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Abstract and Figures

Black garlic is a type of garlic product that is generally produced by heating raw garlic at high temperature with controlled humidity for more than 30 days. Black garlic has appeared on market for many years. It is crucial to investigate the characteristic of quality formation of black garlic during processing of varied temperatures. In this study, fresh garlic was processed to black garlic at temperatures of 60, 70, 80 and 90 °C. Moisture, amino acid nitrogen and allicin contents decreased gradually during thermal processing of various temperatures. Reducing sugar, 5-Hydroxymethylfurfural (HMF), total phenols, total acids contents and browning increased. The changing rate of quality indicators and flavor of black garlic was various at different temperatures. Browning intensity reached about 74 when black garlic aged. Sensory score was significantly higher in black garlic aged at 70 °C (39.95 ± 0.31) than those with other temperatures, suggesting that 70 °C might facilitate formation of good quality and flavor of black garlic during processing. Temperatures had remarkable impacts on the quality and flavor of black garlic. This article is protected by copyright. All rights reserved.
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Research Article
Received: 15 February 2015 Revised: 20 July 2015 Accepted article published: 24 July 2014 Published online in Wiley Online Library: 3 September 2015
(wileyonlinelibrary.com) DOI 10.1002/jsfa.7351
Effects of temperature on the quality
of black garlic
Xinyan Zhang, Ningyang Li, Xiaoming Lu, Pengli Liu and Xuguang Qiao*
Abstract
BACKGROUND: Black garlic is a type of garlic product that is generally produced by heating raw garlic at high temperature with
controlled humidity for more than 30 days. Black garlic has appeared on the market for many years. It is crucial to investigate
the characteristics of quality formation of black garlic during processing at various temperatures.
RESULTS: In this study, fresh garlic was processed to black garlic at temperatures of 60, 70, 80 and 90 C. Moisture, amino
acid nitrogen and allicin contents decreased gradually during thermal processing of various temperatures. Reducing sugar,
5-hydroxymethylfurfural, total phenols, total acids contents and browning increased. The changing rate of quality indicators
and flavour of black garlic varied at different temperatures. Browning intensity reached about 74 when black garlic aged. The
sensory score was significantly higher in black garlic aged at 70 C (39.95 ±0.31) compared with that at other temperatures,
suggesting that 70 C might facilitate formation of good quality and flavour of black garlic during processing.
CONCLUSION: Temperature had a remarkable impact on the quality and flavour of black garlic.
© 2015 Society of Chemical Industry
Keywords: black garlic; temperature; quality; flavour; processing
INTRODUCTION
Garlic (Allium sativum L.) is a species of the onion genus. It has
been used widely both as a culinary seasoning and a med-
ical herb throughout history.1Garlic could promote appetite
and help digestion. The main effective components in garlic
are organosulfur compounds and bioactive enzymes. Among
these, allicin is well known for its pharmacological properties,
including anti-bacterial, anti-hyperlipidaemia, anti-tumour and
immuno-regulatory activity.2
Although garlic has been widely used as one of the popular
seasonings for food and medicinal purpose in China, Korea and
America, consumption of raw garlic is limited due to its unpleas-
ant odour and taste. The unpleasant odour and taste could be
removed by heat treatment. Also, by this way, the palatability of
garlic could be improved.3Accordingly, heating treatment has
been widely used to process black garlic to improve the flavourand
quality of garlic, and further endow garlic with new functions.4,5
When garlic is heated, its bioactive aspects are changed.5Alliin
and deoxidised alliin are decomposed to allyl sulfur-containing
compounds, and some sulfur-containing compounds in ther-
mal degradation have a fragrant smell.6In fermented garlic
much of odorous smell from fresh garlic is removed and many
sulfur-containing compounds are formed, which contribute
to health benefits. Through the heating process, unstable and
unpleasant compounds in raw garlic are converted into stable
and tasteless compounds. As a result, black garlic generally
has a sweet– sour flavour instead of the offensive odour and
taste.5,7Moreover, black garlic does not cause abdominal pain
or other gastrointestinal problems.8It is reported that black
garlic has stronger antioxidant activity than fresh garlic,5,9,10 and
better efficacy in preventing metabolic diseases and alcoholic
hepatotoxicity.11,12 Moreover, the heating process could lead
to non-enzymatic browning reactions, for example the Maillard
reaction, caramelisation and the chemical oxidation of phenols.
