Sous-Vide as a Technique for Preparing Healthy and High-Quality Vegetable and Seafood Products

Sous-vide is a technique of cooking foods in vacuum bags under strictly controlled temperature, offering improved taste, texture and nutritional values along with extended shelf life as compared to the traditional cooking methods. In addition to other constituents, vegetables and seafood represent important sources of phytochemicals. Thus, by applying sous-vide technology, preservation of such foods can be prolonged with almost full retention of native quality. In this way, sous-vide processing meets customers’ growing demand for the production of safer and healthier foods. Considering the industrial points of view, sous-vide technology has proven to be an adequate substitute for traditional cooking methods. Therefore, its application in various aspects of food production has been increasingly researched. Although sous-vide cooking of meats and vegetables is well explored, the challenges remain with seafoods due to the large differences in structure and quality of marine organisms. Cephalopods (e.g., squid, octopus, etc.) are of particular interest, as the changes of their muscular physical structure during processing have to be carefully considered. Based on all the above, this study summarizes the literature review on the recent sous-vide application on vegetable and seafood products in view of production of high-quality and safe foodstuffs.

Interestingly enough, sous-vide, in some cases with nutrient retention, even intensifies characteristic flavors, as it happens with rutabaga and turnip [27]. Additionally, other physical properties of vegetables, mainly texture and color, are greatly influenced by these treatments [28]. Regarding physicochemical properties, after the processing, pH and water activity remain almost the same as in raw vegetables, probably because they are prepared without additives. Even salt addition, as a taste enhancer, does not significantly modify water activity of the product [17]. However, dos Reis et al. [29] reported that pH of broccoli and cauliflower inflorescences diminished half a point after sous-vide processing (90 • C, 20 min) when compared to the fresh samples. This is probably due to some cellular ruptures in the walls that released inner acids.
Sous-vide treatment may reduce the weight loss of cooked vegetable products. Gonnella et al. [23] reported that asparagus spears' weight loss was 2.1% after sous-vide processing (80 • C, 15 min). Further, after microwaving (900 W, 2450 Hz, 1.5 min), weight reduction was 11.9% as a result of a more efficient moisture removal from the vegetable tissues [30].
Besides, it is well known that sous-vide treatment causes changes in the color of vegetables. For instance, L* value of asparagus spears decreased from 54.1 to 42.6, making them darker after the processing [23]. Furthermore, this value was even lower than with boiled asparagus (44.4). A reduction in hues parameter from 115.8 (raw) to 112.8 (sous-vide samples) was noticed due to conversion from Foods 2020, 9, 1537 4 of 28 green to olive-green h • . This was a consequence of chlorophyll transforming to pheophytin [31], which was also lower than the value obtained for asparagus after boiling (113.9). In the case of broccoli florets, a 60% increase in a* value was observed after sous-vide treatment (90 • C, 15 min), while boiling (100 • C, 3.5 min) reduced the value of this parameter by 36% [28]. Moreover, boiling did not modify h • parameter when compared to the raw florets and stems but sous-vide did reduce this value by 13% and 19%, respectively. dos Reis et al. [29] analyzed color parameters in organic cauliflower inflorescences after boiling (100 • C, 5 min), steaming (final temperature 95 • C), microwaving (800 W, 4 min) and sous-vide processing (90 • C, 20 min). In all cases, an L* parameter reduction was observed. As regards parameter a*, sous-vide cauliflower registered lower values compared to the other cooking methods.
Carrot color is attributed to the presence of carotenoid pigments and a* value correlates well with sensory acceptability. Patras et al. [32] reported that when compared to a fresh sample, carrot slices prepared via sous-vide (90 • C, 10 min) decreased 12% in this parameter, while for samples boiled in water (until core temperature was 70 • C), the reduction was 30%. Moreover, the authors also observed that the losses of color red continued during storage. They reported a decrease of 27% for sous-vide carrot disks and a reduction of 32% for boiled samples after 20 and 5 days of storage, respectively. Consequently, it was concluded that sous-vide carrots were superior in color as compared to water-immersed cooked samples.
Trejo Araya et al. [33] compared the appearance of sous-vide carrot sticks (90 • C, 5 min in contact with water) to raw samples. Intensity of orange, visual firmness, brightness, surface moisture and flexibility (judged using finger touch) were used as indices. Panelists assigned higher scores to sous-vide carrots in all categories except for visual firmness. Concerning perception of texture, the processed vegetables were classified as more fibrous than raw samples but not significantly different. Furthermore, release of higher levels of moisture were perceived in the mouth. For instance, when compared to raw carrots the crunchiness and chewing time of sous-vide samples decreased because of water release during mastication.
Relating to textural aspects of broccoli, sous-vide samples (90 • C, 15 min) registered less stem softening than boiled ones (100 • C, 3.5 min). Shear force values decreased by 49.0% for the former and 65.7% for the latter, as compared to the initial stem firmness (36.5 N). It was speculated that when lower temperature and vacuum packaging were applied, cell wall disruption was minimized, and stem firmness was less affected [28]. Similarly, dos Reis et al. [29] measured values of 72.6 N for fresh organic broccoli, 16.8 N for broccoli submitted to sous-vide processing (90 • C, 20 min) and 12.7 N for broccoli subjected to conventional boiling (100 • C, 5 min). In contrast, for cauliflower, the same authors reported that firmness was less affected by boiling (33.5 N) than by sous-vide (15.6 N). The lowest shear force exhibited by sous-vide inflorescences could be associated with 3.2% water losses measured for these samples, against an increment of moisture by 1.0% for boiled samples.
