Chemical Quality Assurance

Module 4. Environmental contaminants in milk and milk products

Lesson 15
MILK CONTACT SURFACES

15.1 Introduction

Most of the foodstuffs sold are packaged. The packaging shall protect the food but shall not contaminate the packed content. Migration refers to the transfer of a substance from the packaging to the foodstuff inside and vice versa. Plastics are most commonly used as packaging material. They are made from monomers and other starting substances which are chemically reacted to a macromolecular structure, the polymer. The polymer as such is an inert high molecular weight structure. As substances with a molecular weight above 1000 Dalton usually cannot be absorbed in the body, the potential health risk from the polymer itself is minimal. Potential health risk may occur from non- or incompletely reacted monomers or other starting substances or from low molecular weight additives which are transferred into food via migration from the plastic food contact material.

15.1.1 Definition

• All materials and articles intended to come into contact with foodstuffs, including packaging materials, cutlery, dishes, processing machines, containers etc.
• It also includes materials and articles which are in contact with water intended for human consumption but it does not cover fixed public or private water supply equipment.
• Food contact materials can be constructed from a variety of materials like plastics, rubber, paper, coatings, metal etc. In many cases even their combinations are used.
• Different types of additives such as antioxidants, stabilizers, lubricants, anti-static, anti-blocking agents, etc. are added to improve performance of polymeric packaging materials.
• Direct contact between the food and its packaging may results in the migration of packaging components into food.
• Recently the packaging has been found to represent a source of contamination itself through the migration of substances from the packaging into food

Fig. 15.1 Migration of molecules from the package to the product

15.1.2 Safety

• The food contact materials shall be safe.
• They shall not transfer their components into the food in quantities that could endanger human health, change the composition of the food in an unacceptable way or deteriorate the taste and odor of foodstuffs.
• During the contact of the food contact materials with the food, molecules may migrate from the food contact material to the food. Because of this, in many countries regulations are made to ensure food safety.
• The transfer of constituents from food contact materials into food is called migration. To ensure the protection of the health of the consumer and to avoid any contamination of the foodstuff two types of migration limits have been established for plastic materials by EU legislation

Specific Migration Limit (SML)

• SML means the maximum permitted amount of a given substance released from a material or article into food or food simulants.
• SMLs are a risk management tool derived from toxicological data, such as tolerable daily intakes (TDIs), or from a limited toxicological assessment ensuring safety only for a low migration
• Plastic materials and articles shall not transfer their constituents to foods in quantities exceeding the set SML.
• The SML are expressed in terms of mg of substance per kg of food or food simulants.
• For substances for which no specific migration limit or other restrictions are provided, a generic specific migration limit of 60 mg per kg shall apply.

Overall Migration Limit (OML)

• OML means the maximum permitted amount of non-volatile substances released from a material or article into food simulants.
• The OML may be seen as a restriction of food contamination by the sum of the substances migrating from food contact materials.
• Plastic materials and articles, intended to be brought into contact with food, shall not transfer their constituents to food simulants in quantities exceeding 60 milligrams of total of constituents released per kg of food or food simulant.
• Therefore, plastic materials and articles shall not transfer their constituents to food simulants in quantities exceeding the stipulated limits.

15.1.3 Methods for testing of packaging materials

• Under mandate of European Commission the European Committee for Standardization has prepared standard test methods required for testing of compliance with the requirements and restrictions in the plastics Directives. Overall migration test methods are published in 1 to 12 parts of EU Commission Regulation on plastic materials and articles intended to come into contact with food.
• Specific migration test methods of seven plastic monomers are published in eighth parts of the EU Commission Regulation.
• Methods of analysis for 35 monomers are developed in a European research project and published in European Commission Regulation.
• Test methods to be used for checking paper and board are prepared and published for

1. preparation of cold and hot water extracts
2. determination of water soluble matter, formaldehyde, polychlorinated biphenyls and metals (cadmium, lead, chromium, mercury)
3. determination of fastness of coloring agents and fluorescent whitening agents
4. transfer of anti-microbial constituents

 

• Testing is carried out using food simulant

Rules for selecting simulant and test conditions are given in the relevant EC directives.

