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ORIGINAL ARTICLE
The impact of freezing and toasting on the
glycaemic response of white bread
P Burton and HJ Lightowler
Nutrition and Food Science Group, School of Life Sciences, Oxford Brookes University, Oxford, UK
Objective: To investigate the impact of freezing and toasting on the glycaemic response of white bread.
Subjects/methods: Ten healthy subjects (three male, seven female), aged 22–59 years, recruited from Oxford Brookes
University and the local community. A homemade white bread and a commercial white bread were administered following four
different storage and preparation conditions: (1) fresh; (2) frozen and defrosted; (3) toasted; (4) toasted following freezing and
defrosting. They were administered randomized repeated measures design. Incremental blood glucose, peak glucose response,
2 h incremental area under the glucose response curve (IAUC).
Results: The different storage and preparation conditions resulted in lower blood glucose IAUC values compared to both types
of fresh white bread. In particular, compared to the fresh homemade bread (IAUC 259 mmol min/l), IAUC was significantly lower
when the bread was frozen and defrosted (179 mmol min/l, Po0.05), toasted (193 mmol min/l, Po0.01) and toasted following
freezing and defrosting (157 mmol min/l, Po0.01). Similarly, compared to the fresh commercial white bread (253 mmolmin/l),
IAUC was significantly lower when the bread was toasted (183 mmol min/l, Po0.01) and frozen, defrosted and toasted
(187 mmol min/l, Po0.01).
Conclusions: All three procedures investigated, freezing and defrosting, toasting from fresh and toasting following freezing and
defrosting, favourably altered the glucose response of the breads. This is the first study known to the authors to show reductions
in glycaemic response as a result of changes in storage conditions and the preparation of white bread before consumption. In
addition, the study highlights a need to define and maintain storage conditions of white bread if used as a reference food in the
determination of the glycaemic index of foods.
European Journal of Clinical Nutrition (2008) 62, 594–599; doi:10.1038/sj.ejcn.1602746; published online 4 April 2007
Keywords: glycaemic response; bread; food storage; food preparation; blood glucose
Introduction
The glycaemic index (GI), first introduced in 1981 (Jenkins
et al., 1981), is a classification of the blood glucose raising
potential of carbohydrate foods. It is defined as the
incremental area under the blood glucose curve (IAUC) of
a 50 g available carbohydrate portion of a test food expressed
as a percentage of the response to 50 g available carbohydrate
of a reference food taken by the same subject, on a different
day (FAO/WHO, 1998).
Recent data support the preventive potential of a low-GI
diet against the development of type 2 diabetes and
cardiovascular disease (Salmeron et al., 1997a, 1997b; Frost
et al., 1999). There is also an interest in the potential of low-
GI diets in body weight management, with studies showing
that low-GI foods, or lowering the GI of a food, may reduce
hunger and result in a lower energy intake (Ludwig, 2000;
Warren et al., 2003).
As consumer interest in GI has grown in recent years, a
major challenge for the food industry is to develop and
produce foods of low glycaemic response. In the UK, bread
forms the most important staple amongst starch foods and is
one of the largest sectors in the food industry, producing
almost 12 million loaves and packs every day (Federation of
Bakers, 2005). Moreover, white bread sales represent over
70% of all bread sales in the UK. In 2004–2005, the
household consumption of white bread was almost 400 g
per person per week compared to 120 and 45 g per person per
Received 3 July 2006; revised 14 February 2007; accepted 19 February 2007;
published online 4 April 2007
Correspondence: Dr H Lightowler, Nutrition and Food Science Group, School
of Biological and Molecular Sciences, Oxford Brookes University, Gipsy Lane
Campus, Headington, Oxford OX3 0BP, UK.
E-mail: hlightowler@brookes.ac.uk
Contributors: PB planned and designed the study and was responsible for the
collection of data and writing the manuscript. HJL advised on the study design
and contributed to the preparation of the manuscript.
European Journal of Clinical Nutrition (2008) 62, 594–599
&
2008 Nature Publishing Group All rights reserved 0954-3007/08 $
30.00
www.nature.com/ejcn
week for wholemeal and brown bread, respectively (Depart-
ment for Environment, Food and Rural Affairs, 2006).
