The L. vannamei shrimp used for this study were of between 60 and 80 shrimp/kg and harvested from three different farms in Selangor, Peninsular Malaysia. The three shrimp farms identified as Farms A, B and C because permission was not given to indicate the names of the farms, was no more than 5 km apart. A total of 30 kg of shrimp were used for the study (10 kg per batch × 3 farms). The harvest of shrimp was done by cast net, which was considered to provide a random sample. The shrimp with signs of visual defects or breakage were removed. It is considered the samples are representative of shrimp harvested in the region.

The schematic overview of the experimental program is presented in Figure 1. The design of this study was adapted from previous research [2] and modified slightly to investigate the physico-chemical differences of untreated Pacific white shrimp during iced storage. Immediately after harvest, the live shrimp were washed using clean flowing water and thereafter placed in sterile Reynolds® zipper (26.8 × 27.9 cm) polyethylene bags (Alcoa Products Inc., Richmond, VA, USA). Then, the shrimp were arranged uniformly in styrofoam boxes between layers of ice at a shrimp/ice ratio of 1:2 (w/w) and transported to the laboratory within approximately 2 h.

Upon arrival the ice-packed shrimp boxes were kept in a walk-in cold room (~4˚C) until required for analyses. Prior to all the analyses, the shrimp were beheaded, peeled and deveined by hand. Time zero was taken as the day of harvest. With the exception of the determination of fat content that began no more than 6 h after harvest, all other experimental analyses were carried out at intervals of no more than two days for up to 12 days. Per batch, approximately 200 g shrimp (that is, between 14 and 17 shrimp) were contained per polyethylene pack. For each day of laboratory analyses, two to three packs per batch were used from which the data of the present study was generated. Thereafter, the data representing each farm (Farm A, B and C) were pooled together (n = 3, where three represented the total number of farms sampled). All chemicals used in the present study were of analytical grade. Unless otherwise indicated, all analyses were repeated four times on different shrimp specimens.

Statistical analysis was applied using one-way analysis of variance (ANOVA) for all data.While the means and standard deviations (SD) were determined, differences between the means were resolved, by post-hoc, usingTukey’s Honestly Significant Difference (HSD). Level of significance was defined at P < 0.05 [2] [3] .

The feed fed to farm-raised shrimp could strongly influence the proximal composition. Usually, fish products are considered to be of higher quality between the end of the main feeding season and the initial stages of gonad development, as well as at their poorest condition just after spawning. In addition, the feeding season could influence their characteristic properties [24] . Table 1 presents the initial proximate composition, pH, colour, WRI, water activity and texture parameters of the Pacific white shrimp freshly harvested from different farms. Generally, the initial proximate composition of freshly harvested shrimp showed very minor differences across the farms. This initial proximate composition of Pacific white shrimp could therefore be compared with other similar shrimp species such as black tiger shrimp (P. monodon) and white shrimp (P. vannamei) previously reported by Sriket et al. [33] , Cao et al. [34] as well as Nirmal and Benjakul [13] . Shortly after harvest, there was no difference in initial moisture and protein contents, as well as a* colour value and water activity (a_w) of shrimp between the farms of this study (P > 0.05). Specifically, the initial moisture contents ranged between 76.379 ± 0.793 and 77.042 ± 0.949 g/kg whereas protein contents ranged between 18.041 ± 0.432 and 18.249 ± 0.432 g/kg across the farms. The results are comparable to moisture (77.21%) and protein contents (18.8%) of Thailand farm-raised Pacific white shrimp reported by Sriket et al. [33] . The Pacific white shrimp reported by Nirmal and Benjakul [13] farmed in Thailand had 13.4% protein which seems below the initial protein content of the present study. Also, the species and variety, state of nutrition, reproductive cycle of specimen as well as parts of organism are useful candidates capable of affecting the protein content of any given fishery product [35] .

