Content uploaded by Wendy Cladman
Author content
All content in this area was uploaded by Wendy Cladman on Dec 08, 2018
Content may be subject to copyright.
,,,,, ",,.1J; ?llili"i**1,""eXh*:|j
ffi p r r : s0e58 -6e46(es)000e4-6 o,rr-**rrr,*,*-r,"* -"',i".l
ELSEVIER
Shelf-life of Milk Packaged in Plastic Containers
@58-6946/98iS-see front matter
With and Without Treatment to Reduce
Light Transmission
Wendy Cladmano, Steven Scheffer', Nina Goodrich'and Mansel W. Griffiths'*
" Department of Food Science, Unioersity of Guelph, Guelph, ON, Canada, NIG 2Wl
hTwinpak Inc., 910 Central Parkway 14., Mississauga, ON, Canada, LSC 2V5
(Received 12 March 1998; accepted 3 July 199E)
ABSTRACT
The eflect of prolonged light exposure on the chemical changes in milk stored at 4"C in clear polyethylene terephthalate (PET)
bottles was compared with milk stored in green PET bottles, containers made from PET incorporating a UV blocker, PET containers
with exterior labels and high-density polyethylene (HDPE) jugs and low-density polyethylene (LDPE) pouches stored under the same
conditions. Data were obtained for lipid oxidation, vitamin A degradation, protein hydrolysis, lipolysis and microbial growth. The
milk stored in green PET bottles experienced less lipid oxidation and vitamin A loss than milk stored in the clear PET bottles, or the
LDPE pouches and jugs. In general, the milk stored in the clear PET bottles was not as well protected from the effects of light as milk
stored in green bottles or LDPE pouches. However, during the first week of storage, only vitamin A loss showed a substantial
difference between the milk stored in green PET bottles, clear PET bottles or LDPE pouches. The PET bottles with UV blockers
slowed vitamin A degradation but had little effect on lipid oxidation. Blocking visibte light with translucent labels helped to inhibit
lipid oxidation and vitamin A degradation. O 1998 Elsevier Science Ltd. Al1 rights reserved
INTRODUCTION
There have been many attempts to improve milk
packaging in recent years. Since milk started to be dis-
played in supermarket coolers under fluorescent lights,
there have been difficulties with maintaining a fresh,
pleasing flavour. Consumers, although concerned with
the nutritional aspects, are probably most influenced by
the flavour of the product (Thomas, 1981). Light expo-
sure, especially to wavelengthi below 500 nm causes the
destruction of light-sensitive vitamins (riboflavin, vita-
mins A and C), induces chemical reactions that affect
milk proteins and lipids, and results in the development
of unpleasant flavours in foods (Fanelli et al., 1985;Sattar
et al., 1977; Schrcider et al., !985). Changes in flavour
can be caused by the destruction of vitamin A, protein
breakdown, lipid hydrolysis, microbial spoilage or the
oxidation of unsaturated fatty acids (Thomas, 1981). Ofr-
flavours may thus be linked to a drop in the nutritional
value of the milk. Light-induced flavour development is
dependent on the availability ofoxygen (Schrirder, 1982).
This can be controlled by reducing the amount of head
space in the container. avoiding agitation ofthe product
and by using oxygen impermeable containers to prevent
further entry of oxygen. Oxidative reactions were re-
ported to be controlled after free oxygen was depleted in
glass containers, but they continued in cartons and high-
density polyethylene containers (HDPE), which were
*Corresponding author. Tel.: 5 19 824 4120 x2269; fax: 519 824
6631; e-mail: mgriffit@uoguelph.ca.
Int. Dahy Journal E (1998) 629-636
irl 1998 Elsevier Science Ltd. All rights rescrved
Printed in GHt Britain
more psrmeable to oxygen than glass (Schrcider er al.,
1985).
Polyethylene terephthalate (PET) has been used in
r€cent years as a very effective packaging material for
carbonated beverages, edible oils and other food prod-
ucts (Feron et a1.,1994).Its use for milk packaging would
help overcome problems that have accompanied other
materials. PET is not easily breakable, is recyclable,
and the contents of PET bottles can be easily poured.
They are easy to open and close and do not require
any ancillary devices to assist opening or pouring. An
opened bottle can be easily resealed, thus minimizing
recontamination and wastage due to spills. PET bottles
may be used for individual servings or for I or 2L
volumes.
