PACKAGING l
For a beer containing PET bottle, oxygen, being approximately 21% of dry air, and having a partial pressure of around 19.6kPa at sea level (ac- counting for the partial pressure of water vapour), O 2 will diffuse into the bottle at a much greater rate than the O 2 inside the bottle will be moving out of it. The partial pressure of CO 2 inside the bottle is, likewise, much higher than that outside of it, resulting in a net loss of CO 2 from the beer. There have also been issues in the past with the migration of flavour taints from acetaldehyde, which is used in the manufacture of PET and remains in the material in low concentrations. Health concerns over the use of antimony during the manufacture of PET would appear to be unfounded, with numerous studies showing that the trace amounts that remain in PET cannot raise the level present in the liquid above the WHO’s safe consumption level. Barrier materials To enable PET to be used successful- ly as a replacement for glass bottles and steel kegs, its barrier properties container wall that we observe a net change (this phenomenon can be ex- ploited to actively remove oxygen from packaged product, something we will return to later).
Polymer
O 2 permeability 23C 0% RH [cm3.mm/m2.day.atm]
1-5 50-100 50-200 100-150
Polyethylene terephthalate (PET) Polypropylene (PP) Polyethylene (PE) Polystyrene (PS) Poly(vinyl chloride) (PVC) Poly(ethylene naphthalate) (PEN) Polyamide (PA) Poly(vinyl alcohol) (PVAL) Ethylene vinyl alcohol (EVOH) Poly(vinylidene chloride) (PVDC)
2-8 0.5
0.1-1 (dry) 0.02 (dry) 0.001-0.01(dry) 0.01-0.3 Figure 1: O 2 permeability of polymers commonly used in packaging (Lange and Wyser, 2003)
Figure 1 , the shelf-life of beer in an untreated PET bottle can be as short as three weeks, due to oxidation. The mechanism by which there is a net change in gas concentration across a permeable solid, such as PET, consists of three processes; the gas is absorbed at the material surface; it then diffuses through the material; and finally, it desorbs from the surface at the other side. The partial pressure of a gas on either side of the material will deter- mine the net direction of gas trans- port, as gaseous exchange proceeds towards equilibrium, in accordance with Henry’s law. An interesting point to note here is that the transport of gas occurs in both directions and it is only due to the great imbalance in concentrations on either side of the
formats (1 ltr plus), which are not practical in glass or can. The image problems that PET bot- tles, in particular, have suffered from may be partly due to this associa- tion with lower-quality products in the past. However, there is evidence that millennials (the generation born between 1982 and 2004) are increas- ingly less likely to make such a link and tend to view shatterproof ma- terials and re-sealable containers as benefits, rather than the sign of a sub-standard product. The wide- spread adoption of one-way kegs and cans in the craft brewing industry has also helped to raise awareness amongst consumers about packag- ing formats – and these have become discussion points around which social media campaigns are being based. It has not been until recently, however, that PET as a beer-packaging material has become more widespread. For while the physical properties of PET are sufficient by themselves for main- taining an acceptable level of quality in the carbonated soft drinks market, beer presents a trickier proposition. The beer problem Beer is, in many ways, a delicate product. It is highly susceptible to staling due to the formation of off flavours from even very low levels (>50ppb) of dissolved oxygen (DO) in package. It’s perceived flavour and mouthfeel can be altered drastically by changes to the carbonation level and exposure to UV light will cause the formation of off flavours through a reaction with hop derived iso- α -acids. Taints that are picked up from the packaging material are easily de- tected in delicately-flavoured styles, and the absorption of compounds from beer by the packaging material can dull its flavour profile. While the O 2 permeability of PET is much lower than some of the other polymers that one might be familiar with, shown in
(a)
Gas molecules
(b)
Gas molecules
The tortuous path barrier slows the rate of gas diffusion through a material
(a)
(b)
Active elements of a barrier react to fix O 2 with the material as it diffuses through it
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