Across the farms, the initial fat and ash contents, pH, the L*, b* and whiteness colour values, water retention index (WRI), as well as fracturability of fresh Pacific white shrimp showed considerable differences (P < 0.05). Specifically, the fat content ranged between 1.203 ± 0.001 and 1.239 ± 0.026 g/kg whereas the ash content

Different letters (a, b, c) in the same row indicate the significant differences (P < 0.05); Moisture, protein, fat and ash expressed on wet weight basis; WRI = Water Retention Index; Values presented as means ± standard deviation (Refer to Figure 1 for the number of replicates during respective analysis).

ranged between 1.783 ± 0.498 and 2.072 ± 0.051 g/kg. The fat and ash result were respectively below the ~1.30% of fat and above the ~1.47% of ash content values previously reported by Sriket et al. [33] . At the beginning of cold storage, fishery products of good condition usually show relatively low pH and this is understood to reflect the degree of freshness [36] . In the present study, the initial pH of fresh Pacific white shrimp across farms ranged between 6.723 ± 0.009 and 6.802 ± 0.018, which fell below the initial pH of 7 of untreated farmed Pacific white shrimp recently reported by Mu et al. [37] . The initial L*, b* and whiteness colour values in the present study ranged between 27.021 ± 4.118 (Farm A) and 33.961 ± 2.972 (Farm C), 2.589 ± 0.790 (Farm A) and 3.903 ± 1.389 (Farm C) as well as 27.201 ± 4.472 (Farm A) and 34.553 ± 2.102 (Farm C), respectively. Generally, the initial colour values obtained at the present study are comparable to the values of other shrimp species (harvested as wild and non-targeted species) previously reported by Eid et al. [38] . Across the farms, the water retention index (WRI) of shrimp ranged between 1.103 ± 0.047 and 1.544 ± 0.461 (P < 0.05). The result suggested that the WRI of shrimp might differ among similar species obtained from different harvest locations. Additionally, the fracturability ranged between 0.110 ± 0.034 and 0.301 ± 0.139 N.s and this might resemble the texture data reported by Tsironi et al. [39] where shrimp prior to iced storage showed some form of elastic and firm texture.

The pH of L. vannamei shrimp significantly increased during iced storage of 12 days (P < 0.05) (Figure 3) in agreement with the behaviour of pH of white shrimp previously reported by Nirmal and Benjakul [12] . Generally, pH of fresh fishery products range between 6.0 and 6.5 and often show increases during cold storage [40] but as the present study tends to suggest this occurrence may not be consistently so for all fishery products. In addition, the action of enzymes to produce ammonia and other amines could cause the pH to increase in shrimp during ice storage [41] . Nevertheless, microbial processes may also be involved with the accumulation of alkaline metabolites that could affect the pH [42] .

Seasonal changes, the feed fed to fishery products as well as degree/stage of maturity are believed to influence the fat content of fishery product [24] . In L. vannamei shrimp particularly within the first three days of iced storage of this study, the fat contents of showed no statistical difference (P > 0.05). Thereafter, fat gradually decreased up to day 9 (P < 0.05) after which it stayed unchanged (P > 0.05) (Figure 4). Some authors believe that ice contributes to the breakdown of fat [14] [18] . During the first 8 days of ice storage of the present study, the protein content of L. vannamei shrimp showed no statistical differences (P > 0.05). Thereafter, there appeared a

gradual decrease in protein content of shrimp (P < 0.05) (Figure 5). This gradual decrease in protein of shrimp here shown could be associated with the decrease in the available amino acids as reported by Fatima et al. [15] . It is also understood that the muscle of shrimp possess thick (myosin) and thin (actin) filaments linked by connective tissues (stroma). For this reason therefore, quantities of moisture might be held between the myofibrillar spaces. During iced storage however, protein contents might then aggregate and result in the formation of filament lattice up to the subsequent gradual shrinking of the collagen [19] . In addition, the break down of protein into trimethylamine oxide that reduces to trimethylamine may result in the emission of ammonia [25] .

