A quality index method for squid Uroteuthis (Photololigo) chinensis (Gray, 1849) preserved on ice

Uroteuthis (Photololigo) chinensis L. is one of the most popular kind of seafood that accounts for about 20% of all cephalopod species caught in Vietnam. This study was aimed at developing a quality index method (QIM) scheme to assess the quality and freshness of Loliginid squids. The new method will be of benefit to consumers, fishers, seafood dealers, seafood industry businesses, and controlling entities. The QIM scheme for Loliginid squids was based on the changes registered for 10 sensory parameters, scoring from 0 to 28. The obtained equation for linear correlation with P-value < 0.05 during storage was Y = 1.083 Xtg + 2.866, with coefficient R2 = 0.99. When preserved on ice, the Loliginid squids proved to have a shelf life of 10–12 days. The QIM program and the quality index equation provided a user-friendly, quick, and efficient scientific-based tool that can specify the storage time and estimate the remaining shelf life for Loliginid squids. The scheme can be combined with other chemical quality parameters of freshness to form a full quality assessment program for Loliginid squids.


INTRODUCTION
Uroteuthis (Photololigo) chinensi L. belongs to Cephalopods, one of the most popular seafoods in the world. Currently, the annual yield of Uroteuthis (Photololigo) chinensi in Vietnam is estimated at 110 000 tons, accounting for over 20% of total squids caught in Vietnam [1]. Cephalopods have no bones, which means that an average Loliginid squid has from 80 to 85% of edible parts, whereas teleosts have 40-75%, crustaceans -from 40-45%, and cartilaginous fish -only 25% [2]. Loliginid squids are associated with a wide range of dried, frozen, chilled, canned, and ready-made products.
Nowadays, consumers pay special attention to high quality and healthy food [3]. To ensure seafood safety, producers have to maintain the product quality on each step of the whole supply chain, from fishing net to fork [4]. The phase between fishing and processing facilities is of high importance because seafood is typically stored on ice, and its quality deteriorates quickly. Therefore, it is necessary to find a quick and effective method to evaluate the seafood freshness quality during this phase.
When seafood undergoes protein degradation during storage, the sequence of changes can be detected by human senses. Sensory evaluation is an easy and effective method of seafood freshness assessment for its economic properties. Sensory evaluation is a scientific method to measure, analyze, and explain the sensory responses as perceived by the senses of sight, smell, taste, touch, and hearing [5]. There are many modern sensory assessment methods to evaluate seafood quality, e.g. the EU-scheme, the quality index method (QIM), the Torry scale, and the quantitative descriptive analysis method [6]. In Vietnam, the sensory properties of Loliginid squids are typically evaluated using the descriptive method and scoring according to the Vietnamese National Standards No. TCVN 3215-79 and TCVN 5652 -1992 [7][8][9]. There is a need to develop a compliance quality control to world standards to help the Vietnamese seafood standards reach the world-class level.
The QIM is a quick and reliable way to measure the freshness of cold storage seafood [10,11]. The QIM method is based on the prespecified meaning of the seafood parameters, e.g. skin, mucus, eyes, belly, smell, etc. Each parameter scores from 0 to 3. The quality index is the total score. When the quality index approaches 0, the seafood is considered fresh, while a higher quality index means some degree of quality deterioration [5]. The quality index increases linearly during ice storage time and is used for quality control in seafood processing. The QIM is developed and applied individually to every seafood species [10,12].
All squids demonstrate sensory changes during storage: they depend on the species, assessment method, fishing area, season, time of fishing, and storage conditions. There were several studies worldwide to evaluate the sensory changes of squids using QIM applied to individual kinds of squids, e.g. common cuttlefish (Sepia officinalis L.), southern shortfin squids (Illex coindetii L.), and common octopus (Octopus vulgaris L.) [13,14]. Still, there have been no similar studies on Loliginid squids (Uroteuthis (Photololigo) chinensi).
Therefore, consumers, fishers, seafood traders, seafood industry businesses, and controlling entities need a reliable tool to determine the quality and freshness of Loliginid squids. This study proposes a sensory evaluation program based on the QIM scheme for Loliginid squids preserved on ice.