Regarding aroma, studies using broccoli florets, green beans and carrots cooked via sous-vide, authors stated that these samples retained more aromatic volatile components than boiled samples [20,34,35]. Moreover, Rinaldi et al. [36] reported that sous-vide Brussel sprouts and carrots had different volatile profiles as compared to the steamed samples. This could be attributed to reduced degradation due to the lower presence of oxygen because of the vacuuming.
In conclusion, findings generally agree that sous-vide vegetables retain more aroma and taste than conventional cooked samples. Certainly, preparation via sous-vide avoids leaking of hydrophilic components into boiling water and these substances are related to the perceived flavor by consumers. Finally, to the best of our knowledge, no data are available regarding the effects of sous-vide on fruits' quality. Even though fruits are usually eaten raw, some varieties can be cooked. For instance, apples and pears could be submitted to sous-vide treatment to potentiate flavor and promote consumer acceptance, hence in this sense more studies are required.

Changes in Nutrients and Phytochemicals of Sous-Vide Vegetables
There are very few available reports about the impact of sous-vide on vegetables on the bioactive compounds such as carotenoids, phenolic compounds, vitamin C and/or their antioxidant capacity. Chiavaro et al. [37] studied the changes in the phytochemical contents and antioxidant capacity of carrot slices and Brussel sprouts after sous-vide processing (100 • C, 20 min) and refrigerated storage at 4 • C for 1, 5 and 10 days, with a reheated final step for 20 min in a water bath at 60 • C. Authors compared the results of sous-vide processing/refrigerated/reheated products with the corresponding raw and oven-steamed products. In this regard, sous-vide carrots and Brussel sprouts at day 1 of storage at 4 • C showed higher contents of carotenoids in comparison to raw and steamed products. For instance, sous-vide carrots exhibited carotenoids content 1.8 and 1.1 times higher than raw carrots and those prepared by steaming. This increase was particularly evident for αand ß-carotenes, and it was probably due to the reheating of the sous-vide processing that may more efficiently release carotenes that normally reside in cellular crystals and are bound by the complexes with protein and/or residual membranes [37,38]. Moreover, carotenoids were released from both, carrots and Brussel sprouts, during storage beyond day 1. Here, a significant increase was observed in sous-vide samples as compared to the raw and steamed carrots (total carotenoid content in sous-vide carrots at day 10 of storage was 2.3 and 1.2 times higher than the total carotenoids in a raw and steamed carrots, respectively). Similarly, sous-vide carrot slices (90 • C, 10 min) preserved the total carotenoid content better than conventional cooking (samples were boiled in water until core temperature reached 70 • C) [32]. Carotenoids are lipophilic compounds are less prone to leakage during sous-vide processing, however they are still sensitive to oxidation. Thus, sous-vide vegetables are more protected against oxidative carotenoid degradation, not only during the cooking, but also throughout the refrigerated storage, as their contact with oxygen is limited due to vacuum-packaging.
Phenolic compounds seem to be better preserved in the vegetable products during sous-vide as compared to conventional cooking methods, e.g., boiling or steaming. Similar to carotenoids, this was probably due to limiting oxidation of these compounds under vacuum conditions [37]. In that sense, Martínez-Hernández et al. found a slight increase (lower than 1.4 times) in the phenolic compounds of kailan-hybrid broccoli cooked by sous-vide (90 • C, 15 min) as compared to the raw produce [28]. Moreover, Štěrbová et al. [39] reported a gain of 5% in the total phenolic content of Sacha inchi kernels cooked by sous-vide at 100 • C for 135 min. Additionally, Chiavaro et al. stated that sous-vide better preserved phenolics in carrots slices with slight increase in their content (of less than 5% as compared to the raw samples). This was particularly emphasized for some flavonoids (quercetin, kaempferol and luteolin) and hydroxycinnamic acid derivates, such as caffeic and ferulic acids [37]. On the other hand, Baardseth et al. [40] reported that sous-vide (100 • C, 15 min) of blanched/frozen green beans had no effect on the concentration of total phenolics. It was concluded that sous-vide or other types of cooking do not increase the content of phenolic compounds in vegetables, rather they facilitate the extraction by increasing the yield in the extracts (not in the vegetables).
However, the high water solubility of phenolic compounds and their thermal instability (of some of them) with the higher temperatures and the negative pressures of vacuum may cause, during sous-vide, forced lixiviation to the extra-cellular media and/or their possible thermal degradation. In this way, the phenolic compounds of inflorescences and co-products of several Brassica vegetables were significantly reduced with sous-vide (T = 80 • C, t = 15 min for inflorescences and leaves, and T = 80 • C, t = 90 min for stems) in relation to raw products [41,42]. Furthermore, Patras et al. [32] reported a decrease of 29.2% of the total phenolic content in carrot slices that were cooked by sous-vide. This was in addition to Chiavaro et al. [37] who found that sous-vide in Brussel sprouts decreased phenolics by 14% in comparison to the raw samples.