Generally, moisture has been an important factor in food quality, preservation and could play a role with regards to resistance to deterioration of foods [43] . In the present study, differences in moisture content of L. vannamei shrimp were inconsequential throughout ice storage. The moisture content ranged between 75.13 ± 0.46 and 76.79 ± 0.98 g/kg (Figure 6(a)). The result can be compared with the moisture differences that were reported by Huidobro et al. [23] concerning iced-stored deep water pink shrimp that were processed on-board a ship. These authors indicated that the moisture content of shrimp from day 1 up to 4 ranged between either 76.10% and 77.10% or 77.10% and 77.50% when kept on flake and liquid ice, respectively. It was after 24 h as well as after day 4 of chilled storage, that minor increases in moisture content was found for the shrimp that were kept on liquid ice [23] . Other authors believed that the fish caught either during or after spawning could exhibit increases in moisture content, yet would be relatively low in lipid and protein contents, compared with those caught during the intense feeding periods [24] .

It has been argued that moisture content alone is not reliable and sufficient food quality indicator considering the differences occurring between associations of water with other food constituents. In addition, when tightly bound with other food constituents, water is less available for microbial growth and biochemical decomposition [43] . According to Richardson and Finley [44] , water activity is able to influence the oxidation of fresh foods particularly during storage. In addition, the water activity of any given food system is an important index worthy to consider particularly because of the resultant chemical effects during food processing. Although the differences in temperature may have a somewhat pronounced effect on the water activity of food products, it is very likely to be of little significance in most food storage situations. Other researchers reported that both aerobic and anaerobic degradation reactions are able to increase exponentially between low and intermediate moisture of food and model systems when the water activity range between ~0.1 and 0.8 [44] . Nonetheless, the water activity (a_w) might be an indicator of choice with regards to food perishability [43] . In the present study, no difference in a_w of L. vannamei shrimp occurred particularly during the first four days of iced storage (P > 0.05). Thereafter, a_w fluctuated significantly reaching a peak of 0.97 ± 0.01 at day 10 (P < 0.05) (Figure 6(b)). The high water activity recorded at this study might be attributable to the direct impact of ice in exerting its influence towards the shelf of the shrimp of this study. A high a_w is understood to potentially hinder the growth of microorganisms that could be harmful on shrimp products [17] .

During the first four days of ice storage, the water retention index (WRI) of L. vannamei shrimp fluctuated some- what widely but with an overall decreasing trend. Particularly from day 6 until the end of storage the WRI remained unchanged (P > 0.05) (Figure 6(c)). The results suggests a slight initial decrease in WRI of shrimp, which might depict that the amount of water the shrimp could hold tight enough might not be involved in syneresis. Further, the decrease in WRI might be associated with the overall reduction in protein reactive groups that are probably involved in the binding of water molecules. Generally, many physical properties such as colour, texture and firmness partly depend on the water holding capacity (WHC) of a given food material [20] .

Colour is a well-known attribute relevant for consumer evaluation of quality of seafood. To understand the colour variations that occur during the processing of seafood is therefore imperative [22] . In a recent study that assessed the quality and shelf life of untreated Pacific white shrimp stored on ice, the colour attributes with respect to both metric chroma and total colour differences were reported, which focused on a holistic quality description of colour and therefore did not consider the individual L*, a*, and b* colour parameters [2] . Metric chroma describes the correlate of saturation as well as measure of vividness of colour whereas the total colour differences

(TCD) indicate the magnitude of colour changes [2] [45] [46] . Notably, Schubring [47] was convinced that the action of freezing could contribute to influence the overall colour change in the brown shrimp. Given that researchers like Du and Sun [48] highlighted that colour could be among essential parameters that determine quality attributes of foods, to comprehend how iced storage would impact on the individual L*, a* and b* colour indices of untreated Pacific white shrimp is crucial and this was undertaken in the present work.