In addition, the oxygen permeability of PET is 4 to
5 cc @ STP.mil 15.5 sq cm- 1 24h-r compared to 100 to
200 cc for HDPE. The relative thicknesses of PET and
HDPE bottles used for beverages are 12 mil and 20 mil,
respectively, indicating that a PET bottle is 20 times less
permeable to oxygen than HDPE (data of Scheffer, Twin-
pak Inc., Mississauga, ON). To reduce the adverse effects
of light on milk quality, coloured or opaque PET bottles
can be used, This is acceptable to the consumer for
products such as cola or beer, but, when these containers
have been proposed for packaging milk, consumers have
shown a preference for clear or translucent white con-
tainers (White, 1985).
Despite the possible advantages, few studies have been
published to determine the shelfJife of milk packaged
in PET bottles. In this study, the quality of milk was
examined under storage conditions that simulated those
629
630 ll. Cladman et al.
\--
\/
expected during display in a supermarket to ascertain
how well clear PET could protect the milk from the
damaging effect of UY and visible light. A comparison
was made for milk stored in clear pET with other
packaging materials. In addition, the study inciuded
the effect of further reducing the amount of visible
light and UV radiation transmitted through pET con-
tainers on the shelfJife of milk by using (i) green pET
bottles, which largely prevent the transmission of light
wavelengths below 500 nm, (i, incorporating UV
absorbing materials into PET, and (iii) by placingllabels
on PET bottles.
MATERIALS AND METHODS
Milk storage and sampling
Eight 4 L bags (each containing three 1.3 L pouches) of
each of whole and 2% freshly pasteurized milk were
obtained _fr9m a local processor. Two 2 L HDpE jugs
each of whole and 2o/o milk were purchased from a l-ocil
convenience store on the same day. All the products had
a similar expected shelf-life, as measured ty the .best-
before'date. The milk was transported to the laboratory
on ice in insulated containers. On arrival at the labora-
tory, the milk was aseptically dispensed lrom the pouches
into 20 clean, green and 20 clean, clear 600 mL pET
bottles for each of whole and 2o/o product. The milk jugs
were aseptically emptied, rinsed with sterile water, and
then refilled with the same milk as the pET bottles. The
remaining pouches were retained in the original outer
bags.. Du.ring the entire procedure, the milk was kept on
crushed ice to maintain a cold temperature.
In another experiment, sixteen 4 L bags of 2oh mllk
with 17 d until the expiry date were obiained from a
local dairy and were kept cold and in the dark until
used. The milk was aseprically dispensed into 22 bottles
e.ach of (i) clean, clear 600 mL bottles made from pET;
(ii) clean, clear 600 mL bottles made from pET combined
with UV absorbers (UV-pET); (iii) clean, clear,
labelled 600 mL bottles made from pET; (iv) clean, clear,
labelled 600 mL botrles made from UV-pET; and (vj
high-density. polyethylene (HDpE) bottles (500 mli
without Iabels.
One of the labels used was placed over the light meter
to assess the degree of translucency of the labels.
_Qne 4 L bag of 2Y, milk (from the same dairy), with
15 d until the expiry date, was used to lill four pijT and
fo.u1 HDPE bottles, which were then completely covered
with aluminum foil.
- After being dispensed, the milk was immediately trans-
ferred to an environmental chamber, 60 cm by iOS cm,
held at a constant temperature of 4 * l.C, fh! tgtrting
was provided by four 60 W cool white Sylvanii
fluorescent tubes for 13 h daily. The intensity of ilght at
the top and sides ol the bottles wa, m.aiured -using
a General Electric type DW-68 light meter and wai
found to be about one-half of the intensity of light
measured on top shelf front row bottles and tartoniin
an.average local sL.Iermarket. The light was evenly dis_
tributed over all of the bottles and fouches within the
chamber.
On each test day, four clear pET, four sreen pET
bottles and 1 LDPE pouch for each milk typejwhole and
2oh) were randomly selected and a fioofia sample
(200 mL) used for analysis, Similarly, 100 mL was re-
moved from each of the 2 HDPE jugs for both milk types,
pooled and used for analysis. The milk was examined for
visual appearance (colour and the presence ofclots) and
odour. Each chemical assay was carried out in triplicate.