Vitamin C includes ascorbic acid and its oxidation product, dehydroascorbic acid [43]. It was believed that both of these compounds have several biological activities that include cancer-protective capacities in the body. The content of vitamin C in fruits and vegetables can be significantly reduced during processing and storage due to its solubility in water, thermal sensitivity and proneness to oxidation. Thus, it is expected that vegetables cooked by sous-vide may have decreased vitamin C content, but still at lower levels than conventional cooking. In a way, this effect is similar to other bioactive compounds, i.e., as the absence or the very low presence of oxygen in the pouches may mitigate ascorbic acid oxidation. So it is somewhat expected that thermal processing by sous-vide significantly reduced the vitamin C content of Brassica vegetables and co-products [41]. For instance, sous-vide of inflorescence and stem of Broccoli cv. 'Parthenon' experienced vitamin C degradation near to 84% and 67%, respectively. However, authors observed that the reduction was significantly higher after steaming when compared to sous-vide for some Brassica vegetables. This was probably due to the reduced amount of oxygen present when cooking by sous-vide [41,42]. On the other hand, good retention of vitamin C was observed for all sous-vide carrot slices with a significant slight reduction at the end of procedure (at day 10 of storage at 4 • C) of about 6%. Moreover, sous-vide of blanched/frozen green beans slightly decreased the levels of ascorbic acid, while conventional cooking by boiled water induced a loss of vitamin C close to 50% [40]. Therefore, sous-vide processing has potential to preserve vitamin C in vegetable foods. Probably, the percentage of retention depends on quality of vacuuming in the package and harshness of thermal treatment.
Lastly, some authors studied the effects of sous-vide on the antioxidant capacity content of several vegetable products. Sous-vide resulted in a significant reduction in the antioxidant potential trough the FRAP (Ferric Reducing Antioxidant Power) assay of all studied inflorescence and leaves of Brassica vegetables. Moreover, for some parts of some varieties, including the inflorescences of broccoli cv. 'Marathon' and broccoli cv. 'Parthenon', sous-vide resulted in losses that ranged from 40% to 50% of antioxidant potential as compared to the raw products. However, the antioxidant activity of the samples measured by the DPPH assay (2,2,1-diphenyl-1-picrylhydrazyl radical scavenging) increased after sous-vide. For instance, this increase in antioxidant activity was higher after the sous-vide processing of the inflorescences of broccoli cv. 'Marathon' (4 times higher than the raw product) and the stems and inflorescences of broccoli cv. 'Pastoret' (5 and 1.3 times higher than raw products). The increase in the antioxidant activity could be caused by the release of insoluble antioxidants or by the formation of new ones from temperature-dependent reactions, and/or due to water loss during processing [41,44]. It is possible that the differences between the antioxidant activities obtained using FRAP and DPPH might be due to the different principles on which these methods are based: acceptance of hydrogen atoms and electrons from antioxidants for DPPH and FRAP assays, respectively [45]. In agreement, Martínez-Hernández et al. [28] reported that cooking kailan-hybrid broccoli by sous-vide increased between 4.7-to 5.4-folds of the initial total antioxidant capacity as compared to uncooked samples. Moreover, sous-vide induced a great increase of the total antioxidant activity in carrot slices and Brussel sprouts by the great enhancement of carotenoids and flavonoids (only for carrots), as well as the marked retention of ascorbic acid shown by these samples [37].
In conclusion, sous-vide vegetables had higher amounts of bioactive compounds (e.g., carotenoids, phenolics and ascorbic acid) than conventionally cooked alternatives (e.g., boiled or steamed). In addition to this, sous-vide vegetables retained higher levels of antioxidant activity. Moreover, high temperature also induced modifications in the vegetable matrix than can be positive for the release of bioactive compounds. The sous-vide packaging provides protection from oxidation, and hence provides consumers a vegetable with richer phytochemical content. Lastly, further investigations are needed to evaluate if changes in the vegetable matrix are due to sous-vide processing, in other words, to clarify if this type of processing provides phytochemicals that are more easily released from the matrix, absorbed in the gastrointestinal tract (bioaccessibility), and available for physiological processes (bioavailability).

Microbiological Concerns of Sous-Vide Vegetable Products
The safety concerns of sous-vide products, particularly concerning spore-pathogen bacteria, needs to be carefully evaluated on a product-by-product basis. The maximum growth temperature for many pathogenic microorganisms growing on food products is between 42 and 49 • C, and some of them have been observed to grow slowly at temperatures between 50 and 55 • C. Therefore, the temperatures being used for sous-vide cooking might be close to, or overlap, the growth temperature ranges of foodborne pathogens [46].
The most often contaminants found in vegetables include E. coli, Enterobacter spp., Klebsiella spp., Salmonella typhi, Serratia spp., Providencia spp., Staphylococcus aureus and Pseudomonas aeruginosa, and other potentially pathogenic microbes [47]. In addition to these, some other types of vegetables are more susceptible to spoilage by other types of microorganisms, like Bacillus cereus, Campylobacter jejuni, Clostridium botulinum, E. coli O157: H7, L. monocytogenes, Salmonella spp., Shigella, Staphylococcus and Vibrio cholera [48]. Most of them are facultative anaerobes, which means that the cells are able to survive and grow in an environment with or without oxygen. Table 1 summarizes selected foodborne bacteria with possible growth and toxin production in temperature ranges below 34 • C; thus, the temperature range is shown to indicate the upper limits of temperature where growth has been observed to occur.