During iced storage, there were significant differences in lightness (L*) of L. vannamei shrimp (P < 0.05). Specifically from day 2 the lightness remained unchanged up to day 8 (P > 0.05) and would increase up to a peak (42.27 ± 4.39) at day 10 after which a decline is shown (P < 0.05) (Figure 7(a)). The increases as well as the peak of lightness of shrimp obtained at day 10 of the present investigation seem to be in agreement with the previous observations of Okpala et al. [2] that reported increases in metric chroma and total colour differences (TCD) of untreated Pacific white shrimp during iced storage. The authors also understood that the colour of shrimp would increase in both magnitude and vividness particularly during the later part of iced storage. Their observation however seems not to agree with the colour data reported by Huidobro et al. [23] with respect to pink shrimp (Parapenaeus longirostris) that were processed on-board under iced storage conditions. Through sensory evaluation, Huidobro et al. [23] recognised the maximum initial colour score to shrimp chilled with flake ice and recognised the lower score to that subjected to liquid ice, considering the much less characteristic colours amidst the more brightness colours. Furthermore, the sensory assessors detected a fair increase in whiteness of shrimp and suggested that it may have decreased the colour quality, coincidental with loss of pink colour of the measured shrimp sample at that study. The authors further suggested that, perchance the colour observations were to be holistically considered, it could contribute to depreciate the commercial value of shrimp product.

Concerning a* colour value, no significant difference with iced storage was observed (P > 0.05) (Figure 7(b)). Concerning yellowness (b*) colour value and particularly during the initial 6 days of storage, no significant difference was observed (P > 0.05). It was after day 6 that the b* colour significantly increased up to a peak (8.43 ± 1.31) by day 10 and thereafter, it decreased noticeably (P < 0.05) (Figure 7(c)). The significant increase in yellowness (b*) observed for the untreated shrimp with iced storage reflects a colur shift, from somewhat red to a more yellow appearance. Probably, this might have occurred due to fair breakdown of astaxanthin, which might attribute to partial up to high pressure exerted by presence of oxygen [49] -[51] .

Generally, the colour of shrimp has been associated with certain groups of carotenoids, such as astaxanthin, canthaxanthin, β-carotene and xanthophylls. Essentially, the astaxanthin is suggested to have much influence on the total carotenoids of shrimp [19] [52] . In the present study, the L*, a* and b* colour values had relatively large standard deviations, which might possibly suggest an uneven washing-out of colour during iced storage. Also worth considering at the present study is that the shrimp were not homogenised prior to the colour determinations. Other contributing factors that could influence the colour of shrimp have been reported, for example, the natural pigment variations with different parts of black tiger (Penaeus monodon) as well as white (Penaeus vannamei) shrimp meat was shown to influence the differences in L*, a* and b* colour scales with iced storage [53] .

Hardness refers to work done to achieve peak force at the first compression of force-time in a typical texture analysis profile [54] . Adhesiveness refers to the combination of both adhesive and cohesive force [55] . The texture of fish flesh could be influenced by a number of factors, such as the age, distribution of muscle lipids, fat content, feeding management, fish species, as well as handling stress prior slaughter [56] . In the present study, the ANOVA test indicated no noticeable difference in the adhesiveness (Figure 8(a)) and hardness (Figure 8(b)) textures of L. vannamei shrimp during iced storage (P > 0.05) even though both texture attributes revealed peak values particularly at day 11. The peak values might suggest the shrimp to be less marketable at later stages of iced storage―a possible and quick signal by retailers/sellers to check the physical condition of shrimp. While investigating thawed white shrimp, Diaz-Tenorio and colleagues suggested that the differences in protein structure of shrimp could be associated with the differences in hardness during processing, which might possibly contribute to affect and define other characteristic properties as well as the overall consumer acceptability [57] . However, in agreement with the present study, the deep water pink shrimp investigated by Huidobro et al. [23] that found differences in hardness and elasticity during iced storage, particularly suggested that ice might have the potential to affect the myofibrillar protein to bring about an immediate up to intense contractions of muscle of the shrimp meat.