Polyvinyl Chloride

Crystalline

LDPE: Low-density polyethylene; HDPE: High-density polyethylene; PP: Polypropylene; PS: Polystyrene; PVC: Polyvinyl chloride; PET: Polyethylene terephthalate; PVDC: Polyvinylidene chloride. Source: Ref. 3.

LDPE: Low-density polyethylene; HDPE: High-density polyethylene; PP: Polypropylene; PS: Polystyrene; PVC: Polyvinyl chloride; PET: Polyethylene terephthalate; PVDC: Polyvinylidene chloride. Source: Ref. 3.

and soft cheese. Paper-based, aluminum-free peelable lid material is prepared with a barrier coating that permits easy peeling without tearing. The lid can provide a barrier against oxygen, water vapors, and odors while offering excellent printing capabilities (6). Another lid technology is a patented polyethylene recloseable tamper-evidence cap for yogurt and cultured dairy products introduced by Portola Packaging, Inc. (San Jose, CA). Using Autoprod (Autoprod Packaging Equipment, Clearwater, FL) heat-form closing technology traditionally used for milk cartons, the cap uses a mechanical external ring with a pull tab for 8-oz dairy containers. To open, a consumer grasps the pull tab and tears away the locking ring.

In the following sections, specific packages for yogurt, cottage cheese, and sour cream, with highlights on the new technologies, will be briefly discussed.

1. Yogurt

Robinson and Tamime (7) and Tamime and Robinson (4) describe the types of packaging materials and constructions for yogurt in great detail. Basically, three types of packaging containers are used for fluid-type and semisolid yogurt: rigid, semirigid, and flexible units. The rigid units include recyclable glass bottles, and metal cans or aluminum foil laminated pouches. Semirigid units are normally manufactured from different plastics and made as tubs. This type of package is the most popularly used today. The flexible units are either in the form of plastic sachets or paper cartons, and used mostly for fluid-type yogurt.

In the United States, yogurt was packed in wax-or polymer-coated paperboard containers with recloseable covers and thermoformed containers. Now, most products are automatically filled in PS tubs and covered with aluminum foil and a polymer cap. Recently, a couple of new packaging designs specially targeted for portability have helped boost yogurt sales. Yoplait Go-Gurt is packaged in a three-sided tube-shape pouch; no spoon is required to eat the yogurt. The pouch is made of Polyester/LDPE/LDPE-based sealant layer and produced on a form/fill/seal machine. The tubes are packaged in solid bleached-sulfate cartons in multiples of eight (Fig. 2). A spoonable yogurt in a cup sporting the Danimals brand was introduced in 1994 and then relaunched in January 1999 (Fig. 2). Part of the relaunch involved new package graphics. The small white high-density PE bottles are extrusion-blown in custom molds. Lidding is aluminum foil. The operation forms the roll-fed foil into a cap complete with pull tab. It heat-seals the foil to the bottle

Figure 2 Typical containers for the packaging of yogurt.

finish and crimps the foil around. Shelf life of the refrigerated product is about the same as any other refrigerated yogurt—30 days.

Efforts to reduce packaging waste and provide environment-friendly operation have been emphasized lately. Glass was rejected by the yogurt industry because the costs of transporting the heavy material outweigh the benefits, although glass is widely recycled and made from recycled material. The energy (fossil fuels) used for manufacture and transport of glass containers exceeds the energy used for plastic containers. Paper yogurt containers are welcomed by consumers, but packaging materials of this type are coated with a thin layer of plastic, making them a combination container and not recyclable. FDA prohibits the use of recycled material in contact with food. Polypropylene containers have become the best option in yogurt package to assure product quality, with the least amount of material, fossil fuel, and environmental impact. One of the most beneficial characteristics of polypropylene is that it can be made with thinner walls while maintaining the same structural integrity, thus significantly less plastic than HDPE can be used. The quart containers are over 30% lighter today than they were 10 years ago. By using this material instead of HDPE, one yogurt manufacturer in 1998 alone prevented the disposal of over 85 tons of plastic. Added environmental protection was achieved through decreased air emissions and resource depletion associated with the manufacture and distribution of the packaging. In addition, polypropylene is manufactured without the use of chlorine, thus eliminating the hazards of deadly dioxin releases during manufacture and incineration, which occurs with certain other plastics.

2. Fresh and Cream Cheeses

Protection against light, oxygen, and loss of moisture is the basic requirement for packages of fresh and cream cheeses. The packaging must provide protection against light transmission. The oxygen in fresh cheeses may be present in the cheese as a result of the processing techniques used (e.g., centrifugation), occupy the headspace inside the package, or permeate the package over time. Because of the high water content of fresh cheeses, they are very sensitive to dehydration. Therefore, they need to be protected from moisture loss by barrier packages (8).