Slow heating of the product to the cook temperature may provoke a heat shock response by the microbes, making them more heat-tolerant to the cooking temperature. Therefore, good operating practice is oriented toward pre-heating the water bath to the appropriate cook temperature. This is especially important when the cook temperature is close to the upper growth temperature of a given microb as this may result in a decrease in the rate of its inactivation. At temperatures below 55 • C, spore-forming bacteria may survive and promote germination, resulting with an increase in bacterial cell number during the cooking, and consequently, increase the incidence of foodborne illness. To date, there is little to no scientific evidence to support any prediction model for foodborne pathogens' inactivation in vegetables and/or seafood at cooking temperatures at or below 55 • C, therefore, the safety of sous-vide cooked at temperatures below 55 • C cannot be assured. An alternative could be to add bio-preservatives (nisin and organic acids) additionally to apply non-thermal hurdles by using some innovative non-thermal technologies or to use time-temperature indicators in the packages for recording the storage history of a product [49,50].
A recent line of investigation [51] introduced the use of rosemary essential oil (REO) as a natural antibacterial and antifungal to process fresh-cut potatoes. Firstly, this essential oil would allow avoiding the use of synthetic preservatives due to the antimicrobial activity related to the presence of components such as 1,8-cineole, α-pinene, borneol, ver-benone, and camphor [52]. Secondly, the combination of REO with the characteristic aroma of vegetables could potentiate the final product flavor. The results showed that the synergistic use of REO and vacuum packaging, combined with refrigerated storage, controlled the growth of mesophilic bacteria and Enterobacteriaceae in minimally processed potatoes destined for cooking with the sous-vide method after 11 days of storage.
Sebastiá et al. [53] evaluated the microbiological quality of broccoli, courgette, potatoes and carrots processed via the sous-vide method (100 • C, 15-20 min, except for courgette, which was heated for 5 min), chilled below 3 • C and stored (0, 15 and 30 days). Broccoli had the highest aerobic plate counts in all the storage periods. Authors attributed these results to the inherent morphology of broccoli. Broccoli inflourescences have hydrophobic pockets, which develop isolated areas that are not reached in the washing process. Therefore, it is necessary to reduce the organic material present in the samples with chlorine and improve the disinfection stage [54][55][56]. Table 1. Temperature range (above 33 • C) of possible growth and toxin production for selected foodborne bacteria.

; Optimun for growth
Foods 2020, 9, x FOR PEER REVIEW Table 1. Temperature range (above 33 °C) of possible growth and toxin production for selected foodborne bacteria.
; T-toxin production in food Foods 2020, 9, 1537 9 of 28 Neither enterotoxigenic staphylococci nor staphylococcal enterotoxins were detected in any of the four sous-vide cook-chill preserved vegetables. Detection of both staphylococci and staphylococcal enterotoxins were done because of the several coagulase-negative staphylococci also detected as enterotoxins [57,58]. Moreover, the presence of coagulase-negative staphylococci as S. epidermidis, a normal skin commensal, in foods reflects poor hygiene. Both enterotoxigenic staphylococci and their toxins can be used as indicators to assess the risks of vegetable contamination by staphylococci [58,59]. Additionally, there are bacteria that grow at low temperatures and survive mild heat treatments, e.g., L. monocytogenes and E. coli [60,61]. However, neither of these species were detected in the studied vegetables. Authors asserted that the absence of these microorganisms in the samples is mainly due to the microbial quality of raw vegetables combined with strict temperature control during the process. This is in agreement with the Sous-Vide Advisory Committee (SVAC, 1991) [62] considerations that indicated that microbiological safety depends on thermal process intensity, cooling speed, the final temperature, temperature monitoring and time of refrigerated storage. Moreover, it is important to check the packaging integrity throughout the entire process to assure storage and safety [53].
Rinaldi et al. [63] carried out microbiological analyses (aerobic and anaerobic total plate counts, mesophilic lactic acid bacteria, yeasts and molds) of steamed and sous-vide carrots and Brussel sprouts (20 min under steam at 100 • C) after 1, 5 and 10 days of refrigerated storage. Both group of carrots showed microbiological counts that were always lower than 1-log colony-forming units per gram (CFU/g), and even after 10 days of storage at 4 • C. Therefore, both thermal treatments appeared efficiently to diminish the initial counts. Similarly, Sebastiá et al. [53] reported aerobic total plate counts of <1 log CFU/g for carrots submitted to the sous-vide process for up to 30 days under refrigerated storage. In the case of Brussel sprouts, both steamed and sous-vide samples showed a decrease of all microbiological counts. The values of aerobic total plate counts were 3.46 log CFU/g for steamed samples and 2.34-3.15 log CFU/g for sous-vide sprouts. Moreover, sous-vide Brussel sprouts exhibited all other microbiological counts (<1 log CFU/g) lower than the values observed for steamed samples for up to 10 days of refrigerated storage. Authors also associated these results with an appropriate time-temperature combination, particularly, controlled heating/chilling steps that reduced initial flora more efficiently than steaming [53].
The table below illustrates the recent literature review on the influence of sous-vide cooking on the quality parameters of vegetables (Table 2).  -Sous-vide appeared to be the most advantageous with regard to caffeic, p-coumaric and gallic acids' stability.
-A significant positive correlation was found between antioxidant activity and the total phenolic compounds in raw and thermally treated vegetables.