STUDY OBJECTS AND METHODS
The research involved fresh Uroteuthis (Photololigo) chinensi L. obtained from Cat Ba fishing port, Hai Phong City, Vietnam, in August 2019.
The squids were layered on a plastic tray with holes and a stainless steel cap. The set was put in an insulated icebox with the squid/ice ratio of 1:2 (w/w) and transported to the Laboratory of Research Institute for Marine Fisheries. In the laboratory, the insulated perforated bottom icebox was kept in the refrigerator at 0-4ºC. Ice was added to the boxes as required. The samples were taken every day, from day 0, when the samples were at their freshest, to day 25, when they got spoilt.
The terms to describe the changes in the sensory parameters related to the texture, smell, and color came from direct observation of the samples, previous studies, and the Vietnamese National Standards No. TCVN 11182-2015 and TCVN 5652-1992 [7,13,14]. An expert committee selected the descriptive terms via discussion and agreement. The terms were short, clear, commonly used, and easy to understand. The members of the committee were chosen and trained according to TCVN 12388-2: 2018.
The establishing of the quality index method (QIM) scheme included three steps: Step 1 -Establishing the initial scheme. Three to five experts observed all the changes in each of the following quality parameters: skin, flesh, and eye color; flesh structure; belly and mouth area smell; eye and tentacle status; and surface and mouth mucus. After that, they developed the terms for the initial scheme. Each property received a score from 0 to 3; a lower score indicated a better quality.
Step 2 -QIM scheme and committee training. The samples were stored at 0-4°C and were evaluated daily during the 25-day period. The committee consisted of six experts. During this phase, the committee members were initially aware of the time of storage to be correlated with the changes of properties during storage. During further phases, the members received no information regarding the storage time of the sample until the results became accurate and reliable.
Step 3 -Applying the QIM scheme. Ten squid samples were evaluated using the QIM scheme established during step 2 (from Mo1 to Mo10). The correlation formula between storage time and quality index helped to estimate the storage time and the remaining shelf life. The estimation was then compared to reality.
The collected data were analyzed using the descriptive statistic method (average, standard deviation). ANOVA 1 factor (P-value < 0.05) made it possible to determine the difference of the factors in the experiments using the Statgraphic XV and MS Excel software. Each test was performed in triplicate and repeated three times with similar results.

RESULTS AND DISCUSSION
The aim of this step was to evaluate the changes in the sensory parameters of Uroteuthis (Photololigo) chinensi L. stored on ice. The parameters included color, texture, smell, etc. After that, the terms were selected to describe the changes to be used in the quality index method (QIM) scheme.
The color was assessed according to the state of the skin, meat, and eyes of the squids. Figs. 1-3 show the color changes in the skin, meat, and eyes of the squids.
The skin was translucent on day 0; translucent white, with black pigment spots on day 5; midly opalescent white, with pink spots, slightly reddish on day 15; and red, with blurry purple-red pigment spots on day 20 (Fig. 1). The meat was described as translucent white on day 0; mildly opalescent white on day 10; pearly white on day 15; and opalescent white, milky white on day 25 (Fig. 2). Changes in the eyes were as foolows: translucent on day 0; mildly opalescent on day 10; opalescent on day 20; and milky white with black fluid on day 25 (Fig. 3).
Smell: property changes and terms. The smell was assessed on the body and mouth area. The changes of the odour on the squid body were described as: seaweedy, fresh, or seafood smell (day 0); mildly fishy (day 10); strongly fishy, mildly sour (day 20); and heavily fishy, foul, spoiled (day 25). The changes of the smell on the mouth areas were seaweedy, fresh, seafood smell (day 0); seaweed, mildly fishy (day 5); strong fishy (day 10); strong fishy, mildly foul (day 15); and heavily fishy, foul, spoiled, respectively (day 25).
Texture and status: property changes and terms. The status or structure assessment was based on the structure of flesh, status of the eyes, texture of the mucus on the body, mouth, and condition of the tentacles.
The shape of the eyes was described as convex, round pupils (day 0); less convex, less round pupils (day 5); flat eyes, pupils not rounded (day 15); and eyes slightly concaved, pupil ruptured (day 20).
The textures of the mucus on the mouth area were transparent (day 0); transparent, stick (day 5); and little and yellowish (day 20).
The tentacles were changed as consistent, strong sucking, suckers intact (day 0); soft, could not suck when touched, suckers start to fall out (day 10); and flaccid, not sucking, suckers falling out (day 15).
QIM scheme for sensory evaluation of Loliginid squids. In this study, we established a QIM scheme for Loliginid squids. It included 10 parameters and recorded the sensory changes during storage time with a score from 0 to 28 (Table 1).
Changes in the Loliginid squid stored on ice. Table 2 features the changes in the sensory parameters of Loliginid squids that occurred during storage.
The Loliginid squids demonstrated obvious changes in the sensory quality during ice storage. It could be divided into four levels: days 0-5, days 5-10, days 10-15, and days 15-25. The levels were numbered from I to IV, respectively. At level I (days 0-5), the squids were raw and fresh, and there was no change in the texture, color, or smell. Most of the sensory parameters scored 0, and the quality index did not exceed 10. At level II (days 5-10), the sensory parameters started to change: the body color and the smell of body and mouth area scored from 0 to 1 with the quality index at about 10-15. At level III (days 10-15), there were apparent changes in the sensory parameters: the skin lost its shine, pinkish pigment spots appeared, the flesh became opalescent, and the smell increased. Most of the sensory parameters got 1-2 scores, and the quality index fluctuated from 15-20, showing that the squids started to deteriorate. At level IV (> 15 days), the sensory parameters changed: the whole body reddened, patchy dark pigment  appeared, the flesh became floppy, and the foul smell became strong. Most parameters scored 2-3, showing clear signs of spoilage. The deterioration became especially evident on days 20-25: the whole body became purple-red and started to emanate a strong foul smell, while the flesh was flaccid and viscous. Therefore, the shelf life for squids preserved on ice equaled 10-12 days. Figure 4 shows changes in the sensory parameters of Loliginid squids stored on ice. The quality index demonstrated a meaningful linearly increase over time with P < 0.05.
The recession equation was: Y = 1.083 X tg + 2.866 (R 2 = 0.9908). Table 3 sums up the results of determining storage time and remaining shelf life of squids using the equation (*) and real remaining time.