15.2 Milk Contact Materials

• Milk is a fluid, therefore, it’s handling while collection, transport, processing, product manufacturing, storage and marketing requires use of containers, equipments
• Some common articles and material likely to come in contact with milk from production to consumption chain; at various stages like milking, storage, transport, processing, packaging, distribution and utilization are:

• Active or intelligent materials
• Adhesives
• Ceramics
• Coatings and lacquers
• Cork
• Glass
• Ion-exchange resins
• Metals and alloys
• Paper and board
• Plastics
• Printing inks
• Regenerated cellulose
• Rubbers
• Silicones
• Textiles
• Varnishes and coatings
• Waxes
• Wood

The brief account of different milk contact surfaces is presented below.

15.3 Metals

Milk has to find its place in any container after leaving the udder of the milk animal. These containers are usually made up of metals. As soon as milk occupies its place in the metal container that we use, it starts reacting with the metal. If it is of good quality, the milk will remain good and if it is not then, it is difficult to maintain the quality of milk produced both from the stand point of clean milk production and good flavors.

With respect to flavor, keeping quality and safety of the product; some of the above important considerations are important to discuss as ‘effect of milk on metals’. The metallurgy of the dairy plant is very different from the other industries. A slight change in the composition of the metal greatly affects the working quality of the equipments and wholesomeness of milk and milk products. The corrosive properties of milk are due not only to various mineral salts in solution but also due to its protein content.
The solubility of metals in milk and other dairy products is of interest both from the standpoint of durability of the equipment and of the effect of the dissolved metals on the flavor, keeping quality, and healthfulness of the product. Aeration favors corrosion and corrosion is greatest at the milk-air junction. Acid milks are more corrosive.

15.3.1 Aluminium

One of the common single metal, second best to stainless steel is used in dairy equipments. Aluminium is not appreciably attacked by milk. It is however strongly corroded by sodium hydroxide and by alkaline solutions. The pure metal is rather soft for use and usually alloys are now employed, especially those incorporating silicon. In cleaning aluminium plant and utensils, care must be taken to incorporate a fair proportion of sodium metasilicate in detergent. Souring of milk and whey also attacks aluminium and cause pitting. Phosphoric acid used for removing milk-stone on pasteurizing plant also attaches aluminium. Aluminium is non-toxic and no flavor is imparted to milk until 9 ppm is reached. The alloys which appear to be most suitable for use in the dairy industry are those of aluminium with silicon, manganese and magnesium. Now-a-days it is used in storage tanks, rail tankers, butter churns and cans.

15.3.2 Stainless steel (SS)

Mostly all the modern dairies extensively use S.S. because of its high resistance to corrosion, easy to clean, bright, good heat conductor, gives nice appearance and imparts no taints and off flavor to milk and milk products. The S.S. consists of 18 parts of chromium and 8 parts of nickel. Addition of 3% molybdenum further improves the resistance of this 18:8 to corrosive influences.

15.4 The Effects of Metals on Milk and Milk Products

The passage into milk of any particular metal may have two undesirable consequences in addition to corrosion. It may impart a bitter, metallic flavor and initiate the cycle of changes leading to an oxidized taints. Instead, of one, if two metals enter milk to the same extent, the effect may be doubled or will be very different considering the sufficient emphasis upon the flavor and keeping quality of milk-products, the effect of metal is of great importance. The classic work done by various workers has shown conclusively that some of the common metals do impart objectionable flavors to milk under certain conditions.

There is considerable weight of evidence to show that extremely small traces of heavy metals, nickel, manganese, chromium in general and copper in particular exercise a powerful catalytic effect to develop an oxidative deteriorations. These metals find their way into milk through the worn coolers in which the tinned surface is no longer intact. Milk is thus in contact with tin and copper at the same time and as a consequence copper dissolves under the influence of protein also favor solutions of copper through the formation of copper-protein complex, which become absorbed by the fat globules and in this manner, the metal is brought into contact with the fat. Iron, cobalt, nickel, chromium and manganese are also effective, but to a much lesser degree than copper. Tin and aluminum have little effect, copper is a normal constituents, being present to the extent of 0.12 ppm but if the concentration rises to 1.5 ppm, an oily taint may result.

