The combination of microbiological, biochemical, and quality index methods in quality evaluation of pacific white shrimps (Litopenaeus vannamei) preserved at 0°C

Life ScienceS | Agriculture
Vietnam Journal of Science,
Technology and Engineering 45June 2021 • Volume 63 number 2
Introduction 
Shrimp are an important source of seafood with 
considerable nutrition and economic value in many 
countries around the world [1]. In Vietnam, Pacific white 
shrimp (Litopenaeus vannamei) have achieved a high 
export turnover rate in the seafood industry in recent years. 
However, the method used to assess the quality of shrimp 
in particular, and seafood in general, is not consistent with 
the current methods in other parts of the world. The quality 
of post-harvest shrimp decreases with storage time due to 
the impact of three main factors, which include the activity 
of endogenous enzymes, microorganisms, and chemical 
reactions. These activities alter the sensory state, chemical 
composition, as well as the total amount of aerobic 
microorganisms in shrimp [2, 3]. Therefore, methods that 
assess sensory, chemical, microbiological, and physical 
qualities are established based on these impact factors [2]. 
The organoleptic quality assessment method is based on 
variations of sensory properties including the color, smell, 
taste, and texture. The intensity rating method is considered 
the most popular at present, followed by the Torry scheme, 
quantitative descriptive analysis (QDA), and QIM, to name 
a few. Among those, QIM is predicted by Hyldig as the 
potential method for quality assessment in the European 
Community. QIM was developed by the Tasmanian Food 
Research Unit in Australia [4] and is continually being 
developed. QIM possesses many advantages such as short 
training time, short evaluation time, and high reliability in 
assessing the freshness of seafood preserved in ice [5]. The 
difference between QIM and other methods is that QIM is 
built to evaluate the sensory changes of specific species. 
QIM focuses on the correlation between the quality index 
The combination of microbiological, biochemical, 
and quality index methods in quality evaluation of pacific 
white shrimps (Litopenaeus vannamei) preserved at 0°C
Nhat Tam Le 1*, Nguyen Que Anh Huynh1, Nhu Khue Doan1, Thuy Xuan Uyen Phan1, Ba Thanh Nguyen1, 
Sao Mai Dam1, Thi Van Thi Tran2, Le phuong Lien Nguyen3, Van Hai Chu4, Huynh Anh Vu Truong4 
1Institute of Biotechnology and Food Technology, Industrial University of Ho Chi Minh city, Vietnam 
2Hue University of Science, Hue University, Vietnam
3Department of Refrigeration and Livestock Product Technology, Szent István University, Budapest, Hungary
4Center of Analytical Services and Experimentation Ho Chi Minh city, Vietnam
Received 28 May 2020; accepted 21 August 2020
*Corresponding author: Email: lenhattam@iuh.edu.vn
Abstract:
In this study, the sensory, microbiological, and biochemical qualities were used to examine the quality of Pacific 
white shrimps (Litopenaeus vannamei) preserved at 0°C during a 10-day period. The sensory quality was evaluated 
by a quality index (QI) that resulted from a quality index method (QIM) scheme. Meanwhile, the total viable count 
(TVC) and K-value were used to assess the microbiological and biochemical qualities of the shrimp. On day 9, the 
results from TVC and QIM have shown that the shrimp showed signs of spoilage, corresponding to a log CFU/g of 
6.4 and a QI of 21.37, which is unacceptable to consumers. The QI increased linearly with storage days therefore the 
remaining shelf-life of the shrimp was estimated from a linear regression equation. In particular, this study found 
a linear relationship between QI, K-value, and hypoxanthine content. Furthermore, hypoxanthine itself could be 
considered as an independent quality index like the K-index. In conclusion, the quality of Pacific white shrimp was 
categorized into four different classes: excellent, good, acceptable, and moderately acceptable, based on its sensory 
and biochemical quality indicators.
Keywords: hypoxanthine, K-value, pacific white shrimp, QIM. 
Classification number: 3.1
DOI: 10.31276/VJSTE.63(2).45-51
Life ScienceS | Agriculture
Vietnam Journal of Science,
Technology and Engineering46 June 2021 • Volume 63 number 2
and the storage time expressed via a linear regression 
equation. Thus, QIM allows an estimation of the remaining 
shelf-life of seafood. So far, QIM-Eurofish has developed 
28 QIM schemes mainly for fish, shrimp, and squid species. 
However, the QIM scheme for Pacific white shrimp has 
not yet been developed [6]. In Vietnam, shrimp quality is 
assessed according to TCVN 3276-89 [7]. This standard is 
generally applied to all shrimp species and is not an intensity 
rating method, however, it is a semi-quantitative assessment 
that describes changes in the properties of shrimp. Therefore, 
quality results from TCVN 3276-89 do not resemble those 
obtained by the rating method used in importing countries. 
This leads to disputes over quality and financial losses for 
exporting enterprises. After shrimp die, the process of self-
decomposition takes place immediately, which is followed 
by the process of decomposition [8]. This autolysis is 
caused by endogenous enzymes in the shrimp and results 
in the release of nucleotides and its derivatives. In 1959, 
Saito and his colleagues introduced the “K-value”, which is 
calculated based on nucleotides and their derivatives. The 
“K-value” has been formulated as an index of freshness 
in seafood [9]. Unlike quality indicators such as TVB-N, 
TMA-N, or histamine that are suitable for assessing quality 
during the decomposition stage, the K-value tends to 
linearly correlate with storage time [10-15]. This allows 
researchers to develop a regression equation between the 
QI and K-value.
Therefore, this study aims to develop a complete 
quality assessment process including the analytical 
methods measuring the microbiological, biochemical, and 
sensory changes. The expected findings includ ... heme for Pacific 
white shrimp and proved that this scheme can be used to 
evaluate the quality and the classification of shrimp quality 
as shown in Table 2.
The change in K-value 
Shrimp undergo the autolysis process quickly after 
harvest. ATP is one of the first metabolites in fisheries after 
death. The changes in nucleotide and related compounds are 
presented in Fig. 1. From this figure, the changes of ATP, 
ADP, AMP, IMP, and inosine became quickly prone due to 
the endogenous enzymes possess in fish and shellfish. In 
contrast, the transformation from hypoxanthine to uric acid 
occurred slowly. Therefore, the amount of hypoxanthine 
increased with the storage time. Fig. 1 depicts the ATP content 
and its derivatives resulting from the ATP autolytic process. 
The ATP components and their derivatives are expressed 
in µM/g. The contents of ATP, ADP, AMP, IMP, HxR, and 
Hx measured on day 1 were 0.4, 1.07, 7.03, 0.4, 0.44, and 
0.59 µM/g, respectively. This proved that the metabolism 
from ATP to AMP occurred quickly at the beginning. The 
amount of ATP and ADP from the 3rd day onward had very 
low values in the remaining days. The amount of AMP 
decreased rapidly until day 4 (4.12 µM/g), meanwhile, the 
amount of IMP, Hx, and HxR increased gradually to 1.24, 
1.09 , and 0.84 µM/g, respectively, on day 4. However, there 
were differences in these components in the following days: 
the amount of AMP decreased steadily; IMP increased until 
day 6 then decreased slightly to day 10; HxR almost did 
not change; while Hx increased gradually over the storage 
time. Thus, among the components considered, the Hx 
value increased linearly with the storage time and the linear 
regression equation between Hx and the storage day was 
Hx=0.177×day+0.33 (R2=0.974). The K-value calculated 
from the equation increased with time of storage. The linear 
regression equation between the K-value and storage day 
was K-value=3.05×day+1.8 (R²=0.992) (Fig. 2). Studies 
[10-15, 31] also show that the K-value and hypoxanthine 
amount increase linearly with storage time. This suggests 
that it is possible to use the amount of hypoxanthine to 
assess the quality change of Pacific white shrimps preserved 
at 0°C using the linear regression procedure between the 
K-value and hypoxanthine presented above.
Fig. 1. Changes in nucleotide content and the composition of 
derivatives during 10-day storage. Adenosine triphosphate - 
ATP; Adenosine diphosphate - ADT; Adenosine monophosphate 
- AmP; Inosine monophosphate - ImP; Xanthine - Hxr; 
Hypoxanthine - Hx.
Fig. 2. Changes of K-value during 10-day storage.
Correlation between the quality indices and 
classification of shrimp quality
The results showed that the QI, K-value, and hypoxanthine 
correlated linearly with storage time. Therefore, these 
quality indicators correlate linearly with each other. The 
linear regression equation between the indicators are as 
follows: QI=0.79K-9.49 (R2=0.956); QI=13.54Hx-7.44 
(R2=0.979); Hx=0.06K-0.13 (R2=0.950). Table 4 presents 
the results of a combined quality classification of Pacific 
white shrimp between the QIM and chemical indicators 
including K-value and hypoxanthine. 
Table 4. Quality classification of Pacific white shrimps based on 
QI, K-value, and hypoxanthine.
Quality class QI K-value Hypoxanthine (Hx)
Excellent 0≤QI≤2.9 0≤K≤18.36 0≤Hx≤0.75
Good 2.9<QI≤8.87 18.36<K≤24.30 0.75<Hx≤1.09
Acceptable 8.87<QI≤14.12 24.30<K≤29.34 1.09<Hx≤1.35
Moderately acceptable 14.12<QI≤18.5 29.34<K≤ 34.11 1.35<Hx≤1.72
components in the following days: the amount of AMP decreased 
steadily; IMP increased until day 6 then decreased slightly to day 10; 
HxR almost did not change; while Hx increased gradually over the 
storage time. Thus, among the components considered, the Hx value 
increased linearly with the storage time and the linear regression 
equation between Hx and the storage day was Hx = 0.177×day + 0.33 
(R2 = 0.974). The K-value calculated from the equation increased 
with time of storage. The linear regression equation between the K-
value and storage day was K-value = 3.05×day + 1.8 (R² = 0.992) 
(Fig. 2). Studies [10-15, 31] also show that the K-value and 
hypoxanthine amount increase linearly with storage time. This 
suggests that it is possible to use the amount of hypoxanthine to 
assess the quality change of Pacific white shrimps preserved at 0°C 
using the linear regression procedure between the K-value and 
hypoxanthine presented above. 
Fig. 1. Changes in nucleotide content and the composition of derivatives 
during 10-day storage. (Adenosine triphosphate - ATP; Adenosine diphosphate - 
ADT; Adenosine monophosphate - AMP; Inosine monophosphate - IMP; 
Xanthine - HxR; Hypoxanthine - Hx). 
0
1
2
3
4
5
6
7
8
0 1 2 3 4 5 6 7 8 9 10
µ
m
ol
/g
 o
f 
m
us
cl
e 
Days 
ATP
ADP
AMP
IMP
HxR
Hx
Fig. 2. Changes of K-value during 10-day storage. 
 Correlation between the quality indices and classification of 
shrimp quality 
 The results showed that the QI, K-value, and hypoxanthine 
correlated linearly with storage time. Therefore, these quality 
indica ors correlate linearly with each other. The linear regression 
equation between the indicators are as follows: QI = 0.79K - 9.49 (R2 
= 0.956); QI = 13.54Hx - 7.44 (R2=0.979); Hx = 0.06K - 0.13 (R2 = 
0.950). Table 4 presents the results of a combined quality 
classification of Pacific white shrimp between the QIM and chemical 
indicators including K-valu and hypoxanthine. 
Table 4. Quality classification of white pacific shrimps based on QI, K-value, 
and hypoxanthine. 
Quality class QI K-value Hypoxanthine 
(Hx) 
Excellent 0≤QI≤2.9 0≤K≤18.36 0 ≤Hx≤ 0.75 
Good 2.9<QI≤8.87 18.36<K≤24.30 0.75<Hx≤1.09 
Acceptable 8.87<QI≤14.12 24.30<K≤29.34 1.09<Hx≤1.35 
Moderately 
Acceptable 
14.12<QI≤18.5 29.34<K≤ 34.11 1.35<Hx≤1.72 
Conclusions 
 A QIM scheme has been developed for Pacific white shrimp 
(Litopenaeus vannamei) stored at 0°C. The scheme employs the 
descriptive terms for quality changes in sensory attributes in 
accordance with the quality requirements required by QIM (from 0 to 
3). The quality of the shrimp was categorized into four classes, 
including Excellent, Good, Acceptable, and Moderately Acceptable, 
which are based on the QI score. The QIM scheme for Pacific white 
y=3.05x+1.77 
R²=0.992 
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9 10 11
K % 
Days
Life ScienceS | Agriculture
Vietnam Journal of Science,
Technology and Engineering 51June 2021 • Volume 63 number 2
Conclusions
A QIM scheme has been developed for Pacific white 
shrimp (Litopenaeus vannamei) stored at 0°C. The scheme 
employs the descriptive terms for quality changes in sensory 
attributes in accordance with the quality requirements 
required by QIM (from 0 to 3). The quality of the shrimp 
was categorized into four classes, including Excellent, 
Good, Acceptable, and Moderately Acceptable, which are 
based on the QI score. The QIM scheme for Pacific white 
shrimp was validated for its accuracy and efficiency. This 
scheme can be combined with the K-value or hypoxanthine 
value for the quality assessment of Pacific white shrimp. 
COMPETING INTERESTS
The authors declare that there is no conflict of interest 
regarding the publication of this article.
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