Note: Descriptions are shown in the official language in which they were submitted.
CA 02354300 2001-07-30
Process for Dehydration of Berries.
Field of the Invention
The present invention relates to the drying of antioxidant-rich berries while
preserving
antioxidant characteristics of the berries.
Background of the Invention.
Many bernes, including blueberries, strawberries, cranbernes, raspbernes,
black currants
and others, contain naturally occurnng chemical constituents with antioxidant
activity and may
therefore be termed "antioxidant-rich berries". Antioxidants are chemicals
that can inhibit
oxidation reactions. Oxidation reactions are known to produce harmful
chemicals within living
1o animals, including humans. Chemically, oxidation is an event in which a
compound loses
electrons. In biological systems, unsaturated lipids are important
constituents that are highly
susceptible to oxidation reactions, especially autoxidation reactions, that is
oxidation reactions
with molecular oxygen .
Autoxidation reactions of complex compounds typically occur in a series or
chain-
15 reaction, which can be divided into three stages described as initiation,
propagation and
termination stages (Pokorny, J. 1999. Antioxidants in Food Preservation. p.
309-337, IN
"Handbook of Food Preservation" M. S. Rahman, ed. Marcel Dekker Inc., NY).
Initiation
requires a free radical, that is a compound with an unpaired electron. Free
radicals may arise
from a number of physiological or degradative reactions within biological
tissues and materials.
2o Free radicals are very reactive and quickly react with unsaturated lipids,
oxygen and other
compounds to form degradation products, some of which are themselves free
radicals. This is
called the propagation stage of autoxidation. Eventually the chain reaction
may terminate when
all oxidizable materials or free radicals are consumed.
Pro-oxidants are compounds that tend to initiate and promote the oxidation
sequence
25 and include metals such as iron and copper. Antioxidants are substances
that can delay the
onset or slow the rate of oxidation of autoxidizable materials (Nawar, W. 1985
Lipids, p. 225-
320. IN "Food Chemistry" O.R. Fennema, ed. Marcel Dekker Inc. NY).
Antioxidants can
function by a number of mechanisms such as chelating metals to inhibit their
pro-oxidant
activity or by combining with and "quenching" free radicals. Other compounds
with antioxidant
CA 02354300 2001-07-30
activity such as ascorbic acid, act as synergists with other antioxidants,
often re-activating spent
antioxidants by reducing them back into an active form. Free radicals are also
generated as
byproduct's of many physiological reactions within living organisms and for
this reason, a
balance must be maintained in the human body between pro-oxidants and
antioxidants.
Due to the toxicity of many autoxidation products, antioxidants are considered
desirable
in the human diet. Many health benefits have been associated with antioxidants
in foods,
including anti-mutagenicity, anti-carcinogenicity and anti-aging (Cook and
Samman, 1996.
Flavonoids chemistry, metabolism, cardioprotective effects and dietary
sources. Nutritional
Biochemistry 7:66-76; Huang and Frankel, 1997. Antioxidant activity of tea
catechins in
to dii~erent lipid systems. J. Agricultural and Food Chemistry. 45: 3033-
3038). Although synthetic
chemical antioxidants are known, natural antioxidants in familiar foods are in
great demand due
to their long historical record in the human diet and presumed safety. Natural
antioxidants
which have be identified in common foods include phenolic compounds such as
tocopherols and
flavonids, including anthocyanins, as well as carotenoids, amino acids, and
ascorbic acid
15 (Pokorny, J. 1999.).
Bluebernes (a prime example of antioxidant-rich berries ) are the fruit of
plants
belonging to the genus haccinium, including h corymbosum, T~ ashei and h
augustifolium
and grow throughout North America. They have been used as food since
prehistoric times and
today are an important food crop. Commercially marketed bluebernes include
both wild, low-
2o bush bluebernes (e.g. h augustifolium) which grow primarily in Maine, Nova
Scotia and
Quebec, and cultivated, high-bush blueberries (e.g. h corymbosum and V.
ashei), grown
principally in British Columbia, Michigan and New Jersey.
Blueberries have a brief harvest season of about one month, after which fresh
bluebernes can only be stored refrigerated for a maximum of 6 weeks. Therefore
fiuther
25 processing is desirable to extend shelf life. Although large amounts of
bluebernes are frozen for
preservation, frozen storage-life is only about 6 months, after which the
fruit develop texture
problems such as woodiness and grittiness (Sullivan et al., 1982. Dehydrated
blueberries by the
continuous explosion-pu~ng method. Journal of Food Science 47: 445-448).