-Sous-vide method is considered as the optimal thermal technique for Brassica vegetables' processing with regard to phenolic compounds' preservation.   -With no effect of the type of processing (conventional vs. sous-vide cooking) and determination method (FRAP vs. DPPH), the antioxidative potential of two vegetables (kohlrabi and red pepper) increased when compared to raw vegetable samples.
-Sous-vide method resulted in higher antioxidative potential after processing for the case of kale, beetroot, red bell pepper, sweet potato, carrot, cauliflower and kohlrabi as compared to their raw samples.
-In comparison to the conventional cooking method, improved antioxidative potential after cooking using the sous-vide method was detected for red onion, shallot, broccoli, tomato, parsley root and cauliflower.
-When comparing the two types of cooking, the obtained results suggest that an increase in the antioxidant potential was higher for the sous-vide technique.

Sous-Vide Applications in Seafood Processing
The demand for natural and lightly processed convenient seafood products is constantly growing, therefore the efforts in the processing of seafood and fish that will ensure safe and high-quality products resulted in the development of alternative process technologies such minimal cooking techniques [68]. Marine organisms have been recognized as a valuable dietary source of high quality bioactive components such as long-chain omega-3 fatty acids (PUFAs), easily digestible proteins, non-protein nitrogen compounds, fiber, taurine, sterol, and pigments. They also contain unique components that are not present in terrestrial organisms [69]. However, because marine organisms muscle lipids are highly prone to oxidation due to their high content of polyunsaturated fatty acids, their sensory and nutritional quality can be rapidly destroyed without suitable handling and processing, leading to rancidity and development of off-flavors. The most common methods that are used for the vegetable processing (e.g. stewing, microwaving, roasting, boiling and steaming) [70] could be used in seafood processing too [71][72][73]. Nevertheless, cooking facilitates several undesirable physicochemical reactions, among which, lipid oxidation is with the most detrimental consequences on cell membranes and denaturation of heme-proteins [74]. During cooking of vegetables, meat or seafood, water-soluble nutrients e.g., vitamins, minerals are typically lost at higher temperatures through evaporation and as exudates leave the food. This includes bioactive compounds and antioxidants, which are essential for maintaining a healthy immune system [75]. Some water-soluble proteins also may be lost with the water during cooking process. Higher cooking temperatures lead to myofibrillar protein shrinkage while decreased binding force between proteins and water that results with the decreased water holding capacity (WHC) in the myofibrils [76]. Sous-vide cooking could better preserve the stability of a secondary structure of proteins as compared to classical cooking procedures. Accordingly, Wan et al. [76] concluded that sous-vide cooking could be used as healthy alternative as it was proved to be helpful in maintaining the quality of largemouth bass fillets. However, in order to preserve sensory and nutritional quality during thermal processing of marine organisms, careful observation of operating technological parameters is required.
It was established that both temperature and cooking time have an effect on lipid oxidation in seafood products [7]. Furthermore, higher temperatures induce various biochemical reactions, protein aggregations and conformations, which change the tissue gaps in fish muscles [76]. Głuchowski et al. [77] by using higher process parameters in the sous-vide cooking of Atlantic Salmon (Salmo salar) achieved a similar intensity of cooked fish odor and flavor without significant degradation in the texture. However, vacuum pretreatment can be utilized to isolate oxygen thus avoid biochemical reactions that require oxygen, but also to minimize the reduction of lipids damage during heating process [76].
Dominguez-Hernandez et al. [10] confirmed that low-temperature over long-time cooking (LTLT) of meat offers multiple advantages over traditional high temperature cooking [6], such as reduced heat degradation of proteins and lipids and lower loss of liquid nutrients [78]. In a line with this, sous-vide might be useful even for processing of marine organisms like cephalopod or fish and seafood maintaining their fresh-like characteristics as with the produce [79]. However, there is a scarcity of literature concerning the sous-vide applications in seafood processing.
Mouritsen and Styrbaek [80] recently reviewed the future perspective of novel trends in cephalopod processing with respect to gastronomy. In this case increased demand for slightly processed seafood with a prolonged shelf life led to the application of sous-vide in order to obtain high quality seafood products [81]. Nevertheless, sous-vide has shown some limitations in seafood processing. When considering vacuuming, its lower degree implies that the higher pressure is applied, and high pressure is not recommended for sous-vide processing of fish fillets as the texture of fish is very delicate and gentle, and high pressure might initiate unwanted damages of the tissue. Consequently, such type of food cannot be completely vacuumed and the residual pressure inside the package is typically around 100-120 mbar, that reflects the difficulties for sous-vide processing [16]. Non-desirable reactions during sous-vide cooking like degradation of vaccum-seal bag, might also take place. Seafood contamination can easily occur by migration of plastic derived compounds into product during sous-vide cooking [82].
Cooking and preparing squid and squid or octopus by sous-vide technology are one of the biggest challenges today, as both of them are not always of uniform quality [83]. Like the abalone and clam, squid and octopus must be cooked very slowly to prevent the muscle fibers from toughening. Moderate temperature during sous-vide cooking of cephalopods retains its structure. Cuttlefish is quite difficult to cook, because it gets tough or rubbery, therefore must be tenderized before cooking. Cooking with sous-vide technology is beneficial as the low temperatures prevent the meat from contracting and gives a soft, tender food while plastic foil and vacuum prevents the loss of aromatic compounds.
