Factors affecting thermally induced furan formation

Article abstract of DOI:10.1021/jf801612c

Furan, a potential carcinogen, can be induced by heat from sugars, ascorbic acid, and fatty acids. The objective of this research was to investigate the effect of pH, phosphate, temperature, and heating time on furan formation. Heat-induced furan formation from free sugars, ascorbic acid, and linoleic acid was profoundly affected by pH and the presence of phosphate. In general, the presence of phosphate increased furan formation in solutions of sugars and ascorbic acid. In a linoleic acid emulsion, phosphate increased the formation of furan at pH 6 but not at pH 3. When an ascorbic acid solution was heated, higher amounts of furan were produced at pH 3 than at pH 6 regardless of phosphate's presence. However, in linoleic acid emulsion, more furan was produced at pH 6 than at pH 3. The highest amount of furan was formed from the linoleic acid emulsion at pH 6. In fresh apple cider, a product with free sugars as the major components (besides water) and little fatty acids, ascorbic acid, or phosphate, small or very low amounts of furan was formed by heating at 90-120 °C for up to 10 min. The results indicated that free sugars may not lead to significant amounts of furan formation under conditions for pasteurization and sterilization. Importantly, this is the first report demonstrating that phosphate (in addition to pH) plays a significant role in thermally induced furan formation.

Full text of DOI:10.1021/jf801612c

9490 J. Agric. Food Chem. 2008, 56, 9490–9494
Factors Affecting Thermally Induced Furan
Formation
XUETONG FAN,* LIHAN HUANG, AND KIMBERLY J. B. SOKORAI
Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture,
600 E. Mermaid Lane, Wyndmoor, Pennsylvania 19038
Furan, a potential carcinogen, can be induced by heat from sugars, ascorbic acid, and fatty acids.
The objective of this research was to investigate the effect of pH, phosphate, temperature, and heating
time on furan formation. Heat-induced furan formation from free sugars, ascorbic acid, and linoleic
acid was profoundly affected by pH and the presence of phosphate. In general, the presence of
phosphate increased furan formation in solutions of sugars and ascorbic acid. In a linoleic acid
emulsion, phosphate increased the formation of furan at pH 6 but not at pH 3. When an ascorbic
acid solution was heated, higher amounts of furan were produced at pH 3 than at pH 6 regardless
of phosphate’s presence. However, in linoleic acid emulsion, more furan was produced at pH 6 than
at pH 3. The highest amount of furan was formed from the linoleic acid emulsion at pH 6. In fresh
apple cider, a product with free sugars as the major components (besides water) and little fatty acids,
ascorbic acid, or phosphate, small or very low amounts of furan was formed by heating at 90-120
°C for up to 10 min. The results indicated that free sugars may not lead to significant amounts of
furan formation under conditions for pasteurization and sterilization. Importantly, this is the first report
demonstrating that phosphate (in addition to pH) plays a significant role in thermally induced furan
formation.
KEYWORDS: Furan; heat; pH; phosphate; temperature; sugars; ascorbic acid
INTRODUCTION
Phosphates are widely used as chelators and emulsion
stabilizers in many processed food products, such as soups and
processed meat products. Phosphates also function as a synergist
with primary antioxidants (11). Our earlier study (6) suggested
that pH had an influence on furan formation due to irradiation
and thermal processing. A more recent study (12) confirmed
the importance of pH in thermally induced furan formation in
model systems containing free sugars and/or ascorbic acid. It
is unknown whether phosphate will have any effect on the
formation of furan as a result of thermal treatment at different
pH values. The present study is the first demonstration of the
important role played by phosphate in furan formation.