An ANOVA, computed using Microsoft Excel, was used
to compare treatments. Treatments were deemed to be
significantly different when P < 0.05,
Chemical testing
Pr ot ein hy dr oly si s as say
_ A,2T!Jryple of milk was deproteinized by adding
5 mL of 0.72 r{ trichloroacetic acid. The precipitate was
removed by filtration through Whatman 442 ashless
filter paper. Two mL of sodium carbonate reagent (15%
sodium_carbonate anhydrous and 1% tri-sodium phos-
phate; Fisher Scientific Ltd, Mississauga, ON, Canada)
was added to 1mL of filtrate and mixed vigorously.
Folin-Ciocalteu's phenol reagent (Sigma Chemical Co.,
St Louis, MO) was diluted 1 :3 with Aistllea warer, and
0.6 mL of the diluted reagent was added, with mixing, to
the filtrate. After standing for 5 min at room temperature,
the reaction mixture was again filtered through a O.+S pm
syringe filter (Millipore Corp.). The absorbance at
650 nm was read in a Pharmacia Novaspec II spectro-
photometer. The level of free tyrosine wajdetermined by
comparing the absorbance values with a standard curvi
of known tyrosine concentrations (Hull, 1947). The ex_
tent of proteolysis was proportional to the level of
tyrosine released during the hydrolysis of proteins.
Lipolysis
A lipid extraction was achieved by the addition of
10 mL of extraction mix (2-propanol, petroleum ether,
4 tt HrSOa; 4O: l0: 1), 6 mL petioleum ether and 4 mL
water to 3 mL of milk, prewarmed to 30"C. The mixture
was shaken vigorously, allowed to stand for the separ-
ation of the two phases, and a 2 mL aliquot of the upper
phase removed to be titrated with 0.-02 N merhan;lic
KOH, following the addition of six drops of colour
indicltor (1% phenolphthalein). The conienrration of
free fatty acid (FFA) was determined as
p equiv. FFA mL- 1 = 1fN7f Z; x 103
where T is the net titration volume; N the normality of
KOH; P the volume titrated/volume of upper layer;
Iz the volume of milk.
A control was run without milk (Deeth et al., 1975).
Vitamin A degradation
Five mL al95V, ethanol were added to 2 mL of milk at
room temperature, mixed and allowed to stand at room
temperature for 5 min. Hexane (5 mL) was added and the
mixture vortexed for 30 s, then allowed to stand for
2min. This was repeated two more times. Three mL of
distilled water were added to the above mixture and
mixed by gentle inversion, followed by centrifugation for
l! mi1 a1 1000 g at l5'C. The ,pp.. iuyer was removed,
filtered through a-,0,!5 ru nyl,:n'filter iVfittipore Corp.j
and stored at - 20'C until HpLC analysis.'
_A Waters HPLC system was used wittra 25 cm Waters
pPorasil column and a Varian UV detector adjusted to
Shelf-hfe of milk packaged in plastic containers 631
313 nm. The mobile phase used was as follows: 49 parts
wet hexane:49 parts dry hexane:2 parts diethyl ether.
Using a flow rate of 2 mL min - 1, 100 pL of sample or of
a standard (retinol palmitate) was injected and the trans
peak eluted at 4 min. The area under the peak was
compared with known standards to calculate the concen-
tration of retinol palmitate in the milk samples (Mar-
shall, 1992).
Lipid oxidation
Lipid oxidation was determined by measuring in-
creasos in conjugated dienes according to the method of
King (1962). Milk (17.6 mL), prewarmed to 30"C, was
precipitated by the addition of 1mL of l gml-1 tri-
chloroacetic acid (TCA) and 2 mL of 95o/o ethanol. Fo1-
lowing a 5 min incubation at 30oC, the precipitate was
removed by filtration through a Whatman #42 filter
paper, and lml- of 1.4oh 2-thiobarbituric acid was
added to 4 mL of the resulting filtrate. This was then
incubated for 60 min at 60'C, cooled and the absorbance
read at 532 nm in a Pharmacia Novospec II spectro-
photometer.