Dehydration is
another popular preservation method for blueberries. If the water activity of
the fruit is reduced
3o by dehydration to below 0.60, spoilage micro-organisms are unable to grow
and a storage life
CA 02354300 2001-07-30
greater than 6 months can be achieved. Furthermore, dehydrated blueberries do
not require
energy-intensive refrigerated storage and are lighter and less fragile during
transportation.
Most commercial dehydration of blueberries is accomplished by hot air forced-
convection drying in which heated dry air is passed over or through a bed of
the fruit. A small
portion of the blueberry crop each year may be freeze-dried, a process by
which the water is
sublimated directly from the frozen state under conditions of very low
absolute pressure.
Although freeze dried fruit are considered very good quality from the point of
view of nutrition
and flavor, the process is used to a limited extent because of its high cost.
Some of the health benefits provided by fruits and vegetables in the human
diet have
1o been attributed to antioxidant activity. In antioxidant-rich berries such
as blueberries, two major
contributors to antioxidant activity are ascorbic acid, also known as vitamin
C, and a complex
group of phenolic compounds. Much of the antioxidant activity of the phenolics
is attributed to
a sub-group known as anthocyanins. Anthocyanins are the primary pigments of
bernes and are
responsible for the color of berries. In fact, various anthocyanins are
responsible for almost all
of the red, purple and blue colors of fruits and flowers. They are known to
have strong
antioxidant activity (Wang et al., 1996. Total antioxidant capacity of fruits.
Journal of
Agricultural and Food Chemistry 44: 701-705). Of all bernes, bluebernes have
the highest
concentration of anthocyanins, followed by cranberries and strawbernes.
Drying of fruits is described in the patent literature and attention is
directed to Kraig et
2o al. US Patent 4,515,822 which teaches a method of coating fruit pieces with
sugars and gums,
then drying rapidly in air above 220°F to puff and dry the fruit
pieces. Koshida et al. US Patent
4,341,803 teaches a method of producing a crisp dry fruit snack by a
sequential combination of
freeze drying, microwave drying and vacuum drying. Nafisi-Movaghar US Patent
5,000,972
teaches a method of drying fruit without sulfiting. Mazin et al. US Patent
5,188,861 teaches a
method of removing natural flavor from dry fruit pieces and introducing a new,
substantially
different flavor and Durance et al. US Patent 5,962,057 teaches a method of
drying mango and
pineapple with fresh flavor and crunchy texture. None of these patents deal
with the specific
problem of drying antioxidant-rich bernes while minimizing the loss of
antioxidant properties of
the dried bernes.
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Bluebernes have recently been the subject of scientific study as potent
antioxidants in
the human diet (Costantino, L. et al. 1996. Anthocyanin inhibitors of xanthine
oxidase.
Pharmazie (50):573-574
Antioxidants can be measured by various methods. Known antioxidant compounds
may
be quantified by standard analytical methods. For example, total phenolics can
be measured by
the Folin-Ciolacalteu method, using gallic acid as a standard (Velioglu et al.
1998. Journal of
Agriculture and Food Chemistry (46): 4113-4117). Anthocyanins may be measured
a
spectrophotometric methods (Fuleki, T. and Francis, F.J. 1968. J. of Food
Science (33):73-83.)
Antioxidant activity can also be measured directly be initiating a standard
oxidation reaction in
1o vitro, such as the reaction measured by the thiobarbituric acid reaction
assay TBAR (Buerge
and Aust, 1978. Methods in Enzymology (52):302-310), then measuring the
inhibition of
oxidation conferred by the test sample. Antioxidant activity in a different
system can be
measured by the DPPH free radical scavenging method (Hu and Kitts. 2000.
Antioxidant
activity of Echinacea root extracts. Journal of Agricultural and food
Chemistry 48: 1466-1472.)
15 DPPH is a stable free radical (2,2-diphenyl-1-picrylhydrazyl) that is
scavenged directly by the
antioxidant substrate. This method therefore measures inhibition properties at
both the initiation
and propagation stages of the oxidation reaction.
Brief Description of the Invention.
It is the object of the present invention to provide a process by which
antioxidant-rich
2o bernes can be dried while retaining a significant portion of the
antioxidant activity of the fresh
bernes and a significant absolute amount of known antioxidant compounds such
as ascorbic
acid, phenolic compounds and anthocyanins.
It is an object of the invention to provide a vacuum microwave process for
dehydration
of antioxidant-rich berries while preserving the antioxidant properties of the
berries (i.e. without
25 destroying the antioxidant properties to the extent they are destroyed
using conventionally used
dying procedures).