Furan, a cyclic dienic ether with a boiling point of 31 °C, is
“reasonably anticipated to be a human carcinogen” according
to the U.S. Department of Health and Human Services (1), while
the International Agency for Research on Cancer classifies it
as “possibly carcinogenic to humans” (2). Furan is found in a
wide range of thermally processed foods with furan levels as
high as 170-5,000 ng/g in some foods (3, 4). Heating solutions
of carbohydrates, ascorbic acid, and fatty acids produces
furan (4-7). Thermally processed fruit juice contained furan
levels ranging from below 2.0 ng/g to 31 ng/g (3, 4) since fruit
juices contain high amounts of free sugars and ascorbic acid.
Both U.S. FDA and European Food Safety Authority are seeking
data on furan (8, 9), particularly related to its occurrence and
formation in food, factors and mechanisms that contribute to
its formation, and its toxicity. The long-term effects of furan to
the health of children is unknown, but the presence of furan in
baby foods is a concern because of their high sensitivity to
carcinogens in addition to the larger amounts (relative to body
weight) of certain foods (such as apple juice) that are consumed.
Many factors can affect the formation of furan. For example,
ferric ions increased the formation of furan in linoleic acid by
79%, while addition of reducing agents such as sulfites reduced
its formation from ascorbic acid (10).
Pyrolysis of carbohydrates and ascorbic acid at very high
temperatures (300 °C) resulted in the formation of furan (4, 13).
However, the conditions used in these studies do not represent
the conditions used in the processing of foods such as juices
and soups. A recent study (14) showed that significant amounts
of furan was formed from fruit and vegetable juices after
treatment at 123 °C for 22 min. The effect of heating time and
temperature on furan formation has not been systemically
investigated under the conditions of pasteurization and steriliza-
tion. Therefore, the objective of the present study was to
investigate the effect of heating time and heating temperature
on furan formation in apple cider and the effect of pH and
phosphate on its formation in solutions of furan precursors. Fresh
apple cider, which contains mostly free sugars besides water
* To whom correspondence should be addressed. Tel: 215-836-3785.
Fax: 215-233-6445. E-mail: xuetong.fan@ars.usda.gov.
10.1021/jf801612c This article not subject to U.S. Copyright. Published 2008 by the American Chemical Society
Published on Web 09/24/2008

Thermally Induced Furan Formation
J. Agric. Food Chem., Vol. 56, No. 20, 2008 9491
and has little phosphate, ascorbic acid, or fatty acids (15), was
chosen because of its simplicity and is ideal to study the
formation of furan from sugars.
MATERIALS AND METHODS
Chemicals and Materials. Furan (99%), furan-d4 (99%), D-fructose,
D-glucose, linoleic acid, and L-ascorbic acid were purchased from
Sigma-Aldrich (St. Louis, MO, USA). Commercial nonpasteurized
(fresh) apple cider was obtained from an apple cider producer (Zeigler
Beverage Co., Lansdale, PA, USA). The cider was either used within
three days of processing or stored at -20 °C for later use. The cider
had a soluble solids content (Brix) of 11.1 and a pH of 3.47.
Thermal Treatments. The samples (1 mL) of apple cider or
solutions were injected, using a syringe, into 1.2 mL ampule vials
(Wheaton, Millville, NJ, USA). The vials were flame sealed and then
placed in an ice water bath. The cooled samples were then submerged
into a heated 350CST silicone oil (Chemistrystore.com Inc. Cayce, SC)
bath (Polystat, Cole-Parmer Instrument Co., Vernon Hills, IL) with pre-
established temperatures. During the first minute of treatment, the vials
were shaken to allow uniform heating. After heating, the vials were
removed from the oil bath and placed into an ice water bath. The
temperature changes of the samples in the vials during heating were not
monitored because it was impossible to flame-seal the vials with a
temperature sensor. To estimate the temperature changes during heating,
a type-T thermocouple (wire size 40 AWG, Model TT-T-40-SLE, Omega
Engineering Inc., Stamford, CT) was inserted through a high temperature
silicone stopper (Nalge Nunc International, Rochester, NY) and a screw
cap into a 3-mL vial (Alltech, Deerfield, IL) containing 1 mL of apple
cider. The temperature of the apple cider sample in the vial during
heating and cooling was monitored through a data acquisition board
(5508-TC, ADAC Inc., Woburn, MA) and a data acquisition tool (Daisy
Laboratory Version 5.0). The temperature data were collected every
second. At temperatures >100 °C, the internal pressure built up inside
of the 3-mL vials and caused small leakages.