Psychrotrophic counts
In order to determine that the milk used in the experi-
ments was of similar microbiological quality and to ver-
ify that the type of packaging material did not affect the
microbiological shelfJife of the product, psychrotrophic
counts were determined in all milk samples. The milk
samples were spread plated on plate count agar (Difco,
Detroit, MI) using a spiral plater model D (Spiral Sys-
tems, Bethesda, MD). When necessary, ten-fold dilutions
were prepared in peptone water (Difco). Colony counts
were obtained after incubation of the plates at 22"C for
25-40h (Griffiths et a1.,1980).
RESULTS
Microbiological quality of the milksand light transmission
during storage
M icrobiological quality
The rate of growth of psychrotrophic bacteria in whole
milk was similar for all of the packaging types (Table l).
The bottles and jugs that were filled in the laboratory did
not appear to have higher counts than the unopened
pouches. Up to the expiry date, there were lew visible
signs olspoilage. After 18 d storage, the bacterial counts
for the 2o/o milk samplos were higher in the jugs and
pouches than in the bottles.
However, in the experiments conducted with con-
tainers treated to further reduce light transmission, the
initial psychrotrophic counts in the milk were also low
(<l0cfuml-'1, but, in this case, they remained low
throughout storage in all containers except the foil-wrap-
ped PET and HDPE bottles (Table 1). Counrs in milk
packaged in these reached counts >1x 105 cfumL-r
after 12 d, The reason for this increase is unclear.
Light tansmission
The amount of light reaching the bottles in the envi-
ronmental chamber was comparable or slightly less than
that encountered by milk stored at the front of the top
rows of chill cabinets in supermarkets. Covering the top
of the light meter with one of the labels from the milk
bottles reduced the amount of light exposure by 75%. All
of the bottles in the environmental chamber received
a similar level of light exposure throughout the test
period.
Comparison of shelfJife of milk packaged in clear
PET bottles, green PET bottles, LDPE pouches and
HDPE jugs
Lipid oxidation
Both milk types in the green bottles and in the pouches
exhibited a similar pattern of lipid oxidation over the
18 d storage period, although minor differences could be
observed (Fig. I ). The oxidation level increased gradually
and the milk was undrinkable after 14 d. The same milk
stored in HDPE jugs showed a high level of lipid oxida-
tion after only 3 d. The clear PET bottles were not as
effective as the green bottles at inhibiting oxidation. The
oxidation levels increased at a faster rate in the clear
bottles and the milk was strongly oxidized by the 10th
day. By the 14th day, there was a significant difference
(P < 0.05) in levels of oxidation for whole milk in all
packaging types, with the lowest level of oxidation being
observed in the green PET bottles (Fig. 1). However, for
the 2o/o milk stored for 14d, there was no significant
difference between oxidation levels seen in pouches and
green PET bottles, but the levels in these two packaging
materials were significantly (P < 0.05) lower than for the
clear PET bottles and the jug (Fig. l). There was a signifi-
cant difference (P < 0.05) in levels of oxidation it the 2Yo
milk stored in green PET bottles and pouches at both 10
and 18 d, with oxidation being lower in the milk
packaged in green PET bottles.
Vitamin A ilegradation
The vitamin A concentration in all whole milk sam-
ples, except for the milk stored in the green PET bottles,
decreased by at least 47% within the first 6 d of storage.
The vitamin A content in the milk packaged in the green
PET bottles fell by 28% (Fis. 2). The milk stored in rhe
HDPE jugs had the most severe vitamin A loss (58%)
and the pouches and clear PET bottles were similar in
their respective levels of degradation.
The pattern of vitamin A degradation in 2% milk was
similar to that observed in whole milk (Fig. 2). The
vitamin A content of 2% milk in the green bottles de-
clined less than the others until 14 d when it dropped to
a level similar to the pouches(57% and 54oh, respective-
1y), The level of vitamin A in milk stored in the clear
bottles declined more rapidly than in pouch-packed milk.
In both clear and green PET bottles, the vitamin A level
decreased more in 2Y' mllk than in whole milk. This was
not observed for the other containers.
AII differences were signiflcant (P < 0.05) for the whole
milk except for the comparison between milk stored for
6 d in the clear bottles and pouches and in the clear
bottles and jugs. Similarly, for the 2Vo milk all diflerences
in vitamin A content were signilicant (P < 0.05) except
between milk stored in green bottles and pouches for
18 d.