Broadly the present invention relates to a process for drying antioxidant-rich
berries
containing an initial mass of water to preserve their antioxidant action and
their antioxidant
compounds comprising subjecting the bernes to vacuum microwave drying (VMD) at
an
CA 02354300 2001-07-30
absolute pressure of 0 to 200 millimeters of mercury (mm Hg) and 0.1 to 2
watts of microwave
power/gram of said antioxidant-rich berries containing said initial mass of
water to produce
dried bernes wherein the moisture content of said berries containing an
initial mass of water is
reduced to a residual moisture content of less than 35% of the dry weight of
said dried bernes.
Preferably said VMD is applied to reduce the moisture content to a residual
moisture
content of between 10% and 25% of the dry weight of the final product of the
process.
Preferably said absolute pressure is between 30 and 60 mm Hg.
Preferably said microwave power is applied at 0,5 to 1 watt/gram of
antioxidant-rich
bernes.
to Preferably said antioxidant-rich bernes are subjected to preliminary drying
step wherein
said berries are partially dried to remove up to 90 % of the initial mass of
water associated with
the bernes then
Preferably said preliminary drying removes less than 70% of said initial mass
of water.
Preferably said berries are agitated during said VMD.
15 Preferably said VMD comprise a cooling step following application of
microwave
power wherein said antioxidant-rich berries are subject to a vacuum without
application of
microwave power
Brief description of the drawings
Further features, objects and advantages will be evident from the following
detailed
2o description of the invention taken in conjunction with the accompanying
drawings in which;
Figure 1 is a schematic flow diagram illustrating the steps in the process.
Description of the Preferred Embodiments
As shown in Figure 1, the preferred process involves if desired, first
partially
dehydrating the antioxidant-rich bernes by conventional means as indicated in
1., such as
25 placing them in a moving air stream at 60°C to 90°C for 1 to
4 hours to remove about one half
of the initial weight of the incoming berries in the form of evaporated water.
This step removes
a portion of the water which is less strongly bound to the berry solids and
which is consequently
readily and quickly removed by the hot air treatment. This step uses
conventional air drying
equipment. Any amount of the original water from zero to about 90% of the
initial water in the
3o berries may in theory be removed by air drying prior to vacuum microwave
drying (VMD). In
CA 02354300 2001-07-30
some cases it may be desirable to begin vacuum microwave drying from the fresh
or frozen
state without first air drying, such as in the situation when it was desirable
to dry the
antioxidant-rich bernes very quickly or to retain the maximum possible content
of antioxidant
activity.
The antioxidant-rich bernes are subjected to VMD by placing them in the vacuum
chamber as indicated in 2. and the chamber air pressure is reduced to 30 to 60
mm of mercury
absolute pressure in the preferred process but to at least as low as 200 mm of
mercury. During
or immediately following the application of vacuum, microwave power is applied
in the amount
of 0.1 watt to 2.0 watts microwave power per gram of initial fresh weight of
berries (with the
1o initial water content) as indicated in 3. In the preferred process
microwave power in the
amount of 0.5 watts per gram to 1.0 watts per gram of fresh berries is applied
in step 3.
The vacuum cooling step 4. is optional depending on whether a puffed. expanded
dry
antioxidant-rich berry product is desired. Due to the rapid evaporation of
water during the
vacuum microwave dehydration step, the berries will expand or pui~ inside the
chamber.
15 Vacuum cooling allows the bernes to grow more rigid before atmospheric air
pressure is
allowed to enter the chamber, thus allowing the berries to remain pui~ed.
Table 1 summarizes the various steps in the process, the sequence in which
they will
occur (if applied) and the conditions or duration of each of the steps.
CA 02354300 2001-07-30
7
Table 1. VMD Process for antioxidant-rich berries
Process Preferred range Acceptable range
step/parameter
1. Initial air To remove 0% to 70% of To remove 0% to 90%
drying initial of
mass of water in blueberries.initial mass of water
in
blueberries
2. VMD chamber 30 to 60 mm of Hg 0 to 200 mm Hg
pressure (Absolute)
3. Microwave 0. S to 1.0 Watts per 0.1 to 2 watts per
power gram fresh gram fresh
density berries berries
4. Agitation Equivalent to agitation Equivalent to agitation
during in a 12 in a 12
periods of inch cylindrical drum inch cylindrical drum
rolling on rolling
microwave power.its axis at 2 to 4 RPM. on its axis at 1 to
10 RPM.
5. Cooling stage2 to 5 minutes under 0 to 10 minutes under
at vacuum vacuum
end of VMD step.without microwave power.without microwave power.
6. Final air None. To remove the last
drying 0 to 6 % of
total initial mass
of moisture.
7. Final moisture10 to 20 % wet weight 3 to 30 % wet weight
of basis* basis*
antioxidant-rich(1.5% to 3% of the total(0.5% to 5% of the
initial total initial
berries mass of moisture) of moisture)
* "wet weight basis" means the weight of moisture divided by the weight of
berries including
the remaining moisture.
Vacuum microwave dehydration of antioxidant-rich berries leads to a dry
product with
excellent berry flavor retention and an expanded, pui~ed, tender texture as
taught in Durance et
al (ITS Patent 5,962,057). Unexpected benefits of vacuum microwave dehydration
of
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antioxidant-rich berries were improved preservation of chemical compounds
related to
antioxidant activity of this fruit.
Examples
Table 2 presents a number of experimental results of chemical analysis related
to
antioxidant activity. Two different types of bluebernes were dried. In the
case of one type (Blue
Crop) two batches of bluebernes from different farms and storage times were
dried by each
method and analyzed separately. Each number in Table 2 represents the average
of at least
triplicate determinations. Each batch of blueberries was purchased frozen,
divided into portions
and individual portions were either air dried, freeze dried, vacuum microwave
dried or dried by
1o a combination process in which half the initial weight was removed by air
drying, after which
drying was completed by vacuum microwave. In each case, antioxidant-rich
berries were dried
sufficiently to preserve the fruit without refrigeration, that is to a water
activity of less 0.60.
Water activity, a thermodynamic property, is defined as the ratio of the vapor
pressure of water
in a system to the vapor pressure of pure water at the same temperature.
Table 2 presents a comparison of composition related to antioxidant activity
of
bluebernes dried by a variety of methods. Air temperature for the air drying
(AD) treatment and
the air portion of the AD/VMD treatment was at 70°C for Blueberry 1 & 2
and 84°C for
Blueberry 3. Freeze drying was accomplished at 0.1 mm Hg absolute pressure, a
condenser
temperature of -SO°C and a shelf temperature of 20°C. The
ADlVIV~ process was Example 3
2o for Blueberry 1 & 2 and Example 1 for Blueberry 3. The VMD process was as
described in
Example 2. Bluebernes 2 and 3 were from the same type of blueberry plant but
from different
farms and stored different periods prior to the experiments
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Table 2...
Drying Air DriedCombinationhMD Freeze Frozen
un-
Method (AD) ADlYMD Dried dried
@
(starting
material)
Antioxidant Antioxidant
concentration or -rich berry
activity
type
Phenolics (mg gallic1. Hardy 2150 2230 3350 2450 3550
acid
equivalents / 100gBlue
dry)
Phenolics (mg gallic2. Bluecrop1490 1520 2050 1780 2450
acid
equivalents / 100g
dry)
phenolics (mg gallic3. Bluecrop1023 1302 1528 1222 2855
acid
equivalents / 100g
dry)
anthocyanins by 1. Hardy 198 218 498 524 530
s~ectro-
photometry (mg/100gBlue
~'Y)
anthocyanins bar 2. Bluecrop117 129 173 319 258
spectro-
photometerv (mg/100g
dry)
anthocyanins bj~ 3. Bluecrop290 520 740 660 Not
liquid
chromatography Determined
~mg/100g dry)
ascorbic acid (mg/100g1. Hardy none none 8 22 37.
dry) Blue detecteddetected
CA 02354300 2001-07-30
to
ascorbic acid (mg/100g2. Bluecropnone none 9 25 23
dry) detecteddetected
ascorbic acid (mg/100g3. Bluecrop0.4 3 10 12 21
dry)
antioxidant activity1. Hardy 20 25 28 30 46
(%
inhibition by TBAR Blue
method)
antioxidant activity2. Bluecrop19 20 23 23 32
(TBARS)
antioxidant activit;r3. Bluecrop32 39 64 52 Not
(%
inhibition ~r free determined
radical
scavenging method)
Phenolics of bernes, as a class of chemical compounds, have been identified as
important antioxidants. Phenolic content of the berries was measured by the
method of Velioglu
et al. (1998). In experiments with two types of bluebernes, VMD berries had
higher
concentrations of total phenolics than the same berries when dried by other
methods and closest
to the concentration found in the un-dried, frozen bernes. The combination
AD/VMD process
yielded an intermediate concentration of phenolics. Freeze dehydration, often
reported to be
the most effective means of dehydration for preservation of chemical
integrity, retained less
to phenolics than VMD.