Figure 1. Changes in temperature of apple cider during heating and
cooling.
a Universal Presstight Connecter (Restek Chromatography Products,
Bellefonte, PA, USA). The temperature of the GC oven was set to 50
°C for 2 min, increased to 130 at 10 °C/min, then to 250 at 15 °C/min,
and held for 2 min at the final temperature. Helium was the carrier gas
and was supplied at a flow rate of 39 cm/s. The transfer line was held
at 250 °C during the entire run. Furan and furan-d4 were identified by
comparing the spectra and the retention time of the sample compounds
with those of standards. The m/z 39 and 68 ions and the ratio of 39/68
were used for the confirmation of furan, and m/z 68 was used as the
quantifier. The m/z 41 and 72 ions and the ratio of 41/72 were used for
the confirmation of furan-d4, and m/z 72 was used as the quantifier.
Furan was quantified using a standard curve established in the individual
matrix (apple cider or other solutions).
Statistical Analysis. The experimental design was a completely
randomized design with four replicates. Data were subjected to statistical
analysis using SAS Version 8.2 (SAS Institute, Cary, NC, USA). The
differences between treatments were analyzed by the least significant
difference (LSD) test using the general linear model. Only significant
differences (P < 0.05) are discussed unless stated otherwise.
Effect of Heating Time and Temperature on Furan Formation
in Apple Cider. Apple cider (1 mL) in the 1.2 ampule vials was heated
to 90, 100, 110, and 120 °C for 0, 2, 4, 6, 8, and 10 min. After heating
and cooling, the vials were opened, and samples were spiked with the
internal standard (furan-d4) and analyzed for furan.
RESULTS AND DISCUSSION
Temperature Profile during Heating and Cooling. The
temperature of apple cider increased rapidly after being sub-
merged in the oil bath (Figure 1). The time for reaching 99%
of targeted temperature was about 1.5 min. During cooling, the
sample cooled down to 3 °C within 2 min. Several types of
container systems were tested for the heating treatments.
Different types of glass vials with either screw or crimp caps
were tested first. They all leaked during heating as indicated
by gas bubbling from the vials. Also tested was a thermal death
time (TDT) disk made of an aluminum cylinder chamber (17
mm id × 4 mm length) (16). We found that the TDT disk
worked at temperatures of 100 °C or lower, but leaks occurred
at higher temperatures (110 and 120 °C). It was decided that
flame-sealed small ampule vials would be used as heating
containers. In many earlier studies when large vials containing
the samples were heated using heating blocks or water
baths (6, 7, 17), it was impossible to study the effect of heating
time because it took more than five minutes to reach the desired
temperature. In our system, we used smaller vials (1.2 mL),
flame sealing, and submerged heating. The desired temperature
of 1 mL apple cider samples in 3 mL vials was reached within
∼1.5 min as indicated by temperature readings. It was therefore
assumed that the temperature in the 1.2 mL vials also reached
the target temperatures in ∼1.5 min. This come-up time was
shorter than the heating times of 2, 4, 8, and 10 min.
Effect of pH and Phosphate on the Formation of Furan from
Solutions of Sugars, Linoleic Acid, and Ascorbic Acid. Solutions of
5% fructose, sucrose, glucose, and 1% ascorbic acid were prepared
either in 100 mM NaCl or Na-phosphate solutions with pH of 6 and 3,
representing pH values of sugar solutions and many fruit juices,
respectively. NaCl served as a control for Na-phosphate as NaCl did
not affect furan formation (data not shown). An emulsion of 0.1%
linoleic acid was also prepared in ∼100 mM Cl- or phosphate solutions
with pH of 6 and 3 by homogenizing the mixture using a homogenizer
(Virtishear, Virtis, Gardiner, NY, USA) at a speed setting of 70 for 1
min. Different concentrations of the compounds were used either to
simulate fruit juices or because of low solubility (linoleic acid). The
pH of the solutions was adjusted using 1 M HCl or phosphoric acid.
