deterioration and preventing mechanical damage to the product by adding a ‘cushion of gas around the product.’ Generally, three types of gas mixtures (N2, CO2 and O2) are used in MAP to slow down respiration rates, reduce microbial growth and retard enzymatic spoilage.
The use of appropriate gas mixtures in MAP offers an alternative to vacuum packaging. MAP has the potential to increase the shelf life of fat-rich milk powders, cheeses and fat spreads. In general, these products spoil due to the development of oxidative rancidity and the growth of microorganisms, particularly yeasts and moulds. The air is replaced with 100% N2 or N2/CO2 mixture and the powder is hermetically sealed in metal cans. Due to the spray drying process, air tends to be absorbed inside the powder particles and diffuses into the container over a period of ten days or so. This typically raises the residual headspace O2 content to 1-5% or higher. Because some markets require product with low levels of residual O2 (<1%), some manufacturers re-pack the cans after 10 days of storage. Obviously, this is both expensive and inconvenient. English territorial cheeses, e.g. Cheddar, have traditionally been vacuum packed. The cottage cheese is a high-moisture, low-fat product that is susceptible to a number of spoilage organisms including Pseudomonas spp. Use of gas mixtures containing 40% CO2 balanced with 60% N2 can increase its shelf life significantly.
The choice of packaging material is an important factor in any MAP operation. A low water vapour transmission rate together with a high gas barrier must generally be achieved. Generally, all MAP packages are based on thermoplastic polymers. The packaging materials need to have mechanical strength to withstand machine handling and subsequent storage, distribution and retailing. The success or failure of MAP for respiring and non-respiring foods would depend on both the O2 and CO2 impermeability of packaging materials so as to maintain the correct gas mixture in the package headspace. In addition, film used in gas packaging should also have low water vapour transmissions rates to prevent ingress or egress of moisture. Individual polymers are laminated to one another to produce films of desired characteristics for gas packaging of both respiring and non-respiring products. The packaging of non-respiring product includes nylon /PE /nylon /PVdC /PE or Nylon / EVOH / PE. These composite structures have all the desired characteristics for gas packaging of non-respiring product specifically; strength is provided by the outer most layer of nylon, gas and moisture vapour impermeability is provided by EVOH or PVdC and heat sealability is provided by PE.
(The writer is dairy technologist, Karnal, and can be contacted at arajoria123@rediffmail.com)
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