Verifying the QIM scheme for Loliginid squids.
In this study, we established a QIM scheme that fully described the sensory changes that occurred in Uroteuthis (Photololigo) chinensi according to their biochemical transformation during ice storage. The scheme involved such parameters as skin and pigment color, flesh and mucus texture, and body and mouth smell. Pink color often appears on the cephalopod bodies. Skin pigments deter Loliginid iorate easily as a result of oxidation and enzyme tyroprotease during the storage period, and the skin goes black, pink, or red [15][16][17]. The flesh texture softens mostly due to the endogenous protease [18,19].
We concluded that the shelf life of Loliginid squids was 10-12 days, which is equivalent to and slightly longer than that of other squid species in the previous QIM studies. For instance, raw cuttlefish and shortfin squids proved to have 9-10 days of shelf life, while that of common octopus was only 8-10 days [13][14][15]20].
The quality index equation introduced in this article correlated with the reports from previous authors that the quality index value increased linearly during ice storage in various kinds of seafood, e.g. fish, shrimps, and octopus [12,15,21,22]. The verifying result also showed that the real remaining shelf life was similar to the calculated one. Therefore, the developed equation proved to be a scientifically based tool that could be used to evaluate the freshness and quality of raw Loliginid squids.
The obtained QIM scheme and quality index equation set a new quality control standard for the Loliginid squid in Vietnam. As they are simple enough to be used on the international seafood market, they also contribute to the QIM schemes that can be used worldwide.

CONCLUSION
In this study, we successfully established a quality index method (QIM) scheme for sensory evaluation of Uroteuthis (Photololigo) chinensi L. stored on ice. The scheme included ten parameters, and the quality index value was 0-28. We also developed a linear correlation equation of quality index and storage time with R 2 = 0.99. The best shelf life for Loliginid squids stored on ice proved to be from 10 to 12 days. The QIM and quality index equation can be combined with other chemical quality parameters of freshness to form a full-quality assessment program for Loliginid squids. The program could provide a user-friendly, quick, and efficient scientific-based tool that could help customers, fishers, seafood traders, seafood industry businesses, and controlling entities to specify the storage time and estimate the remaining shelf life for Loliginid squids. Purple-pink body with pigment spots all over; foul smell; milky white, soft and viscous flesh; mildly concaved eyes with opalescent corneal, broken pupils; no mucus; flaccid tentacles with suckers completely fallen out.