The solutions (or emulsion) were then placed into the 1.2 mL ampule
vials, sealed, and heated for 10 min at 120 °C using the oil bath. After
being cooled in ice water for 5 min, the vials were opened and spiked
with furan-d4. Furan was then analyzed.
Measurements of Furan. Furan was analyzed as described
earlier (6, 7) with minor modifications. After being treated, the samples
were spiked with furan-d4 to a concentration of 5 ng/g and spiked before
being transferred to 10 mL glass vials that contained stir bars. The
glass vials, sealed with septa and caps, were incubated at 35 °C in a
water bath for 25 min on a Corning heat/stir plate (Supelco, Bellefonte,
PA, USA) before a solid phase microextraction (SPME) fiber (85 µm
Carboxen-PDMS) was inserted into the headspace of a vial. After 20
min of extraction time, the SPME fiber was inserted into the GC
injection port maintained at 240 °C and held for 5 min to desorb furan.
Compounds were separated by a Hewlett-Packard 5890/5971 GC-MSD
(Agilent Technologies, Palo Alto, CA, USA) equipped with a 3.5 m ×
0.32 mm (i.d.) GasPro capillary column connected to a 30 m × 0.32
mm (i.d.), 0.1 µm DB-5 column (J & W Scientific, Folsom, CA) using
Effect of Heating Time and Temperature on Furan
Formation in Apple Cider. Heating apple cider at 90 °C for
up to 10 min produced little furan (Figure 2). However, heating
apple cider at 100 °C for 4 min induced a detectable level of

9492 J. Agric. Food Chem., Vol. 56, No. 20, 2008
Fan et al.
Figure 3. Effects of pH and phosphate on furan formation in solutions of
sugars, ascorbic acid, and linoleic acid. One milliliter solutions of 5%
fructose (Fruc), 5% glucose (Gluc), 5% sucrose (Sucr), 1% ascorbic acid
(Asco), and 0.1% linoleic acid (Lino) prepared in 100 mM NaCl or Na-
phosphate of pH 6 (A) and 3 (B) were heated to 120 °C for 10 min. Bars
with the same letter are not significantly different (LSD, P < 0.05).
Comparison was made within the same pH.
been heated at high temperatures to ensure that the product is
microbiologically safe, which again increases furan formation.
For inactivating foodborne pathogens in apple cider, heating at
90 °C for a few seconds is sufficient to achieve a 5-log reduction
of the bacterial population and little furan would be formed
during such processes.
Effect of pH and Phosphate on the Formation of Furan
from Solutions of Sugars, Linoleic Acid, and Ascorbic Acid.
At pH 6 and in 100 mM NaCl solution, 36.7 ng/mL furan was
formed from linoleic acid after 10 min of heating at 120 °C
(Figure 3). No detectable furan was formed from solutions of
any sugar or ascorbic acid. At pH 6 and in the presence of 100
mM phosphate, furan was formed from all solutions except
sucrose solution. These observations suggest that the presence
of phosphate increased furan formation from solutions of
glucose, fructose, linoleic acid, and ascorbic acid at pH 6.
Heating the solutions at pH 3 in the presence of NaCl did not
produce furan from any sugar but induced 8.5 and 12.3 ng/mL
furan from ascorbic acid and linoleic acid, respectively. In the
presence of phosphate, the amount of furan produced from
linoleic acid at pH 3.0 was 8.6 ng/mL, a much lower level than
that (62.9 ng/mL) at pH 6. At pH 3.0, the presence of phosphate
promoted furan formation from solutions of fructose and
ascorbic acid. These results suggest the importance of pH in
furan formation. Whether the pH increased or decreased furan
formation depended on the nature of the substrate. Similar to
our earlier study (6), the results in the present study indicated
that heating primarily induced the formation of furan from
fructose solution among the three sugars tested. Higher amounts
(∼100 ng/g) of furan have been found in products that contained
both meat and vegetables such as vegetable beef soups (3, 18).
Meats and meat products contain as high as 4% linoleic acid
(15) and have a neutral pH. Our results suggest that more furan
is formed from linoleic acid at neutral pH than at low pH during
heating.
Figure 2. Formation of furan in apple cider as a function of heating time
at 90 °C (A), 100 °C (B), 110 °C (C), and 120 °C (D).
furan. As heating time increased from 4 to 10 min, furan
formation increased linearly. Similarly, there was a linear
increase in furan formation as heating time at 110 °C increased
from 4 to 10 min. However, the amount (1.4 ng/mL) of furan
formed after 10 min of heating at 110 °C was not significantly
higher than that (1.3 mg/mL) heated at 100 °C. Compared to
heating at 100 °C, much higher levels of furan were formed at
120 °C. However, only about 3 ng/g of furan was formed even
after heating at 120 °C for 10 min. FDA analysis has found
that furan is present in thermally processed apple juice purchased
from supermarkets with furan levels ranging from nondetectable
to ∼8.2 ng/mL (3). Our results showed that small amounts of
furan accumulated in the vials during the first 2 min of heating
regardless of final temperatures (100-120 °C) because it took
about 1.5 min to reach targeted temperatures. Our results further
suggested that a small amount furan in apple cider was formed
at 90 °C and about 3 ng/mL furan was formed at 120 °C for 10
min. Therefore, it is possible that the presence of furan in the
commercially processed apple juice that contained more than 3
ng/mL of furan is due to prolonged heating at high temperatures.
In addition, U.S. FDA’s survey (3) found that the higher amount
of furan is mainly present in juices intended for consumption
by babies. Those juices are often fortified with vitamin C
(ascorbic acid) prior to thermal processing, which would increase
furan formation as ascorbic acid is one of furan’s precursors.
Furthermore, food intended for consumption by babies may have

Thermally Induced Furan Formation
J. Agric. Food Chem., Vol. 56, No. 20, 2008 9493
Compared to solutions prepared in NaCl, the presence of
phosphate significantly increased furan formation in linoleic acid
solutions at pH 6, but not at pH 3, indicating that the stimulating
effect of phosphate on furan formation also depends on pH.
use of phosphate may be questionable and may have to be re-
examined. In future studies, the concentration and presence of
phosphate would have to be carefully controlled and specified.
Another reason for the contradictory results in furan formation
may be due to the difference in pH and the presence of
phosphate that may be used in buffers. Therefore, it is important
to specify pH and phosphate concentration in the solutions when
publishing results relating to furan formation.
Our results showed that fatty acids upon thermal treatment
produced a much higher amount of furan than sugars and that
heating ascorbic acid solutions often generated more furan than
heating sugars. Furthermore, treating apple cider, which contains
mainly sugars with small amounts of ascorbic acid and fatty
acids (15), at 120 °C for 10 min only produced a low amount
(3 ng/mL) of furan. Heating (123 °C for 22 min) pumpkin puree,
which contains much lower sugars and much higher fatty acids
and ascorbic acid (15), produced 49 ng/g of furan (14). There-
fore, it appears that furan from sugars represented a minor route,
which is in agreement with the earlier study (14) showing that
only about 20% of furan was generated from sugars. Ascorbic
acid is also only a minor precursor of furan (12) in pumpkin
puree juices. Therefore, fatty acids (or a combination of fatty
acids with other compounds) may be responsible for the
formation of high amounts of furan in foods.