In the UK, the baking industry predominantly uses the
Chorleywood Bread Process (CBP), which is responsible for
over 80% of bread produced. The CBP, developed in 1961, is
a mechanical dough development process involving inten-
sive mixing, the use of oxidants, a short fermentation process
and the use of a pan for baking. In addition, the process has
an important impact in the UK as it enables the use of low-
protein home-produced wheat (Belderok, 2000). However, the
result of developments in the bread-making process is a
highly favoured, but medium- to high-GI white bread.
In the UK, an increase in time pressure in family life has
meant that infrequent, bulk shopping and freezing are
widespread. Consequently, the consumption of bread that
has been defrosted following freezing is commonplace. In
addition, consumption of bread in the form of toast, mainly
at breakfast, is also customary.
Although some food processes, for example gelatinization,
may lead to high-GI foods (Granfeldt et al., 2000), some food
manufacturing and handling may have an effect on the
degree of starch retrogradation, involving a rearrangement
or realignment of initially heated starch molecules (Berry,
1986; Lang and Vitapole, 2004). Moreover, retrograded
starch has been shown to reduce glycaemic response and
there is evidence of this happening under conditions of
cooling (Larsen et al., 2000; Frei et al., 2003). Retrogradation
is greater where the degree of gelatinization is more
complete, facilitated by higher temperature and greater
water availability or with greater mixing, for example in
the formation of bread dough. Furthermore, it has also been
shown that overall starch recrystallization reaches a max-
imum at around 41C and below this temperature further
starch recrystallization is minimal (Lang and Vitapole, 2004;
Goesaert et al., 2005). Importantly, the process of freezing
and defrosting bread will involve bread passing through this
point of maximal recrystallization and retrogradation twice,
once while cooling and again during defrosting.
Relatively small differences in the glycaemic response of
regularly consumed starch foods have shown beneficial
effects on health, including reduced cardiovascular disease
risk and glycaemic control (Frost et al., 1998; Brand-Miller et al.,
2003). Thus, investigations into ways of reducing the glycaemic
response to white bread are of important application. In the
light of the above, the aim of the present study was to
determine whether the glycaemic response of white bread could
be changed by everyday storage and preparation conditions
such as procedures of freezing, defrosting and toasting.
Subjects and methods
Subjects
Ten healthy subjects (three male and seven female) were
recruited to participate in the study. Subjects were staff and
students from Oxford Brookes University and from the wider
community. To participate, subjects were required to be
between 18 and 59 years of age, with a body mass index
(BMI) o30 kg/m
2
. Interested subjects were asked to complete
a health screening questionnaire to check against ill health,
including clinically abnormal glucose metabolism (fasting
blood glucose 46.1 mmol/l) and any medical conditions or
medications that might affect glucose regulation, gastric
emptying, body weight, appetite or energy expenditure.
Anthropometric measurements were made in the fasting
state, using standardized methods, on the morning of the
first test. Height was recorded to the nearest centimetre
using a stadiometer (Seca Ltd, UK), with subjects standing
erect and without shoes. Body weight was recorded to the
nearest 0.1 kg using the Tanita BC-418 MA (Tanita UK Ltd),
with subjects wearing light clothing and no shoes. BMI was
calculated using the standard formula weight (kg)/height
(m)
2
. Characteristics of the subjects are shown in Table 1.
Ethical approval for the study was obtained from the
University Research and Ethics Committee at Oxford
Brookes University. Subjects were given full details of the
study protocol and the opportunity to ask questions. All
subjects gave written informed consent before participation.
Study protocol
The method used to measure glycaemic response and to
calculate the GI value was in line with procedures recom-
mended by the FAO/WHO (1998). In addition, on the day
preceding a test, subjects were asked to restrict their intake of
alcohol and caffeine-containing drinks and to refrain from
intense physical activity (e.g., long periods at the gym,
excessive swimming, running, aerobics). To minimize the
possible influence of the second meal effect, subjects were
asked to refrain from eating an extra-large evening meal or
have an unusually high or low food intake throughout the
day preceding a test (Wolever, 1990). Where possible,
subjects ate a similar meal type on the evening before
testing; however subjects were asked to avoid consuming
pulses for this meal to avoid the effects of colonic fermenta-
tion on postprandial glycaemic (Macintosh et al., 2003). All
foods were tested in subjects after a 12 h overnight fast.
