Researcher Li Dajing, Jiangsu Academy of Agricultural Sciences, et al.: Chemical pathway analysis of non-enzymatic browning of preserved strawberries during storage

Preserved strawberries are emerging preserved fruits in recent years, but browning occurs during the storage of preserved strawberries, affecting its commodity value and product appearance
.
Browning is a common phenomenon in food storage and processing, can be divided into enzymatic browning and non-enzymatic browning, browning during the storage of strawberries to non-enzymatic browning, the chemical mechanism generally includes ascorbic acid degradation, oxidative aggregation of phenolic substances, Maillard reaction, sugar cleavage, etc.
, directly involved in non-enzymatic browning chemical components often include ascorbic acid, reducing sugars, polyphenols, amino acids, etc
.

In order to explore the relationship between browning and development and temperature and time and the mechanism of internal chemical composition during the storage of preserved strawberry, Niu Liying, Hu Lili and Li Dajing* of the Institute of Agricultural Products Processing of Jiangsu Academy of Agricultural Sciences measured the color, ascorbic acid, reducing sugar, polyphenols, amino acids and 5-HMF contents of preserved strawberry during storage at different temperatures, analyzed the relationship between color and chemical composition, and used correlation analysis and similarity clustering to explore key pathways.
It is intended to provide a reference
for browning control during storage of preserved strawberries.

1.
Color change of dried strawberries at different storage temperatures

It can be seen from Figure 1 that the L* value in the initial color of dried strawberries is high, and the a* and b* values are positive, and the overall appearance is bright red
.
The decrease of L* value during food storage often indicates that the sample has browning phenomenon, but the change law of the three parameters in the browning process of different foods is not the same
.
During the storage of honey and applesauce, browning showed that the L* and b* values decreased but the a* value increased.
After 6 days of storage, lychee browning occurred, and the L*, a* and b* values of red peel all decreased
.
During the storage of preserved strawberries, the values of L*, a* and b* showed a downward trend, and the higher the temperature, the faster the change, the storage conditions with the largest degree of change were stored at 37 °C for 90 days, and the values of L*, a* and b* decreased to 64.
46%, 17.
95% and 21.
07% of the initial values
, respectively 。 The changes were slow at 25 °C and 4 °C, and even if the storage was extended to 150 d, the L* value decreased to 64.
81% and 74.
37% of the initial value, the a* value decreased to 22.
79% and 60.
51% of the initial value, and the b* value decreased to 37.
22% and 35.
53% of the initial value, respectively, which was still higher than the b* value
when stored at 37 °C for 90 days.
This phenomenon of higher temperature and faster change is consistent
with foods such as honey and fruit juice.

From the above data, it can also be seen that the sensitivity of the three color parameters to temperature changes is different, among which the a* value is the most sensitive to temperature differences, that is, for samples with the same storage time, the difference between different temperatures is the largest
.

2.
Changes in ascorbic acid and total phenolic content of preserved strawberries at different storage temperatures

It can be seen from Figure 2 that the content of ascorbic acid and total phenols in dried strawberries decreased rapidly in the early stage of storage, and then the decline rate slowed down, and the higher the storage temperature, the faster the decline rate, which was consistent
with previous research on fruit and vegetable juice 。 The ascorbic acid content in preserved strawberry decreased to 1.
07%, 16.
16% and 25.
44% of the initial value when stored at 37, 25 and 4 °C for 90 days, respectively, and the total phenolic content decreased to 25.
61%, 32.
23% and 39.
18% of the initial value, respectively, compared with orange juice, prune juice and strawberry juice, the loss rate of ascorbic acid and total phenol in preserved strawberry was higher, although this difference was related to external factors such as packaging, but still showed that preserved fruit as a processed product that effectively extended the consumption period of fruits.
The loss of nutrients during storage deserves attention
.
It is also reported that the fastest rate of ascorbic acid and total phenolic content decline in the early stage of fruit juice storage is related to the consumption of dissolved oxygen, and the later degradation is related to the oxygen barrier of the packaging material, therefore, the characteristics of this ascorbic acid and total phenol content in the storage process of strawberry preserved in ordinary sealed packaging (without oxygen removal measures) may also be related
to the presence and consumption of oxygen in the packaging.

3.
Changes in free sugars and amino acids of preserved strawberries at different storage temperatures

The free sugars in strawberries mainly include fructose, glucose and sucrose, and the sugars in dried strawberries are mainly these three (Table 1).

In the storage process, the content of the three sugars showed a downward trend with time, and the initial storage decreased rapidly at 37 °C and 25 °C, and the fructose, glucose and sucrose content was 25.
18%~59.
01% of the initial value at 30 days, and then the decline rate slowed down, and the content of the three sugars at 60 days was 83.
67%~96.
37%
at 30 days.
At 4 °C, the degradation of the three sugars was slow, and there was no significant difference between the fructose content and the initial value at 30 days, while the contents of glucose and sucrose were significantly higher than those at 37 °C and 25 °C when stored for 30 days
.
There were significant differences in sucrose content at different temperatures at the same storage time, which was 37 °C< 25 °C<4 °C, indicating that the higher the temperature, the faster<b13> the change.
However, the changes of fructose and glucose content at three temperatures all had a plateau phase, that is, 30~75 days at 37 °C, and 90~120 days at 25 °C and 4 °C, and the difference with time was not significant
at the same temperature.
The fructose and glucose content in fruit juice often does not change significantly or even increases during storage, because sucrose is decomposed into fructose and glucose, which compensates for the loss
of fructose and glucose.

4.
Changes of 5-HMF content at different storage temperatures

As shown in Figure 3, the content of 5-HMF increased linearly during the storage of preserved strawberries, and the higher the storage temperature, the faster the increase, and the growth rates were 26.
92 and 293.
88 times
at 25 °C and 4 °C, respectively.
It has been reported that temperature has a significant effect on the browning of sugarcane juice Maillard, and the formation rate of 5-HMF of preserved strawberry at different temperatures is quite different, indicating that temperature is also an important factor affecting the browning of
preserved strawberry.
In addition, since 5-HMF has a variety of precursors and formation pathways, it is of great significance
to explore the main pathway of 5-HMF formation during the storage of preserved strawberries to control its browning.

5.
Correlation analysis results between color change and chemical indexes

In order to better explore the relationship between color change and physical and chemical indexes, this study analyzes the correlation of each index
.
The correlation analysis results of color, ascorbic acid, total phenols, free sugars and amino acids, and 5-HMF levels are shown in Figure 4, and it can be seen that there is an obvious positive correlation between the color parameters L*, a*, b*, and ΔE, indicating that several parameters have consistency in characterizing color changes, among which the correlation between ΔE* and L*, a*, and b* values reaches a highly significant level (P<0.
001).
<b11> 。 The total color change was characterized by ΔE, and it was found that the highly significant chemical indexes were ascorbic acid content (P<0.
001), followed by sucrose, total phenol, 5-HMF and glucose (P<0.
01), followed by aspartic acid, fructose, arginine, alanine and asparagine (P<0.
05), and the glutamine content in the 11 chemical indexes was not significantly<b12> correlated.
However, glutamine content was significantly correlated with L* value, asparagine and ascorbic acid content (P<0.
05), and very significantly correlated with total phenolic content (P<0.
01).
<b13> It is reported that the degradation of ascorbic acid can be divided into aerobic degradation and oxygen-free degradation, and its oxygen-free degradation mainly generates dicarbonyl compounds through hydrolysis, decarboxylation, dehydration and ring-forming steps, which can react with amino acids such as carbonylamide condensation to generate colored substances; Under aerobic conditions, it is cleaved into highly reactive small molecule compounds, which can be further reacted with other components to form brown macromolecular compounds
.
The content of ascorbic acid decreased rapidly during the storage of preserved strawberries, and was significantly correlated with L* value, a* value, b* value, ΔE and sucrose and total phenol content (P<0.
01) or highly significant (P<0.
001), and showed significant correlation (P<0.
05) with other 2 free sugars, 3 amino acids (aspartic acid, glutamine, arginine), and 5-HMF content, indicating that it was closely related to browning reaction, aerobic degradation may occur, Anaerobic degradation and multipathway reactions<b15> such as carbonylamide condensation of amino acids.
In addition to fructose content and b* value, the content of three free sugars was associated with L* value, a* value, b* value, ΔE, ascorbic acid and total phenolic content (P<0.
05<b16>).
However, there was no significant correlation with the content of other four amino acids and 5-HMF except aspartic acid (P>0.
05), indicating that the browning caused by the Maillard reaction with the participation of reducing sugars was inconsistent
with the overall color change.
In general, according to the correlation between color index and various components, it can be considered that ascorbic acid, phenolic substances, sugars and amino acids are involved in the formation of brown substances in preserved strawberries, but different reaction pathways are not synchronized
.

The results of bidirectional clustering of various indicators and samples are shown
in Figure 5.
Fifteen samples with different storage periods at 3 temperatures were clustered into two categories: 5 samples stored at 4 °C, 3 samples with short storage time (30~90 d) at 25 °C, and 2 samples stored at 37 °C for 15 d and 30 d.
The other 5 samples with longer storage times at higher temperatures were grouped into Category II
.
All indexes can be divided into 4 groups: 1) L* value, ascorbic acid content, total phenolic content, b* value, glutamine content; 2) A* value, aspartic acid content, fructose content, sucrose content, glucose content; 3) Asparagine content, arginine content, alanine content; 4) ΔE and 5-HMF content
.
The main difference between the two categories is that the index values of group 1~3 in category I are higher (mainly red/black), and ΔE and 5-HMF as the red areas of group 4 are only distributed in 3 samples stored at 37 °C for 60~90 d and samples stored at 25 °C for 120 d and 150 d, reflecting that the 5-HMF content change and the overall color change have a certain degree of synchronization, and have temperature sensitivity
.
This variation of 5-HMF suggests that the increase in its content may come from the superposition
of several different browning pathways.

Conclusion