How long do oysters stay fresh?

Ah, the age-old question: how long does something perishable last? We have all asked that at some point in our lives, especially in front of our fridge. Sometimes, it’s easy to figure out: a weird smell, visible mold – it’s probably time to throw it out. But what about perishable foods with less obvious signs, like oysters without “Best By” dates?

We have been conditioned to rely on dates to help us decide if something is still fresh or safe to eat, but what does “fresh” really mean for oysters? Have we assessed quality without a date bias? Our industry has become so focused on harvest dates and marketing that the oyster’s actual freshness is being overlooked.

From a food safety perspective, oysters stored at proper temperatures can be safe to eat for months. Oysters were historically stored in pits or cellars during winter and consumed during the winter months. “Pitting” or overwintering oysters in coolers and cellars is still a popular technique for growers to keep oysters safe from winter sea ice.

From a freshness perspective, we at Pangea Shellfish define oyster freshness as the following:

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A fresh oyster is alive, has ample liquor, and maintains its aroma and flavor from harvest.

Based on that definition, there are some signs when an oyster has gone bad:

  • The oyster is gaping open, which means it is weak or dead.

  • The oyster is dry, which means it is weak, injured or dying.

  • The oyster smells or tastes different from harvest.

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We have generally found oysters to maintain our definition of “freshness” for up to 14 days. Our observations, though, have been anecdotal, and we didn’t have concrete proof. So, for the month of May, we decided to put our assumptions to the test by shucking one oyster per day and tracking the changes over time.

A Month-Long Freshness Test

The Sample

For this test, we used a 100-count bag of Salten Rock Oysters from our Blish Point Oyster Farm in Barnstable, MA. We chose this oyster because we knew its seed to market process intimately. If something occurred during the test, we could potentially trace the issue back to the farm.

Procedure

We randomly selected 1 of 14 bags from Lot C-748. The lot was harvested on Saturday, April 27, picked up by our company truck, and received at our Boston facility at 4:40PM the same day. We stored the bag of oysters in a crate, on a shelf, and in our cooler for the entire test period. The cooler temperature averaged 41° F. The oysters received no special treatment. No special handling, no ice or special storage, and no wet storage.

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We evaluated one oyster per day. Before Bekah and I shucked each oyster, we recorded the oyster’s size in inches and weight in grams. Once open, we recorded its temp, captured a photo, and noted its liquor content and flavor. On the last day of the test, May 31, we shucked all the remaining oysters to see how they held up.

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Data & Results

Starting Bag Weight: 13.96 LB
Ending Bag Weight: 13.19 LB
Weight Change: -0.77 LB

Bag Yield: 96% (4 oysters dead or dry)
Size range: 3.0” - 4.0”
Average size: 3.6”

Figure 1: The following shows the weight of each oyster evaluated per day during the test.

one oyster weight was recorded each work day in May. 79 oyster weights were recorded on may 31.

one oyster weight was recorded each work day in May. 79 oyster weights were recorded on may 31.

Figure 2: The following shows the distribution of oyster weights shucked on May 31 (34 days post-harvest). The average weight of these oysters were 56.1 grams.

Most of the oyster weights ranged between 48 to 64 grams. Mean: 56.1 grams; Median: 55.2 grams; mode: 56.4 grams.

Most of the oyster weights ranged between 48 to 64 grams. Mean: 56.1 grams; Median: 55.2 grams; mode: 56.4 grams.

Figure 3: The following photos and notes were captured to assess meat, liquor, and flavor. Click on the photo for the oyster’s details.

Discussion of Results

Change in Overall Bag Weight

At the start of the test, the bag weighed 13.96 lb, and over the 34-day period, it lost 0.77 lb (349 g). If we average the weight loss across the bag, each oyster lost about 0.0077 lb (~3.5 g), about the weight of 2 playing cards. We expected this weight loss and it's within reason. Over time, moisture from the outer shells will evaporate and some oysters will weaken, losing some of its liquor in the process.

Change in Individual Oyster Weights

We hypothesized that the individual weight per oyster would decrease over time due to the expected moisture and liquor loss. What we found, however, was there was no direct correlation between time transpired and weight. In fact, oysters evaluated on May 31 (34 days post-harvest) ranged up to 80.9 grams, heavier than all oysters evaluated prior. Liquor was also visible in most of the oysters on the last day (see Figure 3).

This doesn’t mean that the oysters gained weight after harvest (that’d be highly improbable). Instead, this called out a flaw in our testing method. We were just as likely to draw 5 of the largest oysters or smallest oysters from the bag each week. Doing a more extensive test or having an oyