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Development of a Recirculating Seawater System to Cool Farmed Mussels from Semi-Open Water Held in Live-Wells during their Spawning Period
Final Report
Menu-Mer Ltée.
PIAAM-2010-Q06
Table of contents
- Abstract
- 1.0 Introduction
- 2.0 Methodology
- 3.0 Discussion
- 4.0 Recommendations
- 5.0 Acknowledgements
Abstract
The requirements of the fresh mussel market in Québec oblige conditioning plants to maintain a year-round mollusc production calendar. Consequently, keeping the supply stable throughout the year is a major issue in mussel marketing. The summer period, when it is impossible to keep mussels in conventional live wells and when their physiological condition (spawning) makes it harder to handle the mussels, is the most critical. The characteristics of blue mussels farmed deep (10 m) in semi-open water (Gaspé Peninsula) differ from those of mussels farmed in the Maritimes. In fact, mussels reared in semi-open water experience partial spawning from June to September, which makes it harder to handle them. The ultimate goal being to secure mussel supply during the summer time, this project aims to develop a system of live wells with an integrated seawater cooling mechanism that is suited to the characteristics of mussels farmed in semi-open water.
The system developed through this project is composed of a buffer tank, a refrigeration unit and a heat exchanger, which together serve to cool the seawater. The cold water exiting the tanks is pumped back through the heat exchanger, which recovers the energy before dumping the seawater back into the sea.
The system designed around a 20-HP cooling unit was able to maintain the water at a temperature that was 7°C cooler than the water in the sea at a flow rate of 120 l/min. and a cost of $0.004 to $0.008 per pound of stored raw mussels. The heat exchanger proved to be quite efficient, although efficiency can be limited by the presence of spat if there is no well-established cleaning process.
To determine the optimal wet storage conditions for mussels farmed in semi-open water, the mussels were kept in boxes under a range of different conditions. The following parameters were studied: raw mussel loads of 300 and 600 pounds, and presence versus absence of air in the water. When mussels are stored in tanks during pre-spawning and spawning periods, it is preferable to store a volume of 300 lbs of raw mussels per box and to not add air to the boxes. When mussels are stored in boxes after the spawning period, the volume of raw mussels can be closer to 600 lbs and it is preferable to add air.
1.0 Introduction
Menu-Mer Ltée was founded in spring 2007. This young company took over the activities of a plant that formerly produced salt cured fish, but which had already redirected its operations to new productions to overcome the shortage of groundfish. After the change in ownership, Menu-Mer Ltée continued to diversify while maintaining fish and seafood processing activities. We process primarily farmed blue mussels for the fresh and cooked meat markets. Our blue mussel operations expand by some 20 percent per year.
The requirements of the fresh mussel market in Québec oblige conditioning plants to maintain a year-round supply of mussels if they wish to sell large volumes and reach beyond the local market. Consequently, keeping the supply stable throughout the year is a major issue in mussel marketing. The critical periods are when the water thaws and freezes over, at which time farming sites are inaccessible, and the summer period when it is harder to handle the mussels because of their physiological condition (spawning). It is easy to overcoming constraints associated with the thaw and freeze periods by maintaining mussels in live wells since they tolerate this well if the water temperature is below 8°C. The use of live wells also makes it possible to condition the mussels, and most importantly, to have a stock of ready-to-ship mussels that meet clients’ requirements. This is a great advantage since it offers the best shelf life. In fact, when storing mussels in live wells, the mussels are kept in seawater until they are processed and shipped; in contrast, when not stored in live wells, the mussels can be shipped to market only for several days after being harvested. When selling live products, it is essential to optimise shelf life. To meet market requirements, live mussels must have a shelf life of at least 10 days. However, in the summer mussels are more likely to spawn if the water temperature fluctuates widely. Summer spawning causes stress that shortens shelf life. In our experience, it is impossible to keep mussels that were farmed in semi-open water in conventional live wells without altering their shelf life. So we have no choice but to obtain our mollusc supply on a daily basis.
In the Atlantic Provinces, plants can count on several mussel farmers that are located in different areas to obtain the supplies they need for their daily sales. This reduces the risk of a break in supply due to poor weather conditions, zone closure or mechanical breakdowns affecting producers.