Anthocyanins are a sub-class of phenolics which have been identified as
important to
antioxidant activity of antioxidant-rich berries. Anthocyanin content of VMD
bernes was
consistently higher than that of any drying treatment except freeze drying.
Depending upon the
experimental conditions and assay method, anthocyanin content of VMD
blueberries were
15 either slightly higher or lower than in freeze dried blueberries. The
combination AD/VMD
process yielded anthocyanin content intermediate between VNID and AD.
Ascorbic acid, also known as vitamin C is an important synergist of
antioxidants.
Ascorbic acid may be degraded in drying fruit by the activity of the native
enzyme ascorbic acid
CA 02354300 2001-07-30
11
oxidase or by chemical oxidation. The ascorbic acid oxidase enzyme is active
at the temperature
of bernes during AD but is not active in the dry fruit. Therefore extended
times in the air dryer
may be destructive to this antioxidant. Again, VMD retained more vitamin C
than other drying
treatments except freeze drying.
Antioxidant activity of processed antioxidant-rich berries were assessed using
different
oxidation reactions to evaluate activity related to multiple mechanisms of
action, the pattern of
antioxidant activity associated with specific drying methods was consistent.
Thus, according to
both methods of analysis, the un-dried starting material had the highest
antioxidant activity and
the air-dried antioxidant-rich berries had the lowest. VMD and freeze drying
(FD) provided the
1o greatest retention of antioxidant activity of dried treatments and these
two treatments had
similar activities. The combination AD/VMD treatment yield antioxidant
activity intermediate
between that of all-air dried and all VMD dried berries.
Thus VMD treatments were seen to retain more antioxidant compounds and more
antioxidant activity than air drying, and similar concentrations and
activities as freeze drying
15 treatments. Combination AD/VMD processed antioxidant-rich bernes yielded
intermediate
results. Thus VMD can provide an alternative to freeze drying of bernes to
maintain maximum
antioxidants in the dry product.
Obviously the VMD or AD/VMD processes of this invention could also be applied
to fresh or
frozen antioxidant rich berries that had been pretreated for example by having
been previously
20 infused with sugars by immersion in a solution of sugars.
Example 1.
Frozen bluebernes (2 kg, 86.4 % moisture wet basis) were air dried for 1.5
hours in a
commercial air dryer at 84°C to remove 965 grams of water. Next 600
grams of the partially
dried blueberries were placed in the cylindrical drying basket of a 1.5 kW,
2450 MHz vacuum
25 microwave. Vacuum was applied to an absolute chamber pressure of 40 mm Hg
over a 1.5
minute period. Next, 1.5 kW of microwave power was applied for 16 minutes,
while the drying
basket was rotated on its axis at 3 rpm to agitate the bernes and ensure even
exposure to
microwaves. Finally the berries were allowed to cool under the same vacuum and
rpm but zero
microwave power for 3 minutes. The final moisture content of the bernes was
18% dry basis
3o and the final water activity was 0.50.
CA 02354300 2001-07-30
12
Example 2.
Frozen blueberries (1.5 kg, 86.4 % moisture wet basis) were placed in the
cylindrical
drying basket of a 1.5 kW, 2450 MHz vacuum microwave. Vacuum was applied to an
absolute
chamber pressure of 40 mm Hg over a 1. Sminute pump-down period. Next, 1. 5 kW
of
microwave power was applied for 38minutes, while the drying basket was rotated
on its axis at
3 rpm to agitate the berries and ensure even exposure to microwaves and vacuum
was
maintained. Next the microwave power was reduced to 0.75 kW for 5.5 minutes.
Finally the
berries were allowed to cool under the same vacuum and rpm but zero microwave
power for 3
minutes. The final moisture content of the bernes was 15% dry basis and the
final water activity
l0 was 0.48.
Example 3.
A total of 10.9 kg of frozen blueberries were dried on a commercial belt air
dryer with
an air temperature of 70°C for 4 hours, to remove 6.66 kg grams of
water. Next 2.24 kg of
the partially dried bluebernes were placed in the cylindrical drying basket of
a 1.5 kW, 2450
MHz vacuum microwave. Vacuum was applied to an absolute chamber pressure of 40
mm Hg
over a 1.5 minute period. Next, 1.5 kW of microwave power was applied for 48
minutes, while
the drying basket was rotated on its axis at 3 rpm to agitate the bernes and
ensure even
exposure to microwaves. Finally the berries were allowed to cool under the
same vacuum and
rpm but zero microwave power for 3 minutes. The final moisture content of the
berries was
18% dry basis and the final water activity was 0.49.
Having described the invention modifications will be evident to those skilled
in the art
without departing from the spirit of the invention as defined in the appended
claims