Cream, butter and whole milk powder are similarly sensitive to the catalytic effect of copper. It has been found that a number of factors affect the degree of action of the metal. If the milk is warm, metal surface is not clean and highly polished (un-cleaned and dull surface), high acidity of milk together with sugar the effect is likely to be greater. The effect of nickel, manganese, chromium and iron are not as severe as those of copper but hastened by acidity. In addition to its effect on flavor, fat oxidation is known to cause the losses of vitamin A and vitamin C (ascorbic acid). In case of whole milk powder, concentration of copper as low as 4 ppm has a strong catalytic effect on the oxidation reactions and again this is assisted by presence of moisture.

Fishiness and tallowy flavor defects in butter due to high concentration of acid is further aggravated by metallic contamination. The effect of metallic contamination was studied by adding various quantities of metallic lactates during the salting of butter. Tallowy taint developed so quickly that no first fishiness could be detected.
The possibility of metallic contamination also arises from the metal foil used for capping bottle milk. Aluminum foil is generally used but when lacquered zinc foil is used, normally the foil is in contact with cream layer the tinned lead allows quantities of lead to pass into milk to a degree which would render it unsafe for human consumption.

15.4.1 Pasteurized milk

Milk is brought by the vendors in aluminium containers to the doorsteps of the consumers, where the required quantity is measured and given out in the customer’s container. In the packed form, milk is sold in returnable glass bottles sealed with aluminium foil cap clear glass bottles of 500 ml capacity. However, due to the handling problems of glass and other related issues, an alternate packaging system was evolved in the early 80’s, and thus plastic pouches replaced glass bottles. Plastic pouches are generally made of low-density polyethylene (LDPE film). Co-extruded LDPE-LLDPE film is also used because of its advantage of eliminating pin-hole problems. The films are of 65-70 µm thick.

Another technological breakthrough in processing and packaging of milk is the Aseptic Packaging, commonly known as the Tetra pack milk. In this packaging system, both the package and the product are sterilized separately and the packaging operation is carried out under aseptic (sterile) conditions. This system offers a long storage life of about 3 months, without the need for refrigeration or added preservatives. A tetrapak carton is formed from a composite material, which has 5 to 7 layers including paperboard, aluminium foil and polyethylene.

15.4.2 Flavored milk

For flavored milk drinks the package should be leak and tamper proof, should have sufficient wet strength and should not pass on any odor or taint to the product packed inside. The plastic based material used for sachets is octane LLDPE (O-LLDPE). OLLDPE when blended with 50% LDPE provides excellent puncture resistance, excellent seal strength and hot tack. In India, flavored milk drinks are available in sterilisable crown cork glass bottles, glass bottles with aluminium foil lid or snap-on plastic lid, plastic sachets and aseptic packs (Tetra bricks). Recently 200ml, translucent bottles of HDPE with an aluminium foil cap have also been introduced.

15.4.3 Condensed and evaporated milk

Traditionally, condensed milk was bulk packed in barrels or tinplate containers. In India, sweetened condensed milk is the most popular out of all other concentrated milks and is packed in conventional food cans with double seam ends. Evaporated milk is recently packed in aseptic tetrapaks.

15.4.4 Butter

Because of high moisture content, butter, unlike solid fats is susceptible to mold growth. Flavor and odor are easily affected by absorption from other materials or through spoilage of butter due to oxidative rancidity. The package should, therefore, be opaque and a high barrier against oxygen and foreign odors. The most commonly used butter wrap is the vegetable parchment paper of 45 gsm.

Recently, embossed aluminium foil backed parchment paper has been introduced for UV light protection and sales appeal. A popular packaging style in some countries is to use plastic cups and plastic tubs with lids in different shapes and sizes. For such applications, PP (Polypropylene) and ABS (Acrylo-Butadiene-Styrene) are widely used.

15.4.5 Ghee

Ghee needs to be protected from chemical spoilage and rancidity caused by oxygen, light, heat, moisture and metal ions. A major portion of ghee was packed in lacquered or un-lacquered tinplate containers. Alternate packages, which are plastic based, are now gradually replacing tins. Ghee is also marketed in lined cartons with flexible laminated plastics as inner liner materials and in tetrapaks. In both these packs long shelf-life is achieved. Laminated pouches of metalized polyester based films are also used. For packaging of ghee, laminates of polyester, Nylon-6 and use of high barrier materials such as Ethylene Vinyl Alcohol (EVOH) polymer films with a trade name of EVAL can also be explored, as these materials could provide a fairly long shelf-life.