Lipalysis
There was little difference observed in lipid degra-
dation in milk stored in the four different packaging
632 [4. Cladman eL al.
Table 1. Psychrotrophic Counts (CFU mL- 1) During Storage at 4'C of Whole Milk and 2% Milk Packaged in Various Containers
Packaging material Count (logCFU mL- I) at storage time
18d
r0d
6d0d
Whole milk
Clear PET
Green PET
LDPE pouch
HDPE jug
2% milk
Clear PET
Green PET
LDPE pouch
HDPE jug
PET + tabel
UY.PET
UV-PET + label
HDPE bottle
Foil-wrapped PET bottle
Foil-wrapped HDPE bottle
1.3
1.3
1.3
1.3
0.5
1.3
1.3
1.3
< 1.0
< 1.0
< 1.0
< 1.0
1.0
1.0
1.0
>4.0
2.26
1.48
r.48
1.3
1.0
>4.0
2.48
1.18
t.4
< t.0
1.48 (4 d)
3.41 (4 d)
3.94
5.0
4.0
3.53
1.92
3.23
2.0
4.52
2,0
< 1.0
1.95
< 1.0
4.3 (8 dl
4.3 (8 dt
7.76
7.08
'1.59
7.54
3.74
4.32
6.81
8.0
4.11
1.48
< 1.0
<1.0
>5.0 (12 d)
>5.0 (12 d)
A) whole milk A) whole milk
J
J
I
o
o
o
6
!-
.E
o
o
E
.E
o
E
E
C
N
o
o
e
c
(!
o
!
E
c
N
(,
n
6
o
o
e
G
o
a
0.3
o.25
o.2
0.15
0.1
0.05
0
Storage time (d)
B) 2% milk
0.15
0.1
0.05
Storage time (d)
Fig. l. Lipid oxidation in (A) whole and (B) 2% milk during
storage at 4'C in clear PET bottles (o), green PET bottles (r),
LDPE pouch (r') and HDPE jug (").
materials (Fig. 3), There was an increase in the level of
FFAs after l0 d, which continued until the end of the test.
The rate of lipolysis in the whole milk in the green PET
bottles appeared to increase less than in whole milk in the
other packaging materials.
6 10
Siorag8 time (d)
B) 2% milk
06101418
Storage tim€ (d)
Fig. 2. Vitamin A degradation in (A) whole and (B) 2o/o milk
during storage at 4'C in clear PET bottles (o), green PET
bottles (t), LDPE pouch (t) and HDPE jug (x).
For the 2% milk samples, lipolysis was the greatest in
the pouch and jug packaged milk and, by the end of the
storage trial, the level of lipolysis in the PET bottled milk
was lower than for milk in the other two packagos but the
difference was not significant (P > 0.05).
J
l
ci
c
o
o
o
E
=
E
6
c
o
u
A) whole.rnilk
Shelf-lde of milk packaged in plastic containers 633
Storage time (d)
B) 2% milk
Fig. 3. Rate or riporysis,.ilHjl]:ll,r,,* mlk during
storage at 4oC in clear PET bottles (o), green PET bottles (r),
LDPE pouch (e) and HDPE jug (x).
Proteolysis
The data obtained for protein hydrolysis were incon-
sistent, and it was difficult to establish a clear trend.
However, in all cases, there appeared to be no increase in
tyrosine values, and hence in presumed proteolysis, in
any of the samples when compared with the milk sam-
pled prior to storage (Fig. a).
Comparison of shelfJife of milk stored in PET bottles
and UV-PET bottles with and without labels
To investigate the effects of further reducing light
transmission through PET containers, milk was
packaged in PET bottles that had been treated to incor-
porate a UY-blocker and in labelled PET bottles.
Lipid oxidation
After 6 d of storage, milk in the labelled PET bottles
exhibited the lowest degree of lipid oxidation, followed
closely by milk in the labelled UV-PET bottles (Fig. 5).
At the end of the storage period, there was little dif-
ference in lipid oxidation observed in any of the milk
in the PET bottles, but lipid oxidation levels in milk in
the UV-PET bottles was slightly, but significantly
(P < 0.05), lower than those in the PET bottles. The milk
in the foil covered bottles maintained low levels of lipid
oxidation for the entire test period and there was no
significant difference (P > 0.05) between the packaging
types.
0.025
0.02
0.015
0.01
Storage time (d)
B) 2olo milk
0.035
0.03
0.025
0.02
0.015
0.01"0
Storage time (d)
Fig. 4. Rate of proteolysis in (A) whole and (B) ZYo milk during
storage at 4'C in clear PET bottles (o), green PET bottles (t),
LDPE pouch (e) and HDPE jug (x).
0,25
0.2
0.1 5
0.1
0.05
00510'1520Storage time (d)
Fig. 5. Lipid oxidation in 2% milk during storage at 4'C in
PET bottles (.), PET + label bottles (r), UV-PET boules (e),
UV-PET + label bottles (x), HDPE boules (x), PET (in foil)
bottles (o), and HDPE (in foil) bottles (+).
Vitamin A analysis
The milk in PET bottles treated to block UV radiation
had the least amount of vitamin A degradation through-
out the test, with the labelled bottles permitting 20 to
30% less degradation than those that were unJabelled
(Fig. 6). The milk in the labelled, UV-PET bottles losr
30% of its vitamin A content after 6 d and 50%o after 18 d
(Fig. 6). At the end of the storage period, vitamin A levels
were the highest in milks in the labelled UV-PET bottles
i
J
E
CD
E
d)
!G
o
c
6
P
F
J
E
gJ
E
o
-3
(!
o
E
t-
\ z.s
E
f
q
Oo
a
o
a
< t.c
u-
l!
E
c
o
N
to
G
o
o
E
(!
o
o
o
6
A) whole milk
0
634 ll, Cladman et al
J
E
f
q
o
a
d
c
o
o
L
L
J
fd
c
o
o
E
.E
6
q
6
.E
o
t
.-
J
E
o
E
(,
-l
s
o
c
6
F
Storage time (d)
Fig.6. Vitamin A degradation in 2% milk stored at 4'C in PET
bottles (o), PET + label bottles (r), UV-PET bottles (r), UV-
PET + label bottles (x), HDPE bottles (x), PET (in foil) bottles
(o), and HDPE (in foil) bottles (+).
(P < 0.05). Vitamin A levels in milk packaged in labelled,
untreated PET bottlos were comparable to the milk in
UV absorbing PET bottles. Except for the sampling after
3 d exposure to light, the rate of vitamin A degradation in
milk in HDPE bottles was very similar to the milk in
normal PET bottles. The milk in the foil covered bottles
showed no degradation until 12 d of storage, and then
the level of vitamin A decreased by only 20% for the milk
in HDPE bottles and 7o/o for product in the PET bottles
(Fig. 6).
Lipolysis
The rate of lipolysis was similar in milk packaged in
all the containers, except those covered in foil (Fig. 7),
Neither the labels nor the UY blocker appeared to have
an effect on the level oflipid degradation (P > 0.05). The
milk in the foil covered Lottlei underwent greater lipo-
lysis than in the light exposed bottles.
Protein h.,-drolysis
The level of free tyrosine, which is indicative of pro-
teolysis, remained low in milk contained in all packaging
types except the containers wrapped in foil (Fig. 8). In
milk in the foil-wrapped HDPE bottles, there was a sig-
nificant increase (P < 0,05) in proteolysis. Although the
level of proteolysis remained high in milk in the foil-
wrapped PET bottles, the rate of increase was not as
great as that in milk in the foil-wrapped HDPE bottles.
Packaging did not seem to have a large effect on
lipolysis or proteolysis. This was probably due to the
similar rates of growth of bacteria. However, the milk in
the foil-covered bottles showed evidence of lipoysis and
proteolysis at the end ofthe storage period, and this was
probably due to the higher counts of psychrotrophic
bacteria in containers which resulted in the production of
extracellular proteases and lipases (Phillips and Griffiths,
r990).