There are studies showing interactions of phosphate with
sugar in chemical reactions. For example, it has been found
that sugar solutions became bactericidal after being heated in
the presence of phosphate (21, 22). Recent studies (12, 14) have
shown that when sugars and ascorbic acid were heated in the
presence of phosphate (without realizing the phosphate effect),
many compounds besides furan were produced including acetic
acid, formic acid, acetaldehyde, and glycolaldehyde. It has also
been demonstrated that furan is produced from the recombina-
tion of reactive C2/C3 fragments such as acetaldehyde and
glycolaldehyde (12, 14). Phosphates may increase furan forma-
tion from sugars through the formation of those reactive
intermediate compounds and the subsequent recombination of
the compounds, which leads to the formation of furan.
The reason we used pH 6 for all of the solutions was that the
pH of sugar solutions without any pH adjustment was around
6. Our preliminary results showed that heating induced a similar
amount of furan in sugar solutions either in water or in 100
mM NaCl solutions, suggesting that NaCl had no effect on furan
formation. Changing the phosphate concentration from 100 mM
to 50 mM had no significant effect on furan formation in sugar
solution (data not shown). In addition, changing the pH from 6
to 3 had little effect on the partitioning of furan from solutions
to headspace (data not shown (19)). Furthermore, increasing
ascorbic acid from 1% to 5% did not increase furan formation
(data not shown), indicating that the concentrations of phosphate
and ascorbic acid were not the limiting factors under the
experimental conditions. Earlier studies (12, 14) showed that
there is not always a direct correlation between concentration
of precursors and furan formation. For example, when glucose
concentrations were increased from 2 g/100 g to 4 g/100 g, furan
formation due to pressure cooking increased from 64.4 ng/g to
98.8 ng/g, but further increase of glucose to 6 g/100 g reduced
furan formation from 98.8 ng/g to 61.5 ng/g. Addition of 56
mg/kg ascorbic acid to orange juice, which already contained
48 mg/kg ascorbic acid, reduced thermally induced furan
formation by 16%.
It has been shown that the formation of furan is associated
withthethermaltreatmentofMaillardreactionprecursors(10,23).
Phosphate buffer catalyzes Maillard reactions, and the
browning of solutions increased with increasing phosphate
concentration (24, 25). It was suggested that phosphate can
simultaneously donate and accept a proton and act as a
bifunctional catalyst for the nucleophilic reaction of the amine
with the carbonyl (26, 27). However, Maillard reactions occur
in the presence of amino acids or proteins and reducing
sugars, and accelerates with increasing temperature. In the
present studies, there was no amino acid or protein present,
suggesting that furan can be formed independent of Maillard
reactions. The mechanism for the phosphate-promoted furan
formation needs further study.
In summary, our results suggested that significant amounts
of furan were produced in apple cider only at higher temper-
atures (100 °C or above) and prolonged treatment time (more
than 4 min). The pH and presence of phosphate played a
significant role in furan formation in solutions of individual food
components. This is the first article about the effect of phosphate
on furan formation. The exact mechanism of phosphate and pH
effect needs further study.
Our results suggest that phosphate increased furan formation
in solutions of fructose and ascorbic acid, and this is the first
demonstration of phosphate effect on furan formation. Phos-
phates are common ingredients in many food products, such as
soups. It is unclear why phosphate affected furan formation.
Phosphates are known to enhance the emulsification of fat (20).
Therefore, the increased furan formation in linoleic acid in the
presence of phosphates may be partially due to the increase in
solubility of the fatty acid. An earlier study has found that the
addition of ferric ions promoted furan formation from linoleic
and linolenic acids (17). The authors suggested that furan
formation occurred through a radical chain reaction that proceeds
via a hydroxyperoxide route (lipid peroxidation).
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Received for review May 23, 2008. Revised manuscript received August
15, 2008. Accepted August 24, 2008. Mention of trade names or
commercial products in this article is solely for the purpose of providing
specific information and does not imply recommendation or endorse-
ment by the U.S. Department of Agriculture.
JF801612C