In order to preserve the quality and safety of seafood sous-vide products, combinations with other processing steps such as different packaging technologies are often required. There is a possibility for sous-vide cooked marine organisms to be stored at modified atmosphere packaging (MAP) to achieve prolonged shelf life combined with low storage temperatures [84]. Lightly processed salmon (Salmo salar L.) by sous-vide cooking (45 • C for 15 min, 55 • C for 18 min, 65 • C for 21 min) was stored under modified atmosphere (MA) (60% CO 2 balanced with N 2 ) and soluble gas stabilization (SGS) (100% CO 2 ) at 4 • C for up to 24 days [85]. Authors found that SGS significantly improved shelf life of processed salmon by prolonging the lag-phase and slowing the growth rate of naturally occurring and inoculated bacteria. This study confirms that sous-vide processing at lower temperatures could provide inhibition of the bacterial growth, while improving chemical quality compared to traditional processing.

Changes in Physicochemical Properties and Sensory Quality of Sous-Vide Seafood Products
Recently, sous-vide has been considered as a potential technique that could have an impact on the improvement of taste. In this regard, the intensification of umami taste by producing more glutamate as a result of tenderizing meat upon the sous-vide cooking at 54°C or 64°C during 0.5, 1, 1.5, 2, 3, 4, 6, and 8 h has been explored [86]. However, authors concluded that aside from texture analysis and amino acid composition, full chemical analysis with determination of free nucleotides combined with a sensory evaluation could be useful approach to support this hypothesis. Another research suggested that the preparation of fermented fish products, such as fish sauces, could be used to provide free glutamate and inosinate. Fresh fish such as mackerel, anchovies and sardines are a particularly good source of adenosine triphosphate (ATP) that can be enzymatically turned into inosinate, soon after the fish is captured, and then fermented to create large quantities of glutamate [87].
In contrast to fish, cephalopods can be completely vacuumed inside the package, prior to sous-vide thermal treatment, because, this organisms have strong muscular tissue and firmness. The firmness is probably associated with changes on connective tissue structure, which lies immediately below the skin and is reported to be resistant to processes of the autolysis [88,89]. Muscle firmness is closely related to collagen content in seafood thus the squids are especially difficult to cook. Most methods require either quick frying or poor and slow roasting which can still result in hardening of the hard chewing food. Mouritsen and Styrbaek [80] reported that the mantle from cephalopods, like Sepia officinalis and Loligo forbesii, need very short heating at low temperatures (50-60 • C) to become more tender and ensure succulent structure. Low temperatures in sous-vide cooking of squids may ensures a soft and melt-in-the-mouth texture. On the other side, increased temperatures during sous-vide cooking enhance protein denaturation and coagulation of sarcoplasmic proteins on the surface and significantly alter the color. Moreover, protein denaturation plays a major role in the toughening of the texture of muscle products [85].
Cooking methods were also shown to affect the pH values. For instance, the pH of the largemouth bass increased during sous-vide cooking [76] due to the formation of disulfide bonds during the cooking process [90]. Croptova et al. [7] found increased pH values for sous-vide cooked mackerel samples at 60 • C for 10 min, while lower pH values were recorded in sous-vide cooked samples at 75 • C and 90 • C for 10 min on the seventh day of chilled storage. Authors supported previous explanation of this trend by the generation of trimethylamine and total volatile base nitrogen from either microbial or endogenous enzymatic degradation [91]. In addition, cooking losses were found to be significantly affected by sous-vide operating conditions and chilled storage. Especially, higher temperatures of sous-vide cooking enhanced cooking losses in mackerel samples, consequently negatively influencing the product quality. On contrary, the length of chilled storage had a positive contribution to the retention of water in the fish, probably due to structural changes occurring in protein matrix of the fish muscles and connective tissues after cooking [92], including reabsorption of water released by unfolded myofibrillar proteins and its distribution between intracellular and extracellular compartments [93].
Color changes that may occur during cooking are mainly attributed to protein denaturation that is reflected in meat by a more white color during the cooking process [94]. Both instrumental color parameters, lightness (L*) and yellowness (b*), revealed a significant increase throughout chilled storage of sous-vide as compared to the color parameters of the raw mackerel fillets [7]. Increased temperature of sous-vide significantly affected increased L* value. This is possible due to higher denaturation and aggregation of sarcoplasmic and myofibrillar proteins that reflects in increased light scattering [95]. Wan et al. [76] observed significant increase of whiteness in sous-vide cooked largemouth bass samples as compared with the fresh samples. Moreover, in the same study, authors concluded that the texture attribute of elasticity was greater pronounced in the cooked samples than in the raw ones, but in other indicators (hardness, chewiness and resilience) results were opposite. The decreasing of hardness and chewiness in cooked fish may be attributed to the lipid oxidation, which can produce numerous products (mainly aldehydes) that might cause the myofibrillar protein cross-linkages, thus leading to structural changes in proteins.
Regarding textural changes occurred during sous-vide cooking, recent study demonstrated a significantly lower values of breaking strength during chilled storage in comparison to the fresh samples of mackerel fillets. Interestingly, the fillets' firmness increased in proportion with raised heating temperature (60°C vs. 90°C), that may be explained by the heat-induced hardening of the fish muscles after denaturation of myofibrillar and sarcoplasmic proteins. Water losses from the muscle tissue were more intensive at higher temperature (90 • C), so contributing to mackerel samples toughening [7].