Non-enzymatic browning reactions can give black garlic a typical
darkbrowncolour,andleadtotheformationofsomeantioxidant
compounds.1317
In recent years, many studies have been conducted to investi-
gate the bioactive compounds in black garlic (i.e. total phenols,
5-hydroxymethylfurfural) and their functional activities. However,
limited information is available regarding changes in the quality
indicator content of black garlic and the characteristic of quality
formation during thermal processing. The purpose of this study
was to measure the content of quality indicators in the black gar-
lic during processing with different temperatures. These results
might contribute to our understanding the role of temperature in
the quality formation of black garlic.
EXPERIMENTAL
Chemicals and materials
Garlic (Allium sativum L.) was purchased from Laiwu (Shandong,
China). Fresh garlic was converted to black garlic by heating in
a drying oven for several days at 60, 70, 80 and 90 C with 80%
Correspondence to: Xuguang Qiao, College of Food Science and Engineering,
Shandong Agricultural University, Tai’an 271018, People’s Republic of China.
E-mail: xgqiao@sdau.edu.cn
College of Food Science and Engineering, Shandong Agricultural University,
Tai’an 271018, People’s Republic of China
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relative humidity (RH). Samples were taken to test the substance
content every 3 days.
Reagents used in the study were as follows: potassium ferro-
cyanide, zinc acetate, sodium hydroxide, zinc sulfate, barbituric
acid, sodium carbonate, glacial acetic acid, glucose, phenol,
sodium hydrogen sulfite, isopropanol, formaldehyde, diethyl
ether, methanol, trichloroacetic acid and were purchased from
Kaitong Chemical Technology Co., Ltd (Tianjin, China). Potassium
sodium tartrate was obtained from Yongda Chemical Reagent
Co., Ltd (Tianjin, China). 3,5-Dinitrosalicylic acid was obtained
from Lanji Technology Co., Ltd (Shanghai, China). Folin– Ciocalteu
reagent was purchased from Solarbio Science & Technology Co.,
Ltd (Beijing, China). Gallic acid was purchased from Baishi Chemical
Industry Co., Ltd (Tianjin, China). 5-Hydroxymethylfurfural (HMF)
was purchased from Sigma (St Louis, MO, USA). 4-Methylaniline
was purchased from Jin Shanting new chemical reagent factory
(Shanghai, China). Alliin (>99% purity) was obtained from Xinjiang
Ailexin Pharmacy Co., Ltd (Urumqi, China)).
All other chemicals used in this work were of analytical grade,
and all solutions were prepared with distilled water.
Sensory evaluation
Non-professional evaluators (30 people) who acknowledged the
quality attributes of black garlic well were randomly chosen to
carry out a quality evaluation of black garlic. Every garlic sam-
ple was evaluated in terms of the following characteristics: colour,
flavour, texture, taste quality and general acceptability. Each char-
acteristic was assessed on a nine-point scale, i.e. a midpoint of ‘5’,
the highest score of ‘9’ and the lowest score of ‘1’. The total score
was considered as the score of sensory evaluation. We chose the
optimal processing condition concerning temperature and time
on the basis of the highest score of sensory evaluation of black
garlic.18
Browning intensity
A colorimeter (CM-700d; Konica Minolta, Tokyo, Japan) was used
to measure the colour attributes of samples. Before the determi-
nations, the colorimeter should be calibrated by using a standard
white reflector plate. The standard values appeared as L(white-
ness), a(red– green), and b(yellow– blue) values.19 Garlic was
selected randomly, and then the Lvalue, avalue, and bvalue were
measured for the garlic bulblet and its square section. The colour of
the inside of samples was determined by a colorimeter after each
sample was transected into some flakiness (2–3 mm). Triplicate
measurements were performed for each sample.19 The brown-
ing intensity (ΔE) was calculated as ΔE=[(ΔL)2+(Δa)2+(Δb)2]1
2,
where ΔL=LL,Δa=aa,andΔb=bb.L,a,andbrepresent
the colours of the samples; and L,a,andbrepresent the colours
ofthebaseattimezero.
Measurement of reducing sugar content
The heated garlic sample (5 g) was chopped, ground and diluted
with distilled water, then transferred to a 100 mL volumetric
flask which contained 5 mL of potassium ferrocyanide solution
(106 g L1, w/v) and 5 mL of zinc acetate solution (219 g L1,w/v).
The mixture was extracted by using ultrasound for 30 min, allowed
to stand for a moment, and then filtered. Filtrates (0.8mL) were
added to a tube and 0.6 mL of dinitrosalicylic acid reagent and
1 mol L1NaOH solution (0.8 mL) were added. The solution was
mixed thoroughly and boiled for 15 min, and then 9.5 mL of dis-
tilled water was added after it cooled. The solution was measured
at 500 nm.20
Determination of amino acid nitrogen content and total acids
content
The heated garlic sample (5 g) was chopped, ground and trans-
ferred to a 100 mL of volumetric flask with distilled water. The
solution was extracted by using ultrasound for 30 min, allowed to
stand for a moment, and then filtered. The filtrates (20 mL) were
dissolved in 60 mL of distilled water, and put in a beaker. A glass
electrode of a pH meter was put into the mixture, which was stirred
using a magnetic stirrer. The solution was titrated to pH 8.2 with
0.05 mol L1NaOH, and the amount of NaOH solution consumed
was recorded. Then, formaldehyde (10mL) was added to the mix-
ture. NaOH solution (0.05 mol L1) was used to titrate the mixture
to pH 9.2. The amount of consumed NaOH solution was recorded.
Distilled water (80 mL) served as a blank group. The amount of
NaOH solution consumed was used to calculate the amino acid
nitrogen content and the total acids content.21
Determination of 5-hydroxymethylfurfural content
The heated garlic sample (10 g) was chopped, ground and diluted
with distilled water, and then transferred to a 100 mL volumetric
flask which contained 2 mL of potassium ferrocyanide solution
(150 g L1, w/v) and 2 mL of zinc sulfate solution (300 g L1,w/v).
The mixture was extracted by using ultrasound for 30 min, allowed
to stand for a moment, and then filtered. The filtrates (2mL) were
added to two tubes containing 5 mL of 4-methylaniline solution
(60 g L1, w/v) respectively. Distilled water (1 mL) (blank solution)
was added to one tube and 1 mL of barbituric acid solution
(5 g L1, w/v) (sample solution) was added to the other tube.
The absorbance of the solution was determined immediately at
550 nm.22
Measurement of total phenols content
The heated garlic sample (5 g) was chopped, ground and trans-
ferred to a 100 mL volumetric flask with distilled water. The solu-
tion was extracted by using ultrasound for 30 min, allowed to
stand for a moment, and then filtered. Filtrates (0.5mL), 0.5mL
of Folin– Ciocalteu reagent, 1.5 mL of sodium carbonate solution
(100 g L1, w/v) and 7.5 mL of distilled water were mixed in a 10 mL
volumetric flask. The mixture was incubated at 75 C for 10 min,
and then placed in darkness for 2– 3 h. The absorbance of the solu-
tion was measured at 760 nm.23
Analysis of allicin content
The reduced amount of alliin was used to calculate the allicin
content. An alliin standard solution (0.45 g L1)10mLwaspre-
pared and analysed by HPLC. The HPLC column was an Atlantis C18
column, 5 μm, 4.6 mm ×250 mm. The detection wavelength was
214 nm. The mobile phase was methanol–water (60:40, v/v) at a
flow rate of 0.8 mL min1.24 Alliinase solution (1 mL) was added
to the alliin standard solution. After reaction for 10 min at 35 C,
trichloroacetic acid (100 g L1, w/v) 1 mL was added to the solution
quickly. The residual alliin content was determined. Diethyl ether
(20 mL) was used to extract the generated allicin three times. The
extracts were combined and evaporated under nitrogen, and then
2 mL of methanol was used for redissolving the extracts. The mix-
ture was filtered and 10 𝜇L of test solution was injected into the
HPLC system foranalysis. The amount of allicin (in micrograms) was
calculated as follows:
wallicin =(wallin wres.alliin)×MWallicin
2×MWalliin
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Table 1. Main characteristic parameters of black garlic (mean ±SD, n=3)
Temperatures (C)
Parameter 60 70 80 90
Maturity time (d) 69 30 15 9
Browning intensity 74.14 ±0.56a74.44 ±0.42a74.55 ±0.97a74.60 ±0.28a
Miosture (g kg1) 490.23 ±2.70a460.61 ±22.69a471.19 ±33.87a470.84 ±6.80a
Reducing sugar (g kg1) 335.21 ±0.45c357.96 ±3.44a348.93 ±1.21b154.91 ±2.19d
Amino-N (g kg1)3.15±0.04b3.29 ±0.04a1.49 ±0.04c1.03 ±0.04d
HMF (g kg1)1.88±0.02d4.32 ±0.02b4.82 ±0.06a4.08 ±0.08c
Total phenols (g kg1) 13.00 ±0.09b12.35 ±0.13b15.12 ±0.36a13.27 ±0.87b
Tot al a cid s (g k g1) 33.61 ±0.17c37.50 ±0.17a30.96 ±0.17d36.37 ±0.17b
Allicin (g kg1)0.16±0.07c0.28 ±0.07bc 0.93 ±0.07a0.41 ±0.07b
Sensory evaluation 28.71 ±0.44d39.95 ±0.31a35.66 ±0.46b29.41 ±0.41c
a–cMeans followed by different superscripts in the same row are significantly different at P<0.05.
where wallicin is the amount of allicin, walliin is the amount of alliin,
wres.alliin is the residual amount of alliin, and MW is molecular
weight.
Statistical analysis
The data were analysed by analysis of variance (ANOVA) followed
by Duncan’s multiple range test. Differences among means at
P<0.05 were considered statistically significant. The statistical
analyses of data were performed using SPSS software version 18.0
(IBM Corporation, Chicago, IL, USA)
RESULTS AND DISCUSSION
Quality analysis of black garlic
The typical label of black garlic quality is the formation of the
black colour. Based on reports from other authors25 and our
empirical results, we found that when the browning intensity
was about 74–76, the appearance of black garlic was aterrimus.
This colour was a standard colour of black garlic and can be
used as an indicator for maturation of black garlic. As shown in
Table1, the ageing time of black garlic is significantly different
due to the heat treatment at various temperatures (P<0.05). The
higher the temperature, the shorter the ageing time of black
garlic. Temperature had a significant impact on the quality of
black garlic. Colour formation of black garlic was accelerated under
high-temperature conditions.
As shown in Table 1, when black garlic aged, reducing sugar con-
tent was significantly higher at a temperature of 70 Cthanatother
temperatures (P<0.05). Amino-N content was significantly lower
at high temperatures (80 C, 90 C in this study) than at low tem-
peratures (60 C, 70 C) (P<0.05). HMF content was significantly
different among the samples treated with various temperatures
(P<0.05). The rate of Maillard reaction was accelerated at high
temperature condition, resulting in a high HMF content in black
garlic. Total phenols, total acids, and allicin contents in black gar-
lic were significantly affected by heating temperatures (P<0.05),
which could contribute to different quality of black garlic.
Neither low temperature nor high temperature was beneficial
for the formation of good quality black garlic during the thermal
process. The sensory score was significantly higher in black garlic
aged at 70 C (39.9 ±0.31) than at other temperatures, suggesting
that 70 C might facilitate the formation of the good quality and
flavour of black garlic during processing.
Figure 1. Effect of temperature on browning.
Effect of temperature on browning
During the black garlic processing, development of the black
colour is usually connected with the non-enzymatic browning
reaction and greatly depends on the heating temperature.26,27
There were melanoidins forming in Maillard reaction. The colour
of melanoidins might be connected with the enolisation of sugars
and racemisation of amino acids.28 As shown in Fig. 1, regardless
of the temperature, the browning in all garlic samples increased
gradually, and then levelled off during processing. The browning
rate was faster in the samples treated with higher temperature
than in those treated with lower temperatures. Similar results have
been reported by Benzing-Purdie et al.29
We divided the ageing procedure of garlic samples into three
stages (i.e. early period, middle period, and later period) according
to the colour development of garlic sample during thermal pro-
cessing. Under conditions where the temperatures were 60 and
70 C, during the early period of processing, the colour develop-
ment of the garlic sample was from white to pale brown. The inside
colour of the garlic sample showed an uneven distribution of white
flecks. In the middle period of processing, the colour development
of the garlic sample was from pale brown to dark brown. The inside
of garlic sample had some white flecks. In the later period, the
colour of the garlic sample was from dark brown to black. At tem-
peratures of 80 and 90 C, the appearance of the garlic sample was
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Figure 2. Effect of temperature on moisture content.
from white to dark brown in the early period. The inside of the gar-
lic sample had some white flecks. The colour of the garlic sample
was from dark brown to all black in the later period. The quality
of black garlic was connected with its colour; however, the degree
of blackness of black garlic does not necessarily mean the qual-
ity is better.24 According to sensory evaluation, the quality of aged
black garlic was better and its colour was homogeneous blackness
under 70 and 80 C conditions. Although black garlic formed black
faster at 90 C, it was more bitter and had a sour flavour. In the case
of 60 C, the colour of black garlic did not meet the requirement of
homogeneous blackness.
Effect of temperature on moisture content
Figure 2 illustrates the changes in the moisture content of garlic
samples during thermal processing with different temperatures.
The moisture content decreased continuously under four tem-
perature conditions. The decreased rate of moisture content was
faster at higher temperature.
Under these four temperature conditions, the moisture content
was 500 –700 g kg1at the early stage of black garlic processing,
garlic sample was soft and wet at this period. When the moisture
content reached 400 –500 g kg1, black garlic was soft and elastic.
Similar results have been reported by Sang et al.30 The study by
Lei et al.31 reported that moisture content in black garlic was
436 g kg1. When moisture content reaches 350 – 400 g kg1,black
garlic was drier and its elasticity was not good. When moisture
content reaches below 350 g kg1, black garlic was much harder.
When black garlic aged under 60 C process conditions, the
quality of was not good, with high humidity of the internal surface.
The appearance of black garlic was dry with better elasticity and
quality under 70 C conditions. The appearance of black garlic was
bone-dry with something burning, and its quality was not better
at 80 C. The texture of black garlic was very hard with an obvious
burning smell at 90 C.
Effect of temperature on reducing sugar content
During the black garlic processing, the reducing sugar content in
black garlic depended on two factors. On the one hand, polysac-
charide in garlic was degraded to reducing sugar. On the other
hand, reducing sugar was consumed during the Maillard reaction.
During the heat treatment at various temperatures, the trend
of reducing sugar content was different and the rising curve of
Figure 3. Effect of temperature on reducing sugar content.
Figure 4. Effect of temperature on amino acid nitrogen content.
reducing sugar content increased with the temperature (Fig. 3).
The reducing sugar content showed a rising trend in the samples
heated at 60 and 70 C during the whole process, indicating
that under these temperatures the rate of formation of reduc-
ing sugar was faster than its rate of consumption. The reducing
sugar content increased remarkably at the early stage of 80 and
90 C, suggesting that the accumulation rate of reducing sugar
exceeded its consumption rate. The rate of the Maillard reaction
and caramelisation reaction was improved at high temperatures,
thus leading to the consumption rate of reducing sugar exceeding
its accumulation rate. As a result, the reducing sugar content
showed a downward trend at the later stage of high temperature
(80 and 90 C) processing.
Black garlic did not have the appropriate sweet flavour
because of the large amount consumption of reducing sugar
in high-temperature conditions. The relationship between reduc-
ing sugar content and quality of black garlic might be inferred as
follows. When black garlic aged under 70 C, the accumulation
amount of reducing sugar was very high, relatively, with abundant
sweet flavour, and thus the quality of black garlic was better than
others. The reducing sugar content was the highest when black
garlic was heated at 80 C for 12 days, and black garlic had ripened
basically with a nice flavour by this time. If black garlic continued
to be processed under this condition, its quality would become
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poor with a burning smell. In the case of 60 C, the reducing
sugar content in black garlic was lower than others, thus it was
unsweetened and its quality was also worse. When black garlic
aged at 90 C, the accumulated amount of reducing sugar was
the lowest, and the products had a strong burning smell and hard
texture, suggesting that the quality of black garlic was the worst.
Effect of temperature on amino acid nitrogen content
The effect of processing temperatures on amino acid nitrogen
content of black garlic is shown in Fig.4. During the progression
of the heat treatment process, the amino acid nitrogen content
decreased gradually, and the reduced rate of amino acid nitrogen
was faster at higher temperature.
When the garlic sample was processed at 60 C, the consumption
of amino acid nitrogen was the lowest among all treatments. This
phenomenon might be explained by considering that the rate
of the Maillard reaction at 60 C was very slow so that amino
acid nitrogen was hardly consumed in the Maillard reaction. With
increased temperature (70, 80 and 90 C), the rate of the Maillard
reaction in black garlic was accelerated, and thus led to a faster
consumption of amino acid nitrogen. The consumed amount of
amino acid nitrogen also increased at high temperature (80 and
90 C). Therefore, t he amino acid nitrogen conte nt was much lower
at the final stage of high temperature (80 and 90 C) processing
compared with that at low temperature (60 and 70 C) processing,
which was in accordance with changes in reducing sugar content.
Effect of temperature on 5-hydroxymethylfurfural content
HMF is an important intermediate product in Maillard reaction32
and it is one of the main antioxidant ingredients in black garlic. The
accumulation of HMF was connected with the rate of black forma-
tion of the garlic sample.33 When the HMF content accumulated
to some extent (about 4 g kg1), the colour of the garlic sample
became black.33 35 Therefore, changes in HMF content can serve
as a significant monitoring index to predict the rate of black forma-
tion of garlic samples.
The garlic samples produced a large amount of HMF due to the
Maillard reaction. Regardless of the temperature, HMF contents
in all garlic samples increased during the thermal processing.
The increment speed of HMF content was faster due to higher
temperature treatment (Fig.5). Thus, the change of HMF content
was significantly affected by temperature. When the HMF content
reached around 4 g kg1, black garlic had aged basically and
presented black. The changes in colour of garlic samples was from
dark brown to black when garlic was processed for about 9 days
at 90 C, for about 21 days at 80 C, for about 33 days at 70 C.
HMF content increased very slowly during the whole process at
temperature of 60 C and when black garlic aged, its content was
approximate 0.39– 0.46 times that at other temperatures, which
resulted in worse quality of black garlic. When garlic was heated
at temperatures of 80 and 90 C the HMF was generated faster and
its content was quite high at the final process stage. Although high
temperatures (80 and 90 C) accelerated the browning reaction
and contributed to the formation of a favourable colour and lustre
of black garlic, the black garlic could acquire a bitter flavour more
easily with increasing temperature. In consequence, taste quality
of black garlic was not palatable at high temperatures (80 and
90 C). The increased rate of HMF content of garlic sample was
appropriate when it was heated under 70 C conditions. When
black garlic aged under 70 C conditions, its colour was black, its
mouthfeel was palatable and its texture was soft. As a result, black
garlic could develop a better quality at a temperature of 70 C.
Figure 5. Effect of temperature on HMF content.
Figure 6. Effect of temperature on total phenols content.
Effect of temperature on total phenols content
Garlic is rich in a number of phenolic compounds.36,37 It is sug-
gested that phenolic compounds were increased by about four-
to 10-fold in the black garlic compared with that in the fresh
garlic.38,39 The increase of total phenols content improves the total
antioxidant capacity of black garlic.40
Changes in total phenols content of garlic sample at four tem-
perature treatment conditions are shown in Fig.6. Regardless of
the temperature, the total phenols content significantly increased
during thermal treatments (P<0.05). The increased rate of total
phenols content was faster at higher temperatures. Total phe-
nols content was continuously increased during the processing
at 60 C, indicating that the accumulation rate of total phenols
exceeded its consumption rate throughout the processing. In con-
trast, the total phenols content increased in the early stage of
70 C, 80 Cand90C processing. Additionally, the total phenols
content decreased at the later stage of these conditions, which
demonstrated that the accumulated rate of total phenols was less
than its consumed rate.
Among all the treatments, the black garlic aged at 80 Chadthe
highest total phenols content (15.12 g kg1) during the processing
(P<0.05). The quality of aged black garlic was better at this
time. The maximum of total phenols content was 12.35 g kg1
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Figure 7. Effect of temperature on total acids content.
Figure 8. Effect of temperature on allicin content.
under 70 C process condition and its quality was also better. Total
phenols content was very low and the maturity of black garlic was
not enough at a temperature of 60 C. When total phenols content
reached the maximum at 90 C, black garlic was immature and its
quality was bad. These results suggest that 80 C might facilitate
accumulation of total phenols in black garlic during processing.
Effect of temperature on total acids content
The increase of acidity of garlic samples due to heat treatment was
partly associated with browning substances that formed during
thermal processing. It is reported that the formation of carboxylic
acids can lead to the increase of acidity.41,42
According to sensory evaluation and our experience, when the
total acids content in black garlic is below 15 g kg1, black garlic
has little sour flavour. When the total acids content in black
garlic reaches 15 –30 g kg1, black garlic has a little sour flavour.
When total acids content in black garlic is within 30– 40 g kg1,
black garlic has a good sour flavour. When total acids content in
black garlic exceeds 40 g kg1, black garlic has an unpleasant sour
flavour.
Regardless of the temperature, the total acids content gradually
increased in black garlic during thermal processing. Moreover,
its increasing rate in the samples was elevated with increasing
temperature (Fig.7). By the time of maturity of black garlic at 70
and 90 C, total acids content in black garlic was higher than that
in fresh garlic (4.6 g kg1). Total acids content did not increase at
a temperature of 60 C so the black garlic did not acquire a good
sour flavour. Evaluation of the acid flavour showed that the total
acids content in black garlic aged at 80 C was appropriate, and its
taste quality was better than other samples.
Effect of temperature on allicin content
Allicin is the main and symbolic flavour substance in fresh garlic.
The allicin content in all samples decreased to very little during
the black garlic process (Fig.8). Therefore, in black garlic, the
offensive odour from fresh garlic was removed. The allicin content
rapidly decreased at the early stage of thermal processing, and its
decreased rate was slightly discrepant. The allicin content of black
garlic had declined to about 0.2 g kg1by the time of maturity
of black garlic, significantly lower than that in fresh garlic (about
3.45 g kg1). These results suggest that black garlic hardly had an
irritating odour, and allicin was no longer a significant functional
substance in black garlic.
CONCLUSIONS
When garlic was processed to black garlic under different temper-
atures, biological activity and content of various substances in the
garlic varied significantly. Temperature had a remarkable impact
on the quality of black garlic. The flavour of black garlic was con-
spicuously different from that of fresh garlic. The results showed
that good quality and flavour of black garlic occur under 70 Cpro-
cessing conditions. However, quality formation of black garlic is a
very complex process, which is influenced by many factors besides
temperature, such as garlic varieties and relative humidity during
thermal processing. More effort should be made to elucidate the
effects of these factors on the quality of black garlic products in
future studies.
ACKNOWLEDGEMENTS
This project was supported by the National Natural Science Foun-
dation of China (31371816) and Special Fund for Agro-scientific
Research in the Public Interest of China (201303079).
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... During processing, pH was directly related to L* and b* but inversely proportional to ∆E because of the browning process. At the end of the process, after the garlic had turned perfectly black (∆E = 80) 39 within 1 week at 80 °C and within 5 weeks at 60 °C, the pH of black garlic decreased to below 4.6 (Fig. 3f). A lower pH was found for black garlic, possibly due to the presence of acetic acid and succinic acid. ...
... On the contrary, an increase in the acidity during the process due to the browning reaction also reduced the pH of the black garlic. It dropped below 4.2 after the first week of aging due to carboxylic acids that formed during thermal processing by a browning reaction 39 . The effect of high temperature and acidity may inhibit γ-GTP activity because the optimum pH and temperature for γ-GTP activity are pH 6 and 70 °C 43 . ...
... www.nature.com/scientificreports/ than commercial black garlic, it was more than 80, so it can be considered "perfect black garlic" as well 39 . In addition, the produced black garlic had high antioxidant activity of 5390.02 ± 180.03 and 25,421.11 ...
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