


































Food Science and Nutrition Studies 

ISSN 2573-1661 (Print) ISSN 2573-167X (Online) 

Vol. 3, No. 4, 2019 

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107 
 

Original Paper 

Investigation of Processing Technology for Aged Black Jujube 

Lin Gao1, Duanyin Gu2, Xin Sun1 & Rentang Zhang1* 

1 College of Food Science and Engineering, Shandong Agricultural University, Tai’an, People’s 

Republic of China 

2 Tai’an Academy of Agriculture Sciences, Tai’an, China 

* Rentang Zhang, College of Food Science and Engineering, Shandong Agricultural University, Tai’an, 

People’s Republic of China 

 

Received: August 27, 2019   Accepted: September 9, 2019   Online Published: September 23, 2019 

doi:10.22158/fsns.v3n4p107        URL: http://dx.doi.org/10.22158/fsns.v3n4p107 

 

Abstract 

We investigated how to use jujubes from Xinjiang to make black jujube by temperature-controlled wet 

high-temperature curing. Single-factor and orthogonal experimental designs L9(3)3 were used to 

optimize the color change process of the black jujube. The effect of blackening temperature, blackening 

time and added water amount on the content of cAMP, 5-HMF, polyphenols, total acids, reducing 

sugars and moisture content were investigated and optimized. The results showed that the optimum 

process conditions of red jujube aging and blackening black include blackening temperature of 75°C, 

blackening time of 55 h, and water addition amount of 150 mL per 600 g. Under the best technological 

conditions, the black jujube was fragrant, sweet, and delicious, and the content of all functional 

substances, including cAMP (0.0137 g/100g), 5-HMF (0.103 g/100g), polyphenols (2.71 g/100g), total 

acids (17.09 g/kg), reducing sugars (76.7 g/100g), reached high levels, at a moisture content of 26%. 

Keywords 

Red jujube, aging, blackening, black jujube, processing technology 

 

1. Introduction 

Jujube fruit, also known as big jujube, dry jujube, old Chinese jujube, is the mature fruit of Ziziphus 

jujuba Mill. from the Rhamnaceae family. Jujube is a unique fruit and vegetable resource in China. It is 

native to China and has been cultivated for over 4000 years (Li, Fan, Ding, & Ding, 2007). With an 

area and output of more than 90% of the world’s total, China is the world’s largest producer of jujube 

fruits (Zhao, Zhang, Liu, Xue, & Pan, 2014). Red jujube is a characteristic fruit of China which 

integrates nutrition, health care and medicinal functions. Jujube is rich in vitamins, minerals, 

phytosterols, amino acids, saponins, polyphenols, flavonoids, cAMP, cGMP, and polysaccharides 



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(Pawlowska et al., 2009). Jujube can enhance immunity, help the body resist inflammation, protect the 

liver, intestines and stomach, ameliorate insomnia and act as an antioxidant (Chen et al., 2015; Zhang, 

W. Y., Zhang, L., & Xu, 2016; Almansa, Hernández, Legua, Nicolás-Almansa, & Amorós, 2016; Yue et 

al., 2015). 

At present, there are many red jujube products on the market, such as dry products, fermented products, 

preserved jujube, red jujube beverages and so on. However, there are few reports on aging black jujube. 

Ji et al. (2013) have done relevant research on the aged jujube, but they only investigated the changes 

of composition. There are some conventional jujube products in China, such as Jiaozao, Wuzao, etc., 

but they are produced differently from the production process of aging black jujube. The processing 

method of black jujube is similar to that of black garlic. Black jujube is produced by non-enzymatic 

browning of red jujube at high temperature and high humidity. The color, flavor and nutrition of jujubes 

are changed after blackening. It has been found that the antioxidant capacity of red jujube is also 

enhanced after aging and blackening (Park et al., 2012). There is a lack of research on processing 

technology for black jujube. So, the development of low-sugar aged black jujube and related products 

has broad market prospects. 

 

2. Materials and Methods 

2.1 Plant Material and Reagents 

Jujube fruits (moisture content 15-20%) were obtained from Hami (GPS E93°32′N42°49′) Xinjiang 

Province, China. Chemicals, including zinc acetate, potassium ferrocyanide, NaOH, phenol, 

concentrated sulfuric acid, and ethanol were purchased from Tianjin Kaitong chemical reagent co., LTD 

(Tianjin, China); 5-HMF (98%), cAMP (98%) and gallic acid (98%) were purchased from Shanghai 

Yuanye biotechnology co., LTD (Shanghai, China). HPLC grade methanol and acetonitrile were 

purchased from Shandong Yuwang industrial co., LTD (Yucheng, Shandong, China). 

2.2 The Influence of Blackening Conditions on the Content of Various Index Compounds in Black 

Jujube  

2.2.1 Influence of Blackening Time on the Content of Various Index Compounds in Black Jujube 

Dried jujubes were divided into 6 equal portions, 600 g each, and cleaned. According to the proportion 

of jujube water mass ratio 1:5, the fruits were rehydrate at room temperature for 1 h, removed and drain 

for 15 min, and then 400 mL distilled water added to the bag and sealed. Then, the red jujube were put 

in the 80°C oven for blackening. At 36 h, 48 h, 60 h, 72 h, 84 h, and 96 h. Cyclic adenosine 

monophosphate, 5-hydroxymethylfurfural, polyphenols, total acids, reducing sugars, water content and 

other indicators of each sample were detected respectively. 

2.2.2 Influence of Blackening Temperature on the Content of Various Index Compounds in Black 

Jujube 

Dried jujubes were divided into 6 equal portions, 600 g each, then cleaned. According to the proportion 

of jujube water mass ratio 1:5, rehydrate at room temperature for 1 h, remove and drain for 15 min, and 



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then 400 mL of distilled water was added to the bag and sealed. The portions were separately aged for 

60h at 65°C, 70°C, 75°C, 80°C, 85°C, and 90°C. Cyclic adenosine monophosphate (cAMP), 

5-hydroxymethylfurfural, polyphenols, total acids, reducing sugars, water content and other indicators 

of each sample were measured respectively. 

2.2.3 Influence of the Amount of Added Water on the Content of Various Index Compounds in Black 

Jujube 

Dried jujubes were divided into 6 equal portions, 600 g each, then cleaned. According to the proportion 

of jujube water mass ratio of 1:5, the fruits were rehydrated at room temperature for 1 h, removed and 

drained for 15 min. Then, 100 mL, 200 mL, 300 mL, 400 mL, 500 mL and 600 mL of distilled water 

was added to individual bags and sealed, followed by aging at 80°C for 60 h. The contents of cAMP, 

5-hydroxymethylfurfural, polyphenols, total acids, reducing sugars, water and other indicators of each 

sample were measured respectively. 

2.3 Orthogonal Experimental Design for Optimizing the Blackening Conditions 

On the basis of single factor tests, an orthogonal test was carried out by selecting factors such as 

blackening time, blackening temperature and amount of added water. An orthogonal experiment [L9(3)3] 

test design in the blackening mode was used for optimizing the blackening conditions. The key 

parameters that influenced the contents of key components of black jujube were analyzed, including the 

time of blackening (A), temperature of blackening (B) and amount of added water (C). Every factor 

had three levels. Nine extractions were carried out at blackening times 55, 60 and 65 h, blackening 

temperatures of 75, 80 and 85°C, and added water amounts of 150, 200, and 250 mL. The factor levels 

are shown in Table 1. 

 

Table 1. Orthogonal Design 

Level 
Factor 

A, Blackening time/h B, Blackening temperature (°C) C, Amount of added water (mL) 

1 55 75 150 

2 60 80 200 

3 65 85 250 

 

2.4 Index Measurement Method 

2.4.1 Analysis of Total Acids 

The total acid content of black jujube was determined using the GB/T 12456-2008 method. Black 

samples (20 g) were broken, dissolved in water at 80°C, and then placed in a boiling water bath for 30 

min (shaken 2-3 times). After the solution was cooled, the volume was set to 250 mL. The sample was 

filtered, and 25 mL of the filtrate was added to 50 mL of water and titrated with 0.1 mol/L sodium 

hydroxide solution until the pH was 8.3. 



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2.4.2 Analysis of Reducing Sugars 

The reducing sugar content of black jujube was determined using the GB 5009.7-2016 method. A 

sample comprising 2.5 g of jujube paste was dissolved in 50 mL of water and transferred to a 250 mL 

volumetric flask after stirring on a magnetic stirrer for 10 min. Zinc acetate and potassium ferricyanide 

were added, 5 mL each, to a constant volume, shaken well, and allowed to stand for 30 min. The 

solution was filtered, the primary filtrate discarded, and the subsequent filtrate saved for later use. 

Aliquots comprising 5.0 mL alkaline cupric tartrate solution and 5.0 mL alkaline cupric tartrate solution 

were absorbed, then put in a 150 mL conical flask, 10 mL of water and 2~4 glass beads added. The 

sample filtrate was used for titration, which was heated to boiling within 2 min, and the titration 

continued at a rate of 1 drop per 2s until the blue color just faded as the end point. 

2.4.3 Analysis of cAMP 

A sample comprising 2 g of black jujube paste was added to 80 mL water, stirred with a magnetic 

stirrer for 10 min, transferred to a 100-mL volumetric flask, and filled to the mark with water. The 

sample was liquid incubated at 80°C for ultrasonic extraction for 30 min. After cooling, the liquid was 

passed through a 0.45 μm pore-size membrane and analyzed by HPLC using a sb-c18 column (150 mm 

x 2.1 mm, 3.5 m; Shimadzu, Japan). The mobile phase was composed methanol and 50 mM potassium 

dihydrogen phosphate (10:90, V/V). The flow rate was 1 ml/min. The column temperature was 30°C. 

The detection wavelength was 254 nm. The injection volume was 10 μL. 

2.4.4 Analysis of 5-HMF 

A sample comprising 5 g of black jujube paste was suspended in 10 mL methanol, after which a small 

amount (approx. 5 mL) of water was added. After magnetic stirring for 10 min, the mixture was 

transferred to a 50 mL brown volumetric flask and filled to the mark with water. Then, the sample 

solution was ultrasonicated for 30 min and filtered to the sample bottle through a 0.45 μm organics 

filtering membrane. 

A total of 10 μL was injected into the HPLC column. The analytes were separated on a IntertSustain 

C18 column (250×4.6 mm, 5 μm; Shimadzu, Japan) at 35°C. The mobile phase was a methanol-water 

mixture (2:98, v/v). The absorbance wavelength for determination was 282 nm. 

2.4.5 Analysis of Polyphenols 

This analysis was done based on a previous study[10] with some modifications as follows. 1 g of black 

jujube paste was dissolved in 30 mL 70% ethanol solution and stirred with a magnetic stirrer for 10 min, 

after which the volume was adjusted to 50 mL with 70% ethanol. The sample was then ultrasonicated at 

66°C for 30 min and filtered, after which 0.2 mL of the sample solution was added to 0.5 mL of 100% 

Folin phenol reagent diluted 1 time, and the mixture was evenly mixed. Then, 1.5 mL of Na2CO3 

solution with a mass fraction of 10% was added, and the mixture was mixed evenly with distilled water 

in a constant volume until 10 mL. The mixture was allowed to react at 75°C for 10 min, after which its 

absorbance at 760 nm was measured. 

 



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2.5 Data Processing and Statistical Analysis 

All experiments were conducted in triplicate. The experimental results were presented as means±SE. 

SPSS 20.0 software (IBM Corp., USA) was used for analysis, and Origin 8.0 software (OriginLab 

Corp., USA)was used for drawing charts. 

 

3. Results and Discussion 

3.1 The Influence of Blackening Conditions on the Content of Various Index Compounds in Black 

Jujube  

3.1.1 Effect of Blackening Time on the Biochemical Contents of Black Jujubes 

 

Table 2. Influence of Blackening Time during the Aging Process of Red Jujubes on Important 

Biochemicals 

Index 
Blackening time 

36h 48h 60h 72h 84h 96h 

Total acids (g/kg) 19.76±0.00b 18.66±0.16a 19.59±0.00b 22.40±0.00c 23.86±0.00e 23.45±0.00d 

Reducing sugars (g/100g) 71.6±0.00b 74.60±0.17d 75.67±0.29e 74.10±0.10c 71.6±0.00b 70.27±0.07a 

Polyphenols (g/100g) 2.16±0.01a 2.30±0.09ab 2.32±0.13ab 2.39±0.38b 2.22±0.38ab 2.30±0.09ab 

5-HMF (g/100g) 0.143±0.003a 0.177±0.006b 0.203±0.006c 0.290±0.010d 0.323±0.006e 0.353±0.006f 

cAMP g/100g 0.029±0.006e 0.026±0.001d 0.023±0.001c 0.021±0.000b 0.015±0.001a 0.014±0.001a 

Note. in the same line, numbers marked with the same letters in the upper right corner indicate that 

there is no significant difference between groups (p > 0.05), while those without the same letters 

indicate that there is significant difference between groups (p < 0.05). 

 

 

Figure 1. Influence of Blackening Time on the Moisture Content of Aged Black Jujubes 

 



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The influence of blackening time on the aging indexes of red jujubes is shown in Table 2 and Figure 1. 

Blackening time was fixed at 36 h, 48 h, 60 h, 72 h, 84 h and 96 h. It can be seen from Table 2 that the 

blackening time has a significant effect on the contents of total acids, reducing sugars, furfural, cAMP, 

flavonoids and water in aged jujubes, but has little effect on the polyphenol content. With the extension 

of blackening time, the total acid content decreased significantly at 36-48 h, but increased significantly 

after 48 h. The content of reducing sugars increased significantly between 36 and 60 h, but decreased 

significantly after 60 h. The content of polyphenols did not change significantly with the extension of 

blackening time. Furfural content showed a significant increasing trend with the extension of 

blackening time (p < 0.05), the while cAMP content showed a decreasing trend and leveled off after 84 

h. As can be seen in figure 1, the water content fluctuated greatly with the extension of blackening time, 

with the lowest content of 42% observed at 60 h. 

During the whole process, the total acid content increased, which may be caused by the fact that the 

reduced ketones generated by the Maillard reaction can be lysed to produce acids, resulting in the 

increase of the total acid content (Martins, Jongen, & Van Boekel, 2000; Troise, 2018; Karbasi & 

Madadlou, 2018). In addition, during the browning reaction, the increase of acids is also related to the 

production of carboxylic acids. For example, it was reported that carboxylic acids are produced by the 

oxidation of aldoses (Sang, Cho, Yong, Lee, & Park, 2014). In addition, due to the presence of amino 

and carbonyl groups, organic acids may change during the Maillard reaction. The increase of reducing 

sugars may be due to the fact that sucrose was decomposed into monosaccharides or changed into other 

disaccharides. Our analysis showed that the sucrose content decreased continuously during the aging 

and blackening of jujube, and finally dropped to 0. Therefore, sucrose may be converted into other 

reducing sugars during the blackening process. Since reducing sugars are one of the reactants of 

Maillard reaction, they were continuously consumed and their content decreased with the extension of 

blackening time. The 5-HMF increased with the extension of black time, but the accumulation rate was 

higher in the early stage and decreased in the later stages, which may be because 5-HMF is one of the 

sources of melanoid, the final product of the Maillard reaction. 5-HMF was gradually consumed in the 

process of increasing melanoid accumulation in the later stages. cAMP was consumed in the 

non-enzymatic browning reaction during aging. 

The reducing sugar content reached the highest point at 75.67 g/100g after 60 h of blackening, whereby 

the polyphenols and cAMP still remained at a relatively high level, so as to better maintain the nutrients 

and functional components of jujube. In addition, the moisture content first increased with the increase 

of blackening time, and then decreased to 42% at 60 h, which was suitable for subsequent processing. 

However, the total acid index maintained an upward trend, and too much acidity was not conducive to 

subsequent processing. Furfural has also had a rising trend, but there is still a controversy surrounding 

the impact of 5-HMF at present (Durling, Busk, & Hellman, 2009), and how it affects the taste of the 

final product, especially in terms of bitterness. Therefore, blackening jujube for 60 h had the best 

quality effect. 



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3.1.2 Effect of Blackening Temperature on the Biochemical Composition of Black Jujube 

 

Table 3. Influence of Blackening Temperature on the Aging Process of Red Jujubes  

Index 
Blackening temperature 

65°C 70°C 75°C 80°C 85°C 90°C 

Total acids (g/kg) 13.81±0.01a 15.03±0.01b 17.59±0.02c 20.34±0.01d 21.72±0.17e 25.29±0.15f 

Reducing sugars (g/100g) 75.59±0.03e 74.41±0.05d 67.40±0.03b 77.54±0.03f 71.61±0.03c 63.06±0.00a 

Polyphenols (g/100g) 3.64±0.11a 3.50±0.06a 3.43±0.32a 5.47±0.10c 4.46±0.07b 4.25±0.04b 

5-HMF (g/100g) 0.043±0.001a 0.097±0.003b 0.171±0.006c 0.256±0.012d 0.418±0.005e 0.698±0.027f 

cAMP g/100g 0.034±0.001e 0.027±0.002d 0.021±0.002bc 0.023±0.001c 0.018±0.001b 0.014±0.000a 

Note. In the same line, the numbers marked with the same letters in the upper right corner indicate that 

there is no significant difference between groups (p > 0.05), while those without the same letters 

indicate that there is significant difference between groups (p < 0.05). 

 

 

Figure 2. Influence of Blackening Temperature on the Moisture Content of Aged Black Jujubes 

 

The influence of different blackening temperatures on cAMP, furfural, total acid, reducing sugar, 

polyphenol and moisture indexes was studied, as shown in Table 3 and Figure 2. The blackening 

temperature was fixed at 65°C, 70°C, 75°C, 80°C, 85°C and 90°C. As shown in Table 3, with the 

increase of blackening temperature, the content of total acids and furfural showed a significant 

increasing trend. It is generally believed that 5-HMF is a product of dehydration and decomposition of 

fructose or glucose under acidic conditions, and it is a common intermediate product of the Maillard 

reaction, ascorbic acid oxidation reaction and caramelization. Studies have pointed out that the 

accumulation of 5-HMF is strongly correlated with the browning rate. However, 5-HMF is produced at 

high temperatures and is the most important pollutant among heat-induced products, especially in 



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baked goods (Capuano & Fogliano, 2011). The reducing sugar content increased significantly with the 

increase of blackening temperature. At 80°C the content was 77.54 g /100g. The increase of the 

reducing sugar content may be associated with the decomposition of other sugars. Studies have shown 

that the reducing sugar content is directly proportional to the browning rate in the Maillard reaction. In 

the later stages, with the increase of blackening temperature, the Maillard reaction was accelerated, 

melanoid-like substances accumulated more, and the color of the jujube was deepened (Majid, Mehdi, 

& Moein, 2019). For the polyphenol content, the rise of temperature was within the range 65-75°C, but 

the maximum of 5.47 g/100g reached at 80°C was significantly lower. At temperatures higher than 

85°C the content stabilized. With the increase of temperature, the increase of polyphenol content may 

be caused by the formation of Maillard reaction products or by the cleavage of the esterified and 

glycosylated compounds (Kavita et al., 2015). Overall, cAMP displayed a reducing trend, but increased 

somewhat at 80°C, in spite of generally faster decomposition at higher temperatures. As can be seen in 

Figure 2, with the increase of blackening temperature, water content reduced first reduced and then 

rose again, but was significantly lower at 75°C. This may be because the maillard reaction rate was 

faster in the early stage and the water consumption was faster. When the temperature exceeded 80℃, 

the maillard reaction was weakened by the temperature, and the evaporation of water increased with the 

temperature in the later stage, resulting in the water content of jujube decreasing. Therefore, the jujubes 

blackened at 80°C were optimal, with the functional material content maintaining a high level. 

3.1.3 Effect of Added Water Amount on the Biochemical Contents of Black Jujube 

 

Table 4. Influence of Added Water Amount on the Aging Process of Red Jujubes 

Index 
Added water amount 

100mL 200mL 300mL 400mL 500mL 600mL 

Total acids (g/kg) 21.69±0.12e 21.94±0.01f 21.15±0.00d 18.77±0.01c 19.26±0.01b 17.15±0.08a 

Reducing sugars (g/100g) 63.78±0.02a 70.47±0.09c 66.92±0.11b 72.68±1.66d 76.10±0.16e 77.27±0.13e 

Polyphenols (g/100g) 3.45±0.11b 4.37±0.04e 3.07±0.05a 3.61±0.03bc 4.15±0.06d 3.63±0.03c 

5-HMF (g/100g) 0.337±0.010e 0.314±0.006d 0.281±0.002c 0.264±0.002b 0.260±0.005b 0.245±0.000a 

cAMP g/100g 0.017±0.002a 0.018±0.001a 0.019±0.001a 0.020±0.001a 0.027±0.002b 0.026±0.002b 

Note. In the same line, the numbers marked with the same letters in the upper right corner indicate that 

there is no significant difference between groups (p > 0.05), while those without the same letters 

indicate that there is significant difference between groups (p < 0.05). 



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Figure 3. Influence of Water Addition on the Moisture Content of Aged Black Jujubes 

 

The effects of different amounts of added water on cAMP, furfural, total acid, reducing sugar, 

polyphenols and water indexes in the blackening process were studied, as shown in Table 4 and Figure 

3. The amount of added water was fixed at 100 mL, 200 mL, 300 mL, 400 mL, 500 mL and 600 mL. 

As shown in Table 4, with the increase of water content, the total acids and furfural showed a 

significant downward trend, i.e., the higher the humidity, the lower the content. There was a significant 

difference of reducing sugars between 100 mL and 400 mL of water added, but it tended to be stable 

after 500 mL. The polyphenol content fluctuated greatly, and the increase was the most obvious at 200 

mL, reaching 4.37 g/100g. There was no significant difference in the content of cAMP at 100-400 mL 

and 500-600 mL. As can be seen from figure 3, with the increase of total water content, the water 

content of the jujube also showed an upward trend. When the amount of added water exceeded 400 mL, 

the black jujube not only had a large water content, but also had a large amount of residual water in the 

blackening container, resulting in an incomplete appearance and soft collapse of the black jujube. 

When 100 mL of water was added, the water content of black jujubes was 28%, and the black jujubes 

were hard and bitter. To sum up, when the water content was 200 mL, the water content of jujube was 

appropriate, and cyclic adenosine monophosphate, reducing sugars and polyphenol were all at high 

levels. Therefore, the water content of jujube was the most appropriate with the addition of 200 mL of 

additional water. 

 

 

 

 

 

 

 



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3.2 Optimization of the Blackening Process of Red Jujube Using an Orthogonal Experiment 

 

Table 5. The Orthogonal Experimental Design for Assessing Different Blackening Conditions 

Level A B C 
5-HMF 

(g/100g） 

cAMP 

(g/100g） 

Total acids 

(g/kg） 

Reducing sugars 

(g/100g） 

Polyphenols 

(g/100g） 

1 1 1 1 0.103  0.0137  17.09  76.7  2.71 

2 1 2 3 0.234 0.0116  22.48  72.2  3.605 

3 1 3 2 0.391 0.0082  24.69  66.3  3.122 

4 2 1 2 0.135 0.0150  19.44  72.5  3.42 

5 2 2 1 0.277 0.0124  23.22  72.3  3.302 

6 2 3 3 0.395  0.0092  24.59  61.3  3.296 

7 3 1 3 0.157  0.0158  19.91  76.7  2.799 

8 3 2 2 0.325  0.0100  25.13  70.2  3.509 

9 3 3 1 0.475  0.0217  24.99  66.2  3.525 

5-HMF 

K1 0.728 0.395 0.855      

K2 0.807 0.836 0.851  
 

K3 0.957 1.261 0.786  

R 0.199 0.866 0.069      

cAMP 

K1 0.0335 0.0445 0.0478      

K2 0.0366 0.034 0.0332  
 

K3 0.0475 0.0391 0.0366  

R 0.014 0.0105 0.0146      

Total acids 

K1 64.26 56.44 65.3      

K2 67.25 70.83 69.26  
 

K3 70.03 74.27 66.98  

R 5.77 17.83 3.96      

Reducing 

sugars 

K1 215.2 225.9 215.2      

K2 206.1 214.7 209  
 

K3 213.1 193.8 210.2  

R 9.1 32.1 6.2      

Polyphenols 

K1 9.437 8.929 9.537      

K2 10.018 10.416 10.051  
 

K3 9.833 9.943 9.7  

R 0.581 1.487 0.514      



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Table 6. Variance Analysis Table of Furfural for Orthogonal Test Design 

Factor Sum of squares Freedoms Mean square error F Sig. 

Correction model 0.135a 6 0.023 502.583 0.002 

Intercept 0.690 1 0.690 15409.588 0.000 

A 0.009 2 0.005 100.722 0.010 

B 0.125 2 0.063 1395.859 0.001 

C 0.001 2 0.001 11.169 0.082 

Error 8.956E-005 2 4.478E-005   

Total 0.825 9    

Corrected total 0.135 8    

 

According to the range and variance analysis shown in Tables 5 and 6, the primary and secondary order 

of each influencing factor is: B blackening temperature > A blackening time > C water addition amount. 

The blackening temperature and blackening time had a significant effect on furfural, while the addition 

of water had no significant effect. With furfural as index, an orthogonal test was done to determine the 

best technology of blackening for A1B1C2, which were found to encompass a blackening time of 55 h, 

blackening temperature of 75°C, and added water of 200 mL. 

 

Table 7. Anova Table of Cyclic Adenosine Monophosphate for Orthogonal Test Design 

Factor Sum of squares 

Freedoms 

Mean square error F Sig. 

Correction model 9.333E-005a 6 1.556E-005 0.716 0.682 

Intercept 0.002 1 0.002 70.737 0.014 

A 3.605E-005 2 1.802E-005 0.830 0.547 

B 1.838E-005 2 9.190E-006 0.423 0.703 

C 3.891E-005 2 1.945E-005 0.896 0.528 

Error 4.345E-005 2 2.172E-005   

Total 0.002 9    

Corrected total 0.000 8    

 

According to the range and variance analysis shown in Tables 5 and 7, the primary and secondary order 

of each influencing factor was: C water addition amount > A blackening time >B blackening 

temperature. The effect of blackening temperature, blackening time and water content on cAMP was 

not significant. With cAMP as index, an orthogonal test was conducted to determine the best 

technology of blackening for A3B1C1, which was found to encompass a blackening time of 65 h, 

blackening temperature of 75°C, and added water of 150 mL. 



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Table 8. Total Acid Variance Analysis Table for Orthogonal Test Design 

Factor Sum of squares Freedoms Mean square error F Sig. 

Correction model 67.831a 6 11.305 109.511 0.009 

Intercept 4513.152 1 4513.152 43717.976 0.000 

A 5.551 2 2.776 26.887 0.036 

B 59.646 2 29.823 288.890 0.003 

C 2.634 2 1.317 12.756 0.073 

Error 0.206 2 0.103   

Total 4581.190 9    

Corrected total 68.037 8    

 

According to the range and variance analysis shown in Tables 5 and 8, the primary and secondary order 

of each influencing factor is: B blackening temperature > A blackening time > C water addition. The 

blackening temperature and blackening time had a significant effect on the total acid content, while the 

addition of water had no significant effect. With total acids as index, an orthogonal test was conducted 

to determine the best technology of blackening for A1B1C1, which was found to encompass a 

blackening time of 55 h, a blackening temperature 75°C, and added water of 150 mL. 

 

Table 9. Anova Table of Reducing Sugars for Orthogonal Test Design  

Factor Sum of squares Freedoms Mean square error F Sig. 

Correction model 199.307a 6 33.218 7.759 0.119 

Intercept 44718.151 1 44718.151 10445.454 0.000 

A 15.136 2 7.568 1.768 0.361 

B 176.962 2 88.481 20.668 0.046 

C 7.209 2 3.604 0.842 0.543 

Error 8.562 2 4.281   

Total 44926.020 9    

Corrected total 207.869 8    

 

According to the range and variance analysis shown in Tables 5 and 9, the primary and secondary order 

of each influencing factor is: B blackening temperature > A blackening time > C water addition. The 

time of blackening and the amount of water added had no significant effect on reducing sugars, while 

the temperature of blackening had a significant effect. Using separate raw sugars as an index, an 

orthogonal test was conducted to determine the best technology of blackening for A1B1C1, which was 

found to encompass a blackening time of 55 h, blackening temperature of 75°C, and water addition of 

150 mL. 



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Table 10. Anova Table of Orthogonal Test Design for Polyphenols 

Factor Sum of squares Freedoms Mean square error F Sig. 

Correction model 0.490a 6 0.082 0.503 0.782 

Intercept 95.310 1 95.310 588.045 0.002 

A 0.059 2 0.029 0.181 0.847 

B 0.385 2 0.192 1.187 0.457 

C 0.046 2 0.023 0.142 0.876 

Error 0.324 2 0.162   

Total 96.123 9    

Corrected total 0.814 8    

 

According to the range and variance analysis shown in Tables 5 and 10, the primary and secondary 

order of each influencing factor is: B blackening temperature > A blackening time > C water addition. 

The effect of blackening time, blackening temperature and water content on polyphenols was not 

significant. With the polyphenol content as index, an orthogonal test was conducted to determine the 

best technology of blackening for A2B2C2, which encompassed a blackening time of 60 h, blackening 

temperature of 80°C, and water addition of 200 mL. 

In conclusion, the three factors of blackening time, blackening temperature and water addition had no 

significant influence on the cAMP and polyphenol indexes. Therefore, furfural, total acid, reducing 

sugar indices were used as the basis to determine the optimal processing parameters for A1B1C1, which 

were a blackening time of 55 h, blackening temperature of 75°C, and water addition of 150 mL. 

 

4. Conclusion 

Red jujube is widely planted in China, but there are few reports on the production technology of black 

jujube. In this paper, an orthogonal L9(3)3 test was used to optimize the production process of black 

jujube. The results revealed optimum processing parameters as follows: aging blackening time 55 h, 

blackening temperature 75°C, added water 150 mL. The black jujube produced under the optimal 

technological conditions had a strong fragrance, sweet and sour taste, and contents of various 

functional substances reaching (dry matter meter): cAMP 0.0137 g/100g, 5-HMF 0.103 g/100g, 

polyphenols 2.71 g/100g, total acids 17.09 g/kg, reducing sugars 76.7 g/100g, and water content 26%. 

 

Acknowledgments 

This work was supported by the Shandong Province Key Research and Development Fund 

(2016GNC113015, 2019GNC106061) and Shandong Province major application of technological 

innovation projects. 

 



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