Humaid et al. [81] investigated the use of high pressure processing (HPP) at 150 and 350 MPa for 5 and 10 min for sous-vide cooked lobster tails (Homarus americanus) at 65 • C/10 min. Results showed that the use of moderate pressures significantly influenced the texture and color of lobster tails, whereas processing time had a milder effect. HPP at 350 MPa significantly increased L* values of sous-vide cooked lobster tails, but without considerable impacts on the overall acceptability by a consumer panel. Another study of Humaid et al. [96] established the effectiveness of HPP in extending refrigerated shelf life of vacuum-packaged raw lobster tails while HPP pretreatment did not positively contribute to additional shelf life extension for sous-vide samples. Finally, the authors concluded that the use of HPP to vacuum-packaged lobsters for subsequent sous-vide cooking has a potential in the development of novel ready-to-eat functional seafood products able to be stable during refrigerated storage.

Changes in Nutrients and Phytochemicals of Sous-Vide Seafood Products
Next to the temperature/time regimes with sous-vide, the occurrence and the extent of the reactions related to changes of nutritive quality will depend on the heat transfer medium (liquid water or steam) and from exposure to oxygen (meat vacuum packed or not). Numerous reactions could occur during the cooking, affecting lipids and the volatile profile of seafood such as lipid hydrolysis, lipid oxidation and degradation of nitrogenous compounds (proteins, amino acids and trimethylamine N-oxide, TMAO) via Maillard reactions or other types of deteriorative reactions [82].
Protein denaturation is a major event in the cooking of meat or seafood, which is less pronounced in sous-vide. Hence food structure is somewhat better retained along with color and taste which can be very attractive for consumers [97]. Plastic foil prevents the loss of aromatic volatile compounds and water during the sous-vide, which contributes to juiciness and tenderness of meat and enhanced sensory attributes [16]. Roldán et al. [98] reported the formation of volatile compounds in the amino acid-involved reactions during sous-vide processing (60 • C for 6 and 24 h, 80 • C for 6 h) of lamb meat.
As suggested, the aromatic volatiles were associated with a specific stronger meaty flavor and roast notes, due to which fewer spices and less salt was required.
A healthy diet (lower salt diet) may help prevent certain long-term (chronic) diseases thus, it is of great interest to find the right balance between these different nutrients to achieve maximum health benefits. Djordjević et al. [99] revealed that fish as a food, in addition to the valuable content of proteins, minerals and vitamins, is particularly attractive to the consumers as it represents a very rich source of essential fatty acids, which play a role in the prevention of many human diseases [100].
Marine organisms have noticeable content of valued fatty acids and high temperature treatments may negatively affect the stability of these essential nutrients. Except for octopus, cephalopod is considered as a rich source of unsaturated fatty acids, in particular unsaturated omega-3 docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) [101]. Therefore, cephalopods comprise great source of lean low-fat protein food and knowledge about the physical structure of the cephalopod's muscles (squid, octopus and cuttlefish) can be a useful guide for gastronomy and nutrition.
A recent study evaluated the impact of mild culinary treatments such as boiling (100 • C, 10 min), steaming (100 • C, 10 min), and sous-vide cooking (85 • C, 20 min), on the lipid composition and volatile components of farmed and wild European sea bass (Dicentrarchus labrax) [82]. Obtained results showed that the mild oxidative conditions of the three performed cooking methods did not provoke the hydrolysis of sea bass triglycerides, phospholipids or retinyl esters. In addition, no significant lipid oxidation took place during cooking. However, a slight oxidation of unsaturated acyl groups during cooking has been noticed, yielding several volatile secondary oxidation compounds of low molecular weight. Steaming and sous-vide cooking, in contrast to boiling, resulted in a slight oxidation of unsaturated acyl groups, leading to the formation of alcohols, aldehydes, ketones, alkyl furans and acids, that consequently positively affected aromatic profile. Volatile profiles of cooked samples were also enriched by degradation of nitrogenated fish components (includes Maillard-type reactions) that was considered as acceptable from a sensory point of view.

Microbiological Concerns of Sous-Vide Seafood Product
Microbiological deterioration in perishable products such as seafood occurs rapidly due to neutral pH, high water activity, and nutritional composition. When observing seafood safety of sous-vide seafood products, pathogenic bacteria that must be considered are classified into three groups. Namely: (i) bacteria naturally present in the habitat of the consumed species, such as Vibrio spp., non-proteolytic Clostridium botulinum type B, E and T, Plesiomonas shigelloides and Aeromonas spp; (ii) bacteria present in the environment in general (Listeria monocytogenes, proteolytic Clostridium botulinum type A and B, Clostridium perfringens and Bacillus spp; and iii) bacteria which have their usual habitat in man or animals (Salmonella spp., Shigella spp., Escherichia coli, Campylobacter jejuni and Staphylococcus aureus) [102]. The presence of bacteria group (i) and (ii) in live or fresh raw fish is not a common safety concern because tissue concentrations are too low to produce disease. Heat is lethal to microorganisms, but each species has its own particular heat tolerance, and there are many factors affecting their thermal resistance. The process is dependent both on the exposure time and on temperature required to achieve the desired death rate. Therefore, it is essential to determine the thermal death kinetics (D and z-values) of target bacteria in different food substrates and to characterize the time durations to be applied at certain temperatures according to this data. The decimal reduction time (D value) is the time in minutes at a given temperature required to destroy 1 log cycle (90 %) of the target microorganism. The z-value reflects the degrees of temperature change necessary to change the D-value by a factor of 10 [103]. For example, time (minutes) sufficient to achieve a pathogenic load reduction of six orders of magnitude, or a 6D reduction (six logarithms, e.g., from 10 3 to 10 −3 ) of Listeria monocytogenes for meat, fish, or poultry in water baths from 60 • C to 66 • C based on 2 min of cooking at 70 • C with z = 7.5 • C is given in Table 3. However, the accumulation of large numbers of pathogens (Vibrio spp.) in filter-feeding shellfish represents a risk, especially since shellfish are commonly eaten raw. Pre-harvest contamination with pathogens from the group (iii) may present a risk since in some cases a very low infective dosage is sufficient to cause serious disease [102]. Predictive models for thermal inactivation and growth of microbes under sous-vide conditions were recently reviewed. Here some of the limitations of current modelling approaches were also observed, particularly for a longer processing at lower temperatures [8]. Though regular cooking procedures will eliminate the risks of contamination, advanced knowledge is required regarding their origin, biology, physiology, ecology, survival, growth and prevalence in seafood and related products, along with the epidemiology and symptomatology of the diseases with which they are associated [105].