Test breads were administered to subjects in a randomized,
repeated measures design, with each subject acting as his/her
own control. Subjects travelled to the laboratory by car or
public transport and rested for 10 min before the test
commenced. Test breads were consumed first thing in the
Table 1 Characteristics of study population
Mean7s.d.
Age (years) 45.4714.3
Height (m) 1.7370.97
Weight (kg) 71.3710.1
BMI (kg/m
2
) 23.972.1
Abbreviations: BMI, body mass index; s.d., standard deviation.
Glycaemic response of white bread
P Burton et al
595
European Journal of Clinical Nutrition
morning (i.e., as breakfasts) and were compared with a
reference food (glucose) and were tested in equivalent
amounts (50 g) of available carbohydrate. Available carbo-
hydrate was estimated according to the FAO/WHO procedure
(total carbohydrate minus dietary fibre), using calculated data
of macronutrient content (Table 2). Thus, in this context,
dietary fibre was defined as unavailable carbohydrate.
As blood glucose responses vary within subjects from
day to day, the reference food (glucose) was tested 3 times
in each subject. Thus, subjects tested each test bread once
and the reference food 3 times in random order on
separate days, with at least a one-day gap between measure-
ments to minimize carry over effects. All test breads and the
reference food were served with 200 ml water. A further
200 ml water was given during the subsequent 2 h. Subjects
were asked to eat the test breakfast within a 10–12 min
period to reduce the influence of chewing on particle size
(Hoebler et al. 1998).
Test breads
Two white breads, one homemade bread prepared using a
standard home bread-making machine (Breadman Pro,
Russell Hobbs, Manchester, UK) and one commercially
available bread (Hovis Classic, British Bakeries Ltd, UK),
were tested for glycaemic response. The homemade white
bread was made from 500 g white wheat flour (Carr’s white
strong bread flour; a high-protein wheat that does not
contain enzymes, improvers or bleach), 8 g NaCl, 6 g sugar,
287 ml water, 6 g butter, 7 g skimmed milk powder and 8 g
dehydrated yeast without additives (Fermipan, DSM Bakery
Ingredients, Holland). Details of the macronutrient, dietary
fibre and sodium content of the two test breads are given in
Table 2. Parallel investigations of homemade and commer-
cial breads allowed investigation of any modulation, for
example by additives, in commercial breads not found in the
homemade bread.
Both breads were tested following four different storage
and preparation conditions: (1) fresh; (2) frozen and
defrosted; (3) fresh and toasted; (4) toasted following
freezing and defrosting. Fresh bread was tested on the
morning following baking or purchase. Frozen bread was
defrosted overnight at room temperature before consump-
tion the following morning. Breads were frozen between 2
and 7 days; it has been suggested that further retrogradation
during freezing following temperature reduction to the
frozen state does not occur (Gray and Bemiller, 2003). Bread
was toasted on the morning following baking or purchase
and, where applicable, following defrosting overnight at
room temperature. For the duration of the study, a toaster
was standardized to medium setting, ensuring consistent
moderate toasting.
Blood glucose measurements
Finger-prick blood samples were taken for capillary blood
glucose analysis. Recent reports suggest that capillary blood
sampling is preferred for reliable GI testing (FAO/WHO,
1998; Wolever and Mehling, 2003). To establish blood
glucose stability at the start of the blood glucose response
curve, fasting blood samples were taken at 5 and 0 min
before consumption of the food and the baseline value taken
as a mean of these two values. The reference food/test bread
was consumed immediately after this and further blood
samples were taken at 15, 30, 45, 60, 90 and 120 min after
starting to eat.
Blood was obtained by finger prick using the Glucolet 2
multi-patient lancing system (Bayer HealthCare, Newbury,
UK). Where necessary, before a finger prick, subjects were
encouraged to warm their hand under running warm water
to increase blood flow. Fingers were not squeezed to extract
blood from the fingertip as this may dilute with plasma.
Blood glucose was measured using Ascensia Contour auto-
matic blood glucose meters (Bayer HealthCare). The blood
glucose meters were calibrated daily using control solutions
from the manufacturer and were also regularly calibrated
against a clinical dry chemistry analyser (Reflotron Plus,
Roche, Welwyn Garden City, UK) and the HemoCue Glucose
201 þanalyser (HemoCue Ltd, Dronfield, UK). Using the
Bland–Altman analyses, there was a very strong correlation
(r¼0.980, Po0.001) and good agreement (mean difference
0.2 mmol, 95% CI 0.3 to 0.2, limits of agreement 0.80
and 0.32) between blood glucose measurements for a
random selection of 140 blood samples simultaneously
measured using the Ascensia Contour and the HemoCue
Glucose 201 þanalyser.