DISCUSSION
PET which had been treated to absorb a minimum
of 95o/, of UV radiation appears to be eflective in
Storage time (d)
Fig, 7. Rate of lipolysis in 2Yo milk stored at 4'C in PET bottles
(.), PET + label boules (t), UY-PET bottles (^), UV-
PET + label bottles (x), HDPE bottles (x), PET (in foil) bottles
(a), and HDPE (in foil) bottles (+),
10 15 20
Storage Ume (d)
Fig. 8. Rate of proteolysis in 2% milk stored at 4"C in PET
bottles (.), PET + label bottles (r), UV-PET bottles (r), UV-
PET + label bottles (x), HDPE boules (x), PET (in foil) bottles
(o), and HDPE (in foil) bottles (-t-).
preventing vitamin A degradation and provides similar
protection against lipid oxidation as LDPE pouches. The
green PET bottles were also able to block out much of
the harmful visible and UV light. The clear PET bottles
were not as protective against harmful light effects as the
UV-PET bottles or the green PET bottles but were only
slightly less effective than the LDPE pouches. Although
the clear bottles gave less protection against the effects of
light than the pouches covered with translucent LDPE
bags, they were far less permeable to oxygen than the
LDPE pouches. This may have limited the rate of oxida-
tive reactions (Schrtider et al., 1985). The translucent
HDPE jugs were not very effective in preventing lipid
oxidation due to the higher level of permeability of high-
density HDPE to oxygen (Feron et al., 1994\; Schroder,
1982). The rates of lipid oxidation and vitamin A decline
were considerably higher for the HDPE jugs than for the
other containers. This trend was also observed when
HDPE bottles were used.
All the different containers were at the same average
distance from the light source and received a similar
Shelf-life of milk packaged in plastic containers 635
dosage of light throughout the test period. The difference
in size and in surface aroa may have made a difference in
the amount of milk exposed to light, but milk in the 2 L
HDPEjugs degraded to the greatest extent even though
the ratio of surface area to volume was lower than for the
pouches or 600 mL bottles.
In all the samples, vitamin A decreased by at least 50%
by the time the expiry date was reached. However, the
rate of decline was significantly slower in the milk
packaged in the UV-PET bottles. Vitamin A is destroyed
by light at wavelengths of less than 450 nm (Sattar
et al., 1977). The degradation of both vitamin A and
riboflavin are dependent on the amount of fat in the
milk. Levels ofdegradation have been reported higher for
skim milk than for whole milk (Senyk and Shipe, 1981),
and also higher for added vitamin A than for natural
vitamin A. This was also observed in the milk stored in
PET bottles, where the vitamin A levels in 2Yo milk
declined more than in whole milk. Since the other con-
tainers were not as impermeable to oxygen, light was
probably not the only factor influencing the degradation
of vitamin A,
The green PET bottles did help protect against chem-
ical changes in milk during storage, but they may not be
easily accepted by consumers. Incorporating light ab-
sorbers for wavelengths less than 500 nm in the clear
PET seems to offer a better alternative. Yellow pigment
has been reported to absorb light in the 40O-500 nm
wavelength range, which would protect riboflavin from
detrimental light, and removes wavelengths below
415 nm, which would prevent vitamin A from being de-
graded (Fanelli et al.,1985). However, yellow bottles may
be as unappealing to the public as green bottles for the
storage of milk. When tested, consumers preferred white,
opaque bottles and jugs to coloured containers (White,
198s).
Shielding a major portion of the main body of the
bottle with a material impermeable to the damaging
wavelengths of light has also been suggested (Hoskin,
1988). It was reported that shielding the sides of a milk
bottle from light is more effective than shielding the top,
and it was also recommended that the shield materials be
printed with a background colour that would decrease
the light transmission (Hoskin, 1988). In the present
study, the labels provided for the test covered approxi-
mately 55% of the surface of the bottle and were translu-
cent. Measured on a light meter, the labels reduced
visible light transmission by 75%. Even so, after 6 d
storage, the level of lipid oxidation in the milk in the
labelled bottles was 40-50% lower than in the unlabelled
bottles and the vitamin A content was 20-30% higher in
the labelled bottles. Since milk is rarely on the shelves of
a supermarket for such a long time, and since flavour is
a deciding factor in the purchase of milk, this result could
be considered significant.
The foil-covered control bottles clearly demonstrated
the damaging eflects of light on vitamin content. There
was little or no decrease in vitamin A and no increase in
lipid oxidation when the milk was completely protected
from light exposure. Vitamin A degradation appears to
be largely caused by UV radiation but lipid oxidation
seems to be caused by visible light.
It is recognized that the containers used in this study
were of different sizes and, hence, had different surface:
volume ratios. However, the PET bottles were the
smallest containers used, and, therefore, had the greatest
surface:volume ratio. Yet the milk packaged in these
containers generally were less affected by light and oxy-
gen than those packaged in the larger containers. It is
possible that the protective effects of PET may be Sreater
when it is used to package milk in larger volumes.
CONCLUSIONS
Bottles made from PET incorporating UV blocking
agents can provide an attractive and convenient form of
packaging for milk, offering protection against vitamin
degradation and lipid oxidation. The shelflife of milk of
good bacteriological quality can be further improved by
using a labelled bottle to reduce light transmission. The
use of such labels may provide an unique opportunity to
attract young consumers and, thus, increase the con-
sumption of milk.
ACKNOWLEDGEMENTS
The authors are grateful to Maple Lane Dairies in
Kitchener, Ontario for providing the milk for this test
and to Dr. Y. Kakuda for assistance with the vitamin
A analysis. MWG would like to thank Dairy Farmers of
Ontario and the Natural Sciences and Engineering Re-
search Council of Canada for research funding.
REFERENCES
Deeth, H. C., Fitz-Gerald, C. H. and Wood, A. F. (1975)
A convenient method for determining the extent of lipolysis
in milk. Austr alian J ournal of D air y Te chnolog y 30, i 09- 1 1 1.
Fanelli, A. J., Burlew, J. V. and Gabriel, M. K. (1985) Protection
of milk packaged in high density polyethylene againsl
photodegradation by fluorescent light. Journul of Food Pro-
tection 48, 112-111.
Feron, V. J., Jetten, J., de Kruijf, N. and van den Berg, F.
(1994) Polyethylene terephthalate bottles (PRBs): a health
and safety assessment. Food Adilitiues and Contaminants ll,
5',71-594.
Griffiths. M. W., Phillips. J. D. and Muir, D. D. (1980) Rapid
plate counting techniques for enumeration of psychro-
trophic bacteria in pasteurized double cream. Journal of the
Society of Dairy Technology 33, 8-10.
Hoskin, i. C. (1988) Eflect of fluorescent light on flavor and
riboflavin content of milk held in modified half-gallon con-
tainers. Journal of Food Protection 51, 19-23.
Hull, M. E. (1941) Studies on milk proteins II. Colorimetric
determination of the partial hydrolysis of the proteins in
mllk. Journal oJ' Dairy Science 30,881*884.
King, R. L. (1962) Oxidation of milk fat globule membrane
material. I. Thiobarbituric acid reaction as a measure of
oxidized flavor in milk and model systems. Journal of Dnn'y
Science 45, 1 165-1 17 1.
Marshall, R. T. (1992) StandartJ Methodsfor the Examination of
Dairy Products, 16 edn. American Public Health Association
Inc., Washington, DC.
Phillips, J. D. and Griffiths, M. W. (1990) Pasteurized dairy
products: the constraints imposed by environmental con-
tamination. ln Food Contaminution from Enuironmental
Sources, eds J. O. Nriagu and M. S. Simmons. Wiley, New
York, pp. 387-456.
Sattar, A., deMan, J. M. and Alexander, J. C. (1977) Wavelength
eflect on light-induced decomposition of vitamin A and
p-carotene in solutions and milk fat. Journal of the Canadian
Institutc of Food Science and Technology 10, 56--60.
636 W. Cladman et al.
Schrdder, M. J. A. (1982) Effect of oxygen on the keeping
quality of milk, I. Oxidized flavour developmeot and oxygen
uptake in milk in relation to oxygen availability. Journal of
Dairy Research 49, &7-424.
Schrdder, M. J" A., Scott, K. J., Bland, K. J. and Bishop D. R.
(1985) Flavour and vitamin stability in pasteurized milk in
polyethylene-coated cartons and io polyethylene bottles.
Journal af the Society of Dairy Technology 38' 48-52.
Senyk, G. F. and Shipq W. F. (1981) Protecting your milk from
nutrieut losses. Dairy Field 64, 8l-85.
Thomas, E. L (1981) Trends in milk flavors. Journal of Dairy
Science 64,1023-1027.
White, C H. (1985) Consumer reaction to colored plastic milk
jvgs, Journal of Dairy Scieace fi.261164.