Genuine vegetable parchment or greaseproof paper was frequently used in the past to package fresh cheese. Paper coated with paraffin or PVC/PVDC (polyvinylidene chloride) copolymer is still sometimes used in the form of a banderole (e.g., for packaging an un-ripened cheese intended for consumption within a short time) (9). Today, fresh cheeses, such as cottage cheese, are often packaged in rigid packages because of their soft and semiliquid texture and ready loss of moisture. Vacuum-formed polystyrene (PS) tubs are now used almost exclusively for consumer packaging of cottage cheese. The PS tubs offer superior protection from fat absorption and breakage. Their mechanical and chemical properties have been significantly improved by copolymerization with, for example, butadiene. By co-extruding or extrusion coating with PVC or PV/PVDC copolymer, PS barrier properties have also been improved. In addition, the PS tubs can be pigmented with TiO2 to provide a better barrier to light. The use of PVC is favored due to its inertness and its impermeability to water and gas, as well as its extraordinary resistance to fats (9).

As it is for other tubbed dairy products, tamper evidence is an important package requirement for cottage cheese. PP shrink film that surrounds and totally encloses the tubbed product is suggested for this purpose. This approach also provides additional moisture barrier function and significantly increases the shelf life of the product. However, this approach has not been widely adopted because it is more costly, new investment in shrink packaging equipment is required, and the truncated cone geometry of the tub is a difficult shape to shrink-wrap without leaving unsightly excess film on the package—the so-called dog-ear problem. Vinyl shrink bands have become a popular method for meeting this requirement for cottage cheese.

In addition to plastic tubs, aluminum foil with a thickness of 7-20 Am has been used for packaging fresh cheeses: the thicker foils (15 to 20 Am) are formed into containers, either rectangular in shape with straight walls, or cylindrical in section with corrugated or pleated sides. The corrosion of the aluminum must be prevented, which is usually done by applying a suitable enamel or by laminating with LDPE or PP. Otherwise, aluminum lactate will form due to the lactic acid from the whey coming into contact with the aluminum wall and attack the walls of the container, sometimes perforating them (9). For the manufacture of rectangular containers, combinations of aluminum foil with plastic and paper can be formed from a reel of materials, filled with (generally pasteurized) fresh cheese and heat sealed. Other combinations of paper, wax, and aluminum are also used (7).

Another new packaging technology for fresh cheeses is the application of modified atmosphere packaging (MAP) and processing to improve microbiological safety and to extend shelf life (10-17). Modified atmosphere packaging is a technology of flushing food packaging with a premixed gas mixture—usually elevated CO2 and reduced O2, balanced with N2 (to reach total 100% gases in the headspace)—before sealing. Carbon dioxide, a known antimicrobial agent, has been shown to inhibit the growth of some psychrotropic organisms that contribute to the deterioration of refrigerated dairy products. Significant amounts of CO2 occur naturally in milk. Unfortunately, it quickly dissipates during modern processing. A process for directly adding CO2 into the cream dressing before mixing it with the cheese curd was developed. With moderate barrier packaging, shelf life might be 60 days or more (18-20); i.e., another 2-3 weeks of shelf life can be added to cottage cheese by packaging it in a carbon dioxide-rich atmosphere. The cottage cheese suffers no deleterious effects when the correct gas mixture is used. The CO2 concentration must be limited to 40% (with nitrogen as the balance) to avoid undesirable flavors and collapse of the package during storage. Table 2 provides recommended gas mixture for different types of cheeses using MAP technology. In addition, the CO2 must be present throughout the depth of the cheese: it is not sufficient to simply flush the headspace. This is achieved most simply by bubbling the gas through the creaming mix. The correct temperature control at 5°C is crucial to obtain the full extra 2-3 weeks of shelf life (22).

3. Sour Cream

Similar to the packages used for yogurt and cottage cheese, plastic tubs made of PS or PP are the most popular packages for sour cream. With the rising popularity of meals featuring sour cream as toppings as well as sour cream-based dips, a few changes have come along.

Table 2 Recommended Gas Mixture for Different Types of Cheeses Using Modified Atmosphere Packaging (MAP) Technology

Product

Hard cheeses

Hard cheeses (sliced)

Fresh/soft cheeses

MAP Gas mixture

80-100% CO2

80-90% CO2

20-40% CO2

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