Statistical analysis
Statistical analysis was performed using the Statistical
Package for the Social Sciences (SPSS version 11.0. 1,
Chicago, IL, USA). Data are presented as means and standard
deviations. Before statistical analysis, the normality of the
data was tested using the Shapiro–Wilk statistic. The
Pearson’s correlation coefficient and the method of Bland
and Altman (1986) were used to examine the correlation and
agreement between the automatic analyser and the Hemo-
Cue Glucose 201 þanalyser. Levels of intra-individual
variation of the three reference (glucose) tests were
assessed by determining the coefficient of variation
(CV% ¼100 standard deviation/mean). Repeated measures
analysis of variance, with Bonferroni’s correction, was used
Table 2 Nutritional composition (per 100 g) of test breads, using
manufacturers’ data
Test bread Available
carbohydrate
a
Protein Fat Dietary fibre
b
Sodium
Homemade bread 43.9 8.6 1.7 1.9 0.6
Commercial bread 40.5 9.2 4.5 3.1 0.4
a
Calculated according to FAO/WHO (1998).
b
AOAC International methodology.
Glycaemic response of white bread
P Burton et al
596
European Journal of Clinical Nutrition
to compare glycaemic response between test breads stored
and processed in different ways. Statistical significance was
set at Po0.05.
Results
The mean intra-individual variation in glycaemic response
to the three reference tests in the subjects was 28% CV. This
value is consistent with previously reported data in normal
subjects (Wolever, 2006).
Figure 1 shows the incremental blood glucose response
curves for the homemade white bread. There was no overall
effect of storage and preparation on the peak rise in blood
glucose response (P¼0.64) in the homemade bread.
Table 3 shows the IAUC across the different storage and
preparation conditions for the homemade white bread.
Significant differences in IAUC (F ¼10.503, Po0.001) were
seen. Compared to the fresh homemade bread, IAUC was
significantly lower for homemade frozen and defrosted bread
(P¼0.010), homemade fresh toasted bread (P¼0.007) and
homemade bread that had been toasted following freezing
and defrosting (P¼0.001).
Figure 2 shows the incremental blood glucose response
curves for the commercial white bread. Peak rise in blood
glucose was significantly different across the storage and
preparation conditions (F ¼7.425, P¼0.001). Peak rise in
blood glucose for commercial fresh toasted bread and
commercial bread that had been toasted following freezing
and defrosting was significantly lower than fresh commercial
bread (P¼0.024 and P¼0.018, respectively) and commercial
bread that had been frozen and defrosted (P¼0.020 and
P¼0.006, respectively).
There were significant differences in IAUC (F ¼6.105,
P¼0.003) between the storage and preparation conditions
for the commercial bread (Table 3). IAUC values for fresh
toasted bread and bread that had been frozen, defrosted and
toasted were significantly lower than for fresh bread
(P¼0.001 and P¼0.026, respectively). The IAUC value for
fresh toasted bread was also significantly lower compared to
bread that had been frozen and defrosted (P¼0.046).
Discussion
The current study investigated the impact of storage
conditions and food preparation on the glycaemic response
to white bread. This is the first study known to the authors to
show reductions in glycaemic response as a result of changes
in storage conditions and the preparation of white bread
before consumption. All three procedures investigated, that
is freezing and defrosting, toasting from fresh and toasting
following freezing and defrosting, led to a reduced glycaemic
response and reduction in IAUC values.
The influence of food processing and cooking on glycae-
mic response is well documented (Brand et al., 1985; Bjorck
et al., 1994). Treatments incorporating the generation of
forces such as shearing, compression and extreme heat
treatment increase gelatinization, which results in the
breakdown of the starch granule. Thus, many processing
conditions lead to an increased susceptibility of the starch
Figure 1 Blood glucose response curve for homemade bread:
Glucose (*); fresh (J); frozen, defrosted (K); fresh, toasted (&);
frozen, defrosted, toasted (’).
Table 3 IAUC (mmol min/l) for homemade bread and commercial
white bread with different storage and preparation conditions
Test food Homemade bread Commercial bread
(mean7s.d.) (mean7s.d.)
Glucose 2917100
a
2917100
a
Fresh 2597103
a
2537106
a,b
Frozen, defrosted 179774
b
217799
b,c
Fresh, toasted 193779
b
183796
d
Frozen, defrosted, toasted 157785
c
187795
c,d
Abbreviation: IAUC, incremental area under the glucose response curve; s.d.,
standard deviation.
Values in the same column with different letters are significantly different,
Po0.05.
Figure 2 Blood glucose response curve for commercial white
bread: Glucose (*); fresh (J); frozen, defrosted (K); fresh, toasted
(&); frozen, defrosted, toasted (’).
Glycaemic response of white bread
P Burton et al
597
European Journal of Clinical Nutrition
granule to enzymatic salivary and pancreatic amylases
following consumption, resulting in greater availability of
glucose for absorption and increased glycaemic response.
Examples of such treatments are flour milling, intensive
mixing in bread dough formation, as in the CBP, and bread
baking, which may explain the relatively highGI value of
white bread.
Retrograded starch constitutes RS3, a form of resistant
starch, and there is evidence of the formation of RS3 during
the processes of cooling and freezing (Hoebler et al., 1999;
Goesaert et al., 2005). There has been an increasing interest
in resistant starch in recent years and positive health benefits
have been demonstrated, mediated through effects on colonic
fermentation and on both postprandial glucose and lipid
metabolism (Robertson et al., 2003; Higgins et al., 2004).
An important difference in the constituents of the home-
made and commercial white breads in this study was the
presence of dough conditioners and improvers. These
ingredients are widely used in commercial bread and are
designed to optimize dough formation and quality, reduce
staling rate and maintain water retention during baking
(Goesaert et al., 2005). However, water content and activity
within the dough facilitates the retrogradation process.
Moreover, amylopectin retrogradation, together with moist-
ure transfer between bread components, may be reduced
by the use of dough improvers (Baik et al., 2003). This may
explain smaller reductions in glycaemic response within the
commercial bread, compared to changes within the home-
made bread.
The implications of this study are twofold. First, it is
suggested that simple household methods, such as freezing,
defrosting and toasting, may alter the glycaemic response to
white bread, thus white bread need not always be a highGI
food. There still remains a preference for white bread in the
UK (Department for Environment, Food and Rural Affairs,
2006), therefore the current study will be informative to
consumers in terms of optimal storage and preparation of
white bread to favourably alter glycaemic response. Second,
this study highlights a need to reconsider the use of white
bread as a reference food in GI methodology. Although GI
values determined using white bread as a reference food can
be converted to GI values based on glucose (Wolever, 2006),
the findings from the current study suggest that if bread is
unintentionally treated differently each time it is used as a
reference food, reproducibility and comparisons of the GI of
foods between different studies will be difficult.
Variability of glycaemic response has been a major
criticism of the GI concept. This variability may be owing
to a host of factors, acting independently or together. In this
study, conscientious efforts were made to maximize standar-
dizing of possible factors, to reduce intra- and inter-
individual variability, including a 12 h fast before testing,
restriction of exercise, alcohol and caffeine consumption the
day before a test and time spent chewing the test food.
A limitation of this study was the indirect estimation of
available carbohydrate content of the bread samples. It is
recommended that future studies include such measure-
ments, including resistant and retrograded starch (Englyst
et al., 1992; McCleary and Monaghan, 2002), both before
and after storage and preparation conditions. In addition,
other methods have been proposed to classify the glycaemic
impact of foods based on total carbohydrate rather than
available carbohydrate, such as relative glycaemic effect
(Brouns et al., 2005). However, further research is needed on
the use of alternative indices to measure the glycaemic
response of foods.
In conclusion, this is the first study known to the authors
to show reductions in glycaemic response following different
storage and preparation conditions of white bread and
highlights the need to define and maintain storage condi-
tions of food products when the glycaemic response of foods
is determined. Relatively small differences in the glycaemic
response of regularly consumed starch foods have been
shown previously to have beneficial effects on health (Frost
et al., 1998; Brand-Miller et al., 2003). Thus, considering the
high consumption of white bread, the identification of ways
to reduce the glycaemic response of white bread and other
foods is important in the prevention and management of
chronic disease.
Acknowledgements
Sponsorship: This study was supported by a Biotechnology
and Biological Sciences Research Council studentship.
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