Fish or seafood sous-vide processing requires lower cooking temperatures (50-75 • C) with cooking time for several hours or even days. Sous-vide cooking is considered relatively safe for fish and seafood due to the fact that the food is consumed immediately after preparation, usually with a delay no longer than two hours, during which it is stored at temperatures above 54.4 • C to prevent or slow down the reproduction of pathogenic bacteria [16]. The European Union guidelines recommend that the minimum heat treatment for sous-vide pasteurization should be equivalent to heating at 70 • C which should be reached throughout 2 minutes [15]. Picouet et al. [106] for sous-vide cooked salmon loins established 4.5 log CFU/g for TVC (Total viable aerobic count) and 3.0 log CFU/g for Enterobacteriaceae under at 40.7 ± 0.1 • C for 19 min of cooking conditions. Such a successful sous-vide cooking at a lower temperature was attributed to the influence of additional high pressure processing (210, 310 and 400 MPa for 5 min at 10 • C).
However, insufficient heat treatment is the major problem in sous-vide processes applied at low temperatures. Therefore, sous-vide cooking has been lately combined with the use of natural antioxidants to improve the efficiency of cooking process in terms of food safety during storage [107,108]. This is in compliance with the latest trends in the food industry where different processing procedures could be combined to promote the process efficiency [50]. Also, there is an increased awareness towards replacing the use of synthetic antioxidants in food processing by the addition of natural ones [109], accordingly sous-vide cooking supplemented by the addition of natural antioxidants could be perspective tool for foods preservation. Several studies have been shown this concept as a promising alternative in preserving quality of fish products. Alves et al. [110] among oregano, basil and rosemary extract confirmed oregano as the most effective for preservation of sous-vide cooked tilapia fillets. Though the addition of oregano essential oil (EO) lowered the pH of sous-vide cooked salmon, which may favor the microbial inactivation, Dogruyol et al. [108] found that L. monocytogenes was more rapidly inactivated due to antibacterial effect of different compounds present in EO, like carvacrol and thymol [108].
Limiting storage times is another way to control the growth of pathogens in sous-vide seafood. Table 4 provides the key findings regarding the impact of shelf life on the quality parameters of sous-vide seafood products. Here, storage for extended periods of time is not recommended unless the product is frozen [111]. Cooled sous-vide seafood products should be stored in cold holding units and maintained at an internal temperature of 3 • C or lower to prevent growth of anaerobic spore-forming pathogens such as non proteolytic Clostridium botulinum, Clostridium perfringens and Bacillus cereus. If the food has a pH > 4.6 or available water ≥0.92, cold holding times should be limited, whereas Baldwin [6] recommends that such sous-vide products can be kept for maximum of 30 days. Table 5 summarizes the recent literature review on the influence of sous-vide cooking on the quality parameters of seafood.     -Based on obtained results from kinetics analysis, the estimation of TPD under different processing conditions for each protein can be evaluated.
-Texture changes were more induced by actin denaturation than myosin denaturation, while myosin denaturation was mostly responsible for changes in color and appearance.

Conclusions
Sous-vide cooking is becoming increasingly popular as a convenient and reliable method to produce healthy and high-quality vegetable and seafood products in the home, food service environment or food industry. Lately, due to the lack of time available for the consumption and preparation of meals and foods, sous-vide is gaining in popularity among consumers as an advantageous approach over conventional thermal treatments. Sous-vide cooking employs much lower temperatures than traditional cooking, and therefore it is possible to obtain more nutritious food products with well-retained bioactive compounds, which has significance from the health perspective. Moreover, studies have shown that sous-vide cooking could provide foods with higher nutritive value and more pronounced color characteristics, texture properties and sensory attributes than corresponding raw untreated food. Currently, marine organisms are much more in demand than vegetables for sous-vide processing due to higher risk of spoilage during shelf life. Therefore, major concerns related to sous-vide processing involve the microbiological safety of the products. In order to improve food safety, recent research combines sous-vide technology with natural antioxidants or innovative (non)thermal technologies that resulted in beneficial effects for both quality and safety issues. In conclusion, sous-vide has a great potential for future applications and to achieve safe foods with improved sensory and nutritive characteristics.

Conflicts of Interest:
The authors declare no conflict of interest.