Professor Zhang Min of Beijing Technology and Business University: Study of flavor release during oral processing of indica rice based on gas chromatography-ion mobility spectroscopy and electronic nose technology

The oral processing of food is closely related
to sensory perception, especially the texture and flavor of food.
With the improvement of people's living standards, people's requirements for the taste and quality of rice have gradually increased
.
In previous research on the flavor of rice, rice was only used as an isolated experimental material, and the flavor substances were studied by material science methods, and chewing in the mouth when eating rice was a dynamic process
.
With the rapid rise of food oral processing research, many researchers have begun to explore the dynamic release of flavor during the chewing process of food in the mouth
.

Gas chromatography-ion mobility spectroscopy (GC-IMS) technology combines the advantages
of GC separation power and fast IMS response.
Based on the advantages of GC-IMS technology such as high separation efficiency, short analysis time, more accurate qualitative analysis, low cost and easy operation, it has been widely used in the food field in the past few years, including School of Food and Health, Beijing Technology and Business University, Beijing Food Nutrition and Human Health High-precision Innovation Center (Beijing Technology and Business University), Beijing Food Additive Engineering Technology Research Center (Beijing Technology and Business University), Serimhan Asmi, Ren Xin, Zhang Min*, etc.
combined with GCIMS and electronic nose technology.
To explore the changes of flavor substances and components of indica rice during chewing, and to distinguish whether there are differences in flavor components of indica rice at different stages of chewing, this paper aims to study the flavor release law of indica rice during oral processing, and provide scientific basis and theoretical reference
for the improvement of indica rice varieties and the quality improvement of convenient rice.

1.
Electronic nose analysis

It can be seen from Figure 1 that the flavor intensity of most types of flavor substances is not much different at different chewing stages, and the alkane represented by W1S has the largest intensity.
The alcohols, aldehydes, ketones, organic sulfides and nitrogen oxides represented by the three sensors of W2S, W2W and W5S have great differences, among which aldehydes, ketones and alcohols change the most
.
Through electronic nose analysis, it can be roughly determined that the content of aldehydes, ketones and alcohols changes the most during chewing, followed by some aromatic components
.
Aldehydes, ketones and alcohols mainly come from Indica rice itself and are the main contributors to the flavor of
Indica rice.

2.
GC-IMS analysis of flavor substances of indica rice during oral processing

As shown in Figure 2, the red line parallel to the Y-axis represents the reactive ion peak at the 1.
0 scale on the X-axis, and each data point represents a volatile compound whose intensity information is represented by color (i.
e.
, white represents a lower concentration of volatile compounds, red represents a higher concentration).

In order to better compare and analyze volatile compounds at different chewing stages, a differential comparison mode
is used.
Take 0% stage samples as a reference, the remaining 4 groups of samples deduct the reference in turn, if the content of volatile compounds in the sample is higher than the reference, the substance is displayed in red, if the volatile compounds in the sample are lower than the reference, the substance is displayed blue
.
The change of color of the data points in Figure 2b can directly see the concentration change of flavor substances at different chewing stages, mainly for compounds
concentrated in regions A and B between different chewing times.
In contrast, the color of some compounds in region A was darkened, and the color of compounds in region B became lighter, indicating that with the extension of chewing time, the compound content in region A increased and the compound content in region B decreased
.

3.
Fingerprint analysis of flavor substances in the process of oral processing of indica rice

As shown in Figure 3, the same row represents the signal peaks of volatile compounds of the food mass at the same chewing stage, and the same column represents the signal peaks
of the same volatile compound at different chewing stages.
Light to dark color indicates low to high
levels of volatile compounds.
There are obvious differences
in the flavor substances of indica rice at different chewing stages.
The contents of the substances in box A tend to decrease with the extension of chewing time, including polyhexanal, valeraldehyde, heptaldehyde, octanal, 2-pentylfuran, (E)-2-hexenal, 2,3-pentanedione and (E)-2-pentenal, mostly aldehydes and ketones
.
This result is cross-verified
with the electronic nose result.
Box B shows the changes in acetone, with increasing acetone concentrations as oral processing progresses, while 1-propanol and 2-propanol show a trend
of increasing first and then decreasing.
The ethanol (dimer) in box C is maintained at a relatively high concentration
throughout the chewing phase.
Some unknown flavor compounds require further identification
at a later stage.

The peak intensity values of volatile compounds in the food mass at different chewing stages of indica rice were used as parameter variables, and the five stages of the food mass were PCA
.
As shown in Figure 3b, the contribution rate of the three PCs (PC1, 47.
6%; PC2，26.
0%； PC3, 18.
6%) is 92.
2%, which reflects the overall characteristics of
the sample.
Unchewed indica rice and different chewing stage food masses can be distinguished
on PC1.
The chewing 25%~100% stage of the food mass showed a gradual change
on PC3.

4.
Qualitative analysis of flavor substances of indica rice during oral processing

The results are shown
in Table 2.
Some volatile compounds appear dimers and even polymers, which have similar retention times and different migration times
.
A total of 33 flavor substances were detected during the oral processing of indica rice, including 8 aldehydes, 5 alcohols, 5 ketones, 2 furans, 1 organic acid, sulfide, ester, amine and terpene
.
The flavor substances identified in this study were higher than those of 7 indica rice species such as Zhang Min
.
The relative content of aldehydes (valeraldehyde, hexanal, octanal and heptaldehyde), ketones (2-heptanone and 2,3-pentanedione), furan (2-pentylfuran) decreased significantly with increased chewing time (P<0.
05<b14>).
Similar results have been found in previous studies on flavor release during oral processing of white bread with significantly reduced levels of flavor substances after chewing
.

It can be seen from Figure 4A that compared with 0%, the relative content of aldehydes in 25%, 50%, 75% and 100% of the food mass decreased by 62.
01%, 47.
60%, 69.
19% and 71.
78%,
respectively.
This phenomenon of first decreasing and then increasing, mainly related to the secretion of saliva, preliminary studies have shown that rice will be secreted quickly after chewing, at this time saliva will wrap the food mass thus affecting the release of flavor substances, as chewing progresses, food is crushed and salivary enzymes act, a large number of flavor substances are released
.
In this experiment, a total of 8 aldehydes were identified, (E)-2-pentenal, (E)-2-hexenal, valeraldehyde, hexanal, octanal, heptaldehyde, acetaldehyde and propionaldehyde.

Among them, (E)-2-pentenal, (E)-2-hexenal and n-valeral (dimer) are only detected in the 0% stage, and 25%~100% are not detected in the stage, which may be because the content of these aldehydes is relatively low, and the rapid secretion of saliva when chewed in the mouth will cover, absorb and dilute the flavor compounds
.

The results of this study showed that the relative content of hexanal in 25%, 50%, 75% and 100% of the masses decreased by 45%, 25%, 54% and 57%, respectively, compared with 0% during oral processing.
The relative content of octanal decreased by 31%, 25%, 41% and 44%, respectively.
The relative content of heptaldehyde decreased by 55%, 23%, 55% and 59%,
respectively.

Alcohols are the second major flavor substances
identified in indica rice.
The odor threshold of alcohols is higher, and the contribution to the flavor of indica rice is less than that of aldehydes
.
Compared with 0%, the relative content of alcohols in 25%, 50%, 75% and 100% food masses increased by 59.
89%, 42.
59%, 29.
94% and 17.
73%, respectively
.
A total of 5 alcohols were identified: 3-furanmethanol, 1-propanol, 2-propanol, 1-butanol, and ethanol
.
Alcohol gives rice bananas a sweet taste and a slight alcohol aroma
.
These alcohols have only appeared in ethanol in previous studies on rice flavor and have low levels, and contribute less to rice flavor (odor activity value (OAV) <1), so ethanol in this study may mainly come from oral gases and have a positive effect on the flavor of rice, forming a mellow taste<b15>.

Ketones are the third major flavor substance
identified in indica rice.
Compared with 0%, the relative ketone content of 25%, 50%, 75% and 100% of the food mass decreased by 31.
92%, 17.
87%, 34.
92% and 38.
45%, respectively
.
As the key flavor substance of indica rice, ketones are the oxidative decomposition products of oleic acid and linoleic acid, which give rice a fruity flavor and a pleasant spicy flavor
.