15.4.6 Milk powder

Milk powder is hygroscopic in nature and has a tendency to gain moisture from the atmosphere, which results in lumping or caking of the powder. Whole milk powder is highly sensitive to oxygen as well. Presence of oxygen causes spoilage of the product due to oxidation and rancidification, therefore the packages are required to be vacuum or nitrogen flushed. Milk powder is bulk packed in 25 kg capacity multiwall paper sacks with plastic liner made of polyethylene. Alternatively, the polyethylene liner can be laminated directly to the inner wall of the paper sack.

The flexible materials have evolved through polyethylene bags to sophisticated multi-ply laminates. Stand–up pouches of metalized polyester/LLDPE laminates and polyester/LLDPE laminates are used for skimmed milk powder. For whole milk powder, a typical structure for a plastic pouch is 12 μ polyester/9 μ Al foil/50 μ PE, and when gas flushed, these pouches are found to be as effective as canning to prolong shelf-life of milk powder. Latest development is the increasing use of pouches made from co-extruded film of LLDPE–Nylon–LLDPE with gas flushing and laminates of Polyester/Al foil/surlyn/Pd catalyst/surlyn, as oxygen scavenger.

15.4.7 Ice-cream

Conventional form of packages include paperboard cartons, paper cups and in some cases even metal containers. The various types of packages for ice-cream include:

• Paper board carton which is poly coated is poly-coated
  
• Thermoformed/injection moulded plastic containers made from HIPS (high impact polystyrene), PP (Polypropylene) or HDPE (high density polyethylene). The materials used for the lids are LDPE (low density polyethylene) or PS (polystyrene). The lids are of snap on type.
• Laminates of BOPP (biaxially oriented polypropylene) or PET (polyethylene terephthalate) are used for candies.

15.5 Malted Milk Food

Malted milk food is highly sensitive to moisture and is prone to oxidative changes in the presence of light, heat and oxygen. Aroma retention of the product and prevention of moisture and oxygen ingress, therefore, is very critical in protecting the product, and in selection of the right packaging material. The types of packages used conventionally are glass jars, tinplate containers, which are now slowly being replaced by plastic containers and flexible laminated pouches

15.5.1 Cheese

Cheese needs to be protected against moisture loss and ingress of oxygen in order to maintain the desired quality characteristics. In India, the traditional package of cheese is a hermetically sealed printed tin-plate container. Today, the flexible packaging films and laminates generally used for packaging of cheese to provide adequate moisture and oxygen barrier properties and to retain the vacuum are:

• Co-extruded LLDPE – TIE – Nylon – TIE – LLDPE
• Co-extruded LLDPE – TIE – EVOH – TIE – LLDPE
• Co-extruded film based on PVDC as the core material
• Laminates of metalized polyester/co-extruded nylon based film

15.5.2 Dahi/Yogurt

Yoghurt has a very short shelf-life at room temperature. The traditional pack so far was the earthenware pot with a loose cover of glassine (a very thin and smooth paper that is air and water resistant) or greaseproof paper. The earthenware pots are very heavy, easily breakable and because of oozing of water from its body, the product inside develops shrinkage cracks. Recently, injection molded polystyrene and polypropylene cups have been introduced with aluminium foil based peelable lids.

15.5.3 Traditional dairy products

Traditionally, indigenous products have been packed in leaves, paper cartons or paper-board boxes. These materials do not provide sufficient protection to the product from atmospheric contamination and manual handling. Consequently, the sweets soon lose their typical body and texture, absorb foreign odor and lose their aroma characteristics and show mold growth, when products are stored in open metal trays.

Use of coated films, laminates of aluminium foil with various substrates, metalized films and combinations of various packaging materials need to be tested for suitability for these products. For instance, products like, gulabjamun and rasgolla need protection from light, oxygen, ingress or egress of moisture and micro-organisms; lacquered tinplate can is the most protective material, but this is very expensive.

15.6 Milk Capping

The well-proven method of bottle capping using aluminium foil introduced hygiene and high speed mechanization to milk packaging and contributed to one of the most efficient forms of re-used pack. The used glass bottles are collected and re-filled again and again – so saving valuable material resources. The compatibility of aluminium foil with heat-seal lacquers coupled with the metal’s excellent heat conductivity and stability makes it the ideal material for capping and heat-sealing of all types of plastic milk containers. Whether the milk is fresh, aseptically filled or sterilized in the container, the foil/coating combination can be designed to meet the demands of processing and distribution.