Meat Packaging

Meat packaging is an important topic since bacteria will quickly grow on the surface of the meat products because the ideal environment of moisture, nutrients and mild acidity are present.  Food poisoning will result from ingestion of certain bacteria or toxins which are produced by the bacteria. 

Cooking will kill microorganisms and destroy enzymes.  On the other hand, warm ambient temperatures accelerate microbial growth; conversely low temperatures retard the growth of microorganisms and under deep freeze conditions all growth is virtually halted.  Oxidation is also retarded at low temperatures but can be easily prevented by using packaging materials with very low oxygen permeability.

Fresh meat can be mixed with spices, herbs, vegetables, salt, vinegar, fat to produce products such as pork sausages, beef sausages, minced meat, meat patties, boerewors and the like.  These products are fresh by definition in that they have not been subjected to a cooking process.

Displayed at chilled conditions, 2 to 8°C, an oxygen permeable wrapping of medium moisture vapor impermeability is required.  A balance of properties to retain color and minimize drying out (which results in darkening and mass loss) is best achieved with LLDPE or 19 mm plasticized thin PVC over a foam polystyrene tray.  Shelf life will not exceed four days before discoloration and souring of the meat become evident.

Table 1: Typical oxygen and moisture permeability figures of wrapping materials described.

Material	Trade name	Oxygen (cm3/m2/24 hrs/atm at 23°C 50 % RH)	Moisture (g/m2/24 hrs/atm at 25°C 75 % RH)
20 µm LLDPE	Liner low polyethene	4-5000	20-30
19 µm PVC	Resinite	4 200	200-250
50 µm LDPE	Polythene	4 200	5-6

Processed meats normally have much longer shelf lives and can be eaten without further cooking.  These products, however, rely on refrigeration to extend their shelf life, in conjunction with packaging material that will prevent oxygen ingress and drying out. 

Always stored at chill temperatures, a normally evacuated pack has main functions of maintaining product mass and color, holding vacuum, keeping clean and identifying the product. 

Table 2: Processed meat packaging examples

Sliced bacon (pouch)	Nylon/PE
Sliced meats	Coated polyester/PE coated PP/PE
Viennas, Russian (pouch/bag)	Coated nylon /PE shrink bag of PVDC
Sliced bacon/meats	Reels – lidding: coated polyester/PE or coated polyester/ionomer (total seal) Reels – base: cast nylon/PE or cast nylon/ionomer (total seal)

Vacuum Packaging

Vacuum packaging involves enclosing large joints in flexible plastic containers (usually bags) to prevent moisture loss and exclude oxygen from the meat’s surface.  Packing under a vacuum reduces the volume of air sealed in the meat. 

Reported O₂ permeabilities of packaging films are usually measured at ambient temperatures and moderate humidities (typically 23°C 75% RH), but both temperature and humidity can affect the rates at which gases are transmitted through films.  Data on the O₂ permeabilities of packaging films at chill temperatures are sparse, and those that do exist often do not include a complete specification of the film under test or the test conditions.  This is further complicated by the test methods and units used.  The O₂ permeabilities at subzero temperatures of two plastic films used for the vacuum packaging of meat have been reported.  One film was a PA-LDPE laminate, while the other was an EVA copolymer-PVDC copolymer laminate.  Their OTRs at -1°C were reported as 2.0 and 0.6 ml/m²/24 hrs/atm, respectively, approximately 1/50 of the values obtained at 23°C 90 % RH.

Three basic methods are available fro vacuum packaging meats, which are now discussed in turn.

1. Shrink bag

The system involves placing the meat into a heat shrinkable barrier bag (typically triple layer coextruded film constructed from EVA copolymer-PVDC copolymer-EVA copolymer, but sometimes PA is used the barrier layer with an ionomer as the inner or outer layer).  The bag is then evacuated prior to sealing.  In the past, this was achieved by applying a metal clip around the twisted neck of the bag, but today, heated jaws are used.  The bag is then heat shrunk by placing in water at 90°C.  After shrinking, the bag conforms closely to the meat and produces a tight vacuum pack.  Very high vacuum levels are achieved on rotary single-chamber machines, which also heat seal shrink bags, and owing to their improved productivity and versatility, these machines have become the industry standard. 

2. Nonshrink bag

In this technique, meat is placed into e preformed plastic bag, which is then put in an enclosed chamber that is evacuated.  When a predetermined low pressure has been reached, heated jaws close and weld the mouth of the bag.  Typical bag constructions consist of laminates or coextrusions, which include PET as the outside layer to provide strength, PA as the middle layer to provide a good O₂ barrier, and inner layers of LDPE, ionomer or EVA copolymer, which are good moisture barriers and can be easily heat sealed.  A typical structure would be ionomer – PA – EVA copolymer. 

3. Thermoforming

In this method, deep trays are thermoformed in-line from a base web of plastic.  Meat is placed in the trays and an upper web of plastic is heat sealed under vacuum to form a lid.  Generally, the materials used for thermoforming laminates of PA, PET or PVC, sometimes with a PVDC copolymer coating and heat sealing layers such as LDPE, EVA copolymer or ionomer. 

Modified Atmosphere Packaging

As an alternative to vacuum packaging, attempts have been made to store meat under various gaseous atmospheres, a process referred to as modified atmosphere packaging (MAP).  The strain on the packaging material can be alleviated by introducing another gas or mixture of gases after evacuation and before sealing.  Typical polymers used for the packaging of chilled meat (both vacuum and MAP) are presented in Table 3.

Table 3: Typical materials used for packaging chilled meat

Pack type	Bottom web materials	Top web materials (where applicable)
Flexible vacuum pack	PA – LDPE, coextruded as 5 layer film
	PA – LDPE	OPA – LDPE
	PA – EVOH – LDPE
	PA – EVOH – PA – LDPE	PET – PVDC – LDPE
	PP – EVOH – LDPE
	LDPE – EVOH – LDPE
Rigid vacuum pack	APET	OPA – LDPE
	PVC or PVC – LDPE	PET – PVDC - LDPE
	PS – EVOH – LDPE	OPA – LDPE – EVOH – LDPE
		PET – LDPE – EVOH – LDPE
Rigid MAP pack	PVC	OPA – LDPE
	PVC – LDPE or PVC – EVOH – LDPE	PET – PVDC – LDPE
	APET	OPA – LDPE – EVOH – LDPE
	APET – LDPE or APET – EVOH – LDPE	PET – PVDC – LDPE
	PS – EVOH – LDPE
Skin pack	PVC – LDPE	Several combinations of up to seven or more alyers but incorporating EVOH as gas barrier
	PS – EVOH – LDPE
	APET
	APET – LDPE

Please read the article on Polyamide, EVOH and PVDC on this blog for detailed information about those materials.