The situation in Québec, however, is different, essentially because there are few mussel farming businesses and especially, because of the limited number of mussel harvesting areas. In fact, our plant obtains its supply from three different areas: Gaspé Bay located 30 km away, Chaleur Bay located 300 km away and the Magdalen Islands, which lie over 1,000 km away. At present, the plant received mussels from two producers per region. The Gaspé Bay harvest is subject to a management plan because of the bacteriological quality of the water and the presence of toxic algae, which make summer harvesting impossible. On the Magdalen Islands, summer harvesting is also difficult because mussels spawn massively during that time; also it is hard to ship small volumes of mussels given the distance and because the mussels cannot be kept in live wells. Consequently, only Chaleur Bay producers can supply the plant on an ongoing basis during the summer. However, mussel farming in that area is done in semi-open water and mussel harvesting at those sites is greatly influenced by weather conditions. It is not unusual for high winds to make it impossible to reach the sites for three to four days in a row. Also, because there are only a few producers, the risk of a break in supply due to mechanical problems is not insignificant.
In addition, since mussels were first processed in Rivière-au-Renard (2003), it has been observed that blue mussels farmed deep (10 m) and in semi-open water (Gaspé Peninsula) have characteristics that differ from those of mussels originating in the Maritimes. In fact, mussels inhabiting semi-open water experience partial spawning from June to October. This means we had to adapt our practices to take into account the fact that these mussels require considerable icing from the moment they are harvested at sea to slow their basic metabolism and make sure their quality remains optimal. Mussels from semi-open water also tend to gape (they open to access the oxygen present in the air) more than mussels from the Maritimes.
To stabilise supply for our markets during the summer (mid-June to mid-October), we believe it is essential to extend the period during which mussels are kept in live wells. To this end, we have developed a seawater cooling system to supply water to the live well and more importantly, we have developed a holding method that takes into account the particularities of mussels farmed in semi-open water.
1.1 Project objectives
The principal object of this project is to:
- Develop a live well system adapted to the characteristics of mussels farmed in semi-open water that features an integrated seawater cooling mechanism and can be used to store these mussels during the period from June to October.
The project also has specific objectives:
- Determine the optimal storage conditions needed to obtain a long shelf life for marketable mussels farmed in semi-open waters.
- Keep the energy costs associated with the cooling system to a minimum to make the operation profitable.
2.0 Methodology
This project took place on the Menu-Mer Ltée premises over a period of one year, from September 2010 to September 2011.
2.1 Description of live-well system
The company’s existing live well has a capacity of 175,000 pounds of raw mussels and is made of insulated boxes, an air supply system and a seawater supply system, with seawater being provided by the municipal industrial park. The water is pumped in from a spot 300 m from the shore and 10 m down in the Gulf of St. Lawrence. Inside the plant, the water used to keep the mussels in the live well flows back into the sea. Water temperature varies from about 15°C in the summer to 0°C in winter. The ideal temperature for maintaining mussels in the live well is from 0 to 5°C. To maintain these values throughout the season, the water temperature needs to be cooled by more than 10°C during the summer.
In a conventional live well, mussels are kept in insulated 900-litre boxes with holed, false bottoms; each box holds 600 pounds of live mussels and, to keep the mussels in optimal conditions, water is pumped through at a rate of 20 litres per minute per vertical stack of three boxes. Each box has its own air supply.
2.2 Description of seawater cooling system
The system is equipped with a 10,000 litre buffer tank that contains seawater which must first be cooled by a 20 HP cooler. To this end, a pump circulates water between the buffer tank and the cooler to lower the temperature of the water and then keep it at that temperature, taking into account that fresh seawater enters the buffer tank continuously. Since this buffer tank is made of plastic, it was wrapped in insulation to keep the water from being influenced by the environment.
The cooler is composed of a heat exchanger tube designed for saltwater use that is made of PVC, titanium and stainless steel, and a 20 HP cooling unit. The water that enters the heat exchanger passes through a network of smaller tubes carrying a refrigerating gas. The gas is refrigerated by the cooling unit.
Once cooled, the seawater is pumped from the buffer tank to a live-well section in the plant that has been isolated from the rest by valves. This section can hold 27 insulated boxes nine columns of 3 boxes stacked vertically.
Under conventional conditions, as described in the previous section, it would be possible to store 16,200 pounds of raw mussels, which corresponds to approximately the volume sold weekly during the summer. Starting from this basis, the system should cool at least 200 litres of water per minute.
2.2.1 Cold recovery system
To keep energy costs to a minimum and since the live well operates on a closed circuit, the already cooled water leaving the live wells is sent to a plate-type heat exchanger placed at the inlet of the buffer tank so it can cool the seawater entering the system. This type of exchanger has a recovery rate of about 60 to 65%. The water leaving each stack of three insulated boxes is recovered when it runs into a 5-inch diameter drain on floor. Each stack of 3 insulated boxes can be lifted using plastic palettes measuring 6 inches in height. The water recovered via the drain flows towards a buffer tank and is then pumped to the plate heat exchanger before being returned to the sea via the plant’s piping system.
2.3 Evaluation of the efficiency and costs associated with operating the cooling system – protocol 1.
We evaluated how efficient the system was at maintaining water temperature at 5°C at four different periods when the temperature of the seawater was around 8°C to 15°C. To measure the efficiency of the device, four thermographs were installed in the following places: upstream to and downstream from the heat exchanger, at the outlet of the buffer tank and at the outlet of the boxes. We measured water temperature at these four places on an ongoing basis. The system was tested for a while before the mussels were added until it stabilised.
The cost of using the system was evaluated by measuring how much electricity the various components used as well as how much seawater was used during these first four trials.
2.4 Storage of mussels in live wells – Protocol 2
Inspired by the literature and current practices, we chose the experimental parameters that would define optimal storage conditions: the volume of mussels per box (300/600 lbs), and the addition of air to the boxes (absence/presence). Experimental wet storage was set to last seven days.
Three hundred pounds (136 kg) and six hundred pounds (272 kg) of raw mussels were put into individual boxes. The boxes were placed in stacks three boxes high in the live well room. The boxes were supplied via a seawater intake (UV treated) at the top of the stack and a cascade system that let the water leaving the upper box flow down through the second and then the third boxes. Water flow was adjusted to around 20 l/min. The oxygenated boxes each had their own air supply with air entering those boxes via a perforated pipe installed in the false bottom. Air flow was adjusted at between 0.5 and 3 CFM/min.
Half way through the experiment, samples were taken to analyse mortality in the boxes and evaluate the out-of-water shelf life (SL) of the mussels. The mussels were cleaned and debearded by hand for the shelf-life study.
At the end of the experiment, the same analyses were conducted but this time, the shelf-life study was conducted using mussels that had been cleaned, sorted and mechanically debearded (the usual method used to process mussels for marketing).
The experiment was repeated four times, in June, August, September and October, these four periods representing different conditions in terms of mussel physiology and water temperature.
2.4.1 Monitoring of live-well storage conditions
Water flow, air flow and oxygen saturation in the water were all measured daily at the outlet of each box, at which time the presence of spat in the boxes was also verified. These data were recorded and analysed.
2.4.2 Meat yield and the presence of spat
To determine the impact that holding the mussels in live wells had on their tendency to spawn, each wet storage box was observed daily (to see whether orangish particles were present on the surface of the water and around the edges of the boxes). The mussels were also observed when at the time of cooking (to see whether there were orangish particles in the shells).
Meat yield was evaluated by checking the mussels when they arrived at the plant and during the shelf-life evaluation study.
Meat yield calculations were done on one-pound (454 g) samples of commercial size mussels measuring at least 50 mm in length. After being cleaned, the mussels were steamed for 15 to 20 minutes until the meat was firm. The mussel samples were weighed before and after being cooked. The meat was then separated from the shell and weighed separately, which allowed us to calculate meat yield using the following two methods:
North American yield:
(Cooked meat weight X 100) / (Cooked meat weight + shell weight)
European yield:
(Cooked meat weight X 100) / (Pro-cooking weight)
2.4.3 Evaluation of mortality in live-well storage boxes
Mortality in the boxes at mid-experiment and after a seven-day period of storage in the live wells was evaluated only in the commercial size mussels. Only the mussels without any shell breakage were taken into account. A single sample was taken from the surface of each box (bottom, middle and top boxes) at mid-experiment and at the end of the experiment. Dead mussels (whose shells remained open even after a physical shock) and live mussels were counted to calculate the proportion of dead mussels.
2.4.4 Evaluation of shelf life
In this report, the end of sample’s shelf life is determined as being the day when 5% mortality is reached in the sample.
To evaluate shelf life, two pounds of mussels were placed in individual net bags. The bags were well iced and placed in a basket stored inside a cold storage room (0-4˚C).
Shelf life was monitored on days four and ten (or the nearest day that it was possible to conduct the analysis) in two two-pound bags per evaluation day. The sample analysis consisted of counting the closed mussels (presumed to be alive), the gaping mussels (the ones that closed after being tapped), and the dead mussels (the ones that didn’t close after being tapped). The bags were also checked daily for evidence of spat.
2.4.5 Processing yield
To evaluate mussel mortality (including dying and gaping mussels) in another way at the end of each trial, the mussels were cleaned, declumped, debearded and mechanically sorted before being manually sorted and weighed. The weight of the finished product (commercial mussels), rejected mussel waste and small mussels were compiled and analysed separately for each of the treatment boxes. The waste produced by the declumping and debearding operations was not weighed.
3.0 Discussion
The cooling system as originally designed did not meet the objectives initially established for this study, lowering the water temperature by 10°C at a flow rate of 200 l/m. Instead, we obtained a difference of 7°C at a flow rate of 120 l/m. The supplier did not take into consideration in his calculations the time it took to initially fill the live wells, during which time the heat exchanger did not operate since there was not yet any water returning from the live wells; this explains in part the difference in results. The 20 HP cooling unit is probably also too small and to be able to reach the objectives, it would be better to have a 30-HP or even a 40-HP unit. Nevertheless, given its performance in this study, using a heat exchanger proved to be a good choice. However, the presence of spat can greatly limit its performance. Consequently, a procedure needs to be adopted to clean it or filter the water better to limit fouling of the exchanger plates by mussel spat.
Our results indicate that it is possible to store mussels wet between June and October at an affordable cost without reducing mussel quality while sometimes even extending their shelf life. However, it also appears that the optimal parameters for keeping mussels in wet storage will have to be adjusted to take into account the time of year.
For instance, as the spawning period approaches (trial 3/June-July), it seems that it would be better not to add air to the boxes. It appears that it would even be advantageous at that time to store the mussels wet using cooled seawater before shipping them to the fresh market. The shelf life of mussels harvested and sent directly to the fresh market is shorter than 3 days; in contrast, the mussels in the 300-pound, no-added-air lots had a shelf life that exceeded 10 days. In the case of the 300-pound, no-added-air lots, the results obtained after 3 and 7 days of wet storage were similar; in contrast, for all the other treatments, if appeared to be preferable to not exceed 3 days of wet storage.
A little later, beginning in late July, it appears to be more advantageous to add air to the boxes and load the boxes with 600 pounds of raw mussels per unit. It is, however, hard to explain why the largest volume of mussels generates the best results. In this case, it is also better to keep the mussels in wet storage for longer than 3 days.
During the trial in late September, no 600-pound lots were treated. The results for the 300-pound lots, with or without added air, were similar although it would appear to be preferable to not extend wet storage beyond 8 days. This was, however, more obvious in the evaluation conducted after a shelf life of 3 days.
The analysis of the shelf life of the mussels held in the lower boxes produced results that are often lower those obtained for the other boxes. Although this is not a general phenomenon, it would be prudent to keep a close eye on this. We could always stack the boxes only two high.
As for the trial that took place near the spawning period, our results suggest that the presence of air in the boxes could be a factor that stimulates spawning in mussels. This was not as obvious during the next trial, but it is important to remember that many of the mussels had already spawned by then.
It is interesting to see that the mussel mortality observed corresponded to the results obtained for the shelf life of these mussels. For instance, during the period close to spawning, mortality in the mussels stored in the wet storage bids was similar for all treatments after 3 days of wet storage but lower in the case of the 300-pound, no-added-air lot at the end of the 7-day wet storage period.
After spawning, mortality in the mussels in the wet storage boxes was similar for all the treatments after 3 days of wet storage. In contrast, it was the 600-pound, added-air treatment that fostered low mortality in the boxes after six days of wet storage.
The oxygenation conditions meet the needs of mussels in wet storage. The mortality observed in the boxes and the shelf life results are consequently not attributable to oxygenation conditions in the seawater in the boxes.
Based on our results, we cannot determine whether one treatment is more effective than the others in terms of the yield available for processing.
Finally, wet storage for at least three days in cold water that meets certain parameters tends to improve the shelf life of mussels on markets. This effect is similar to that produced by the “hydro-cooling” operations used by some plants in the Maritimes and in Europe. It is now recognised that maintaining the cold chain during the mussel marketing process is a key element essential to their survival out of the water. By slowing the metabolism of mussels before they are marketed, wet storage in cold water serves to initiate the cold chain essential for mussel conservation as soon as they enter the plant.
4.0 Recommendations
- When storing mussels in boxes during the pre-spawning and spawning period, it is preferable to store the mussels in 300-pound lots without adding air to the boxes;
- When storing mussels in boxes during the period following spawning, the volume of raw mussels per lot can be closer to 600 pounds and it is preferable to add air to the boxes;
- Fill the boxes slowly (at half the usual flow rate) when the mussels are first put into storage and once certain that the water entering the boxes is cooled water coming from the heat exchanger, adjust the flow rates to 20 l/m per stack;
- An effective cleaning process must be established to make sure the system runs efficiently at all times.
5.0 Acknowledgements
The authors would like to thank their funding partners: the Department of Fisheries and Oceans for financial support provided via the Aquaculture Innovation and Market Access Program (AIMAP) and the Société de développement de l’industrie maricole (SODIM). Thanks also go to Sophie Gauthier-Clerc and Robert Vaillancourt for their contributions as we were designing the experimental protocol. Finally, the authors would like to thank the staff at Menu-Mer Ltée for their collaboration as the equipment was being installed and the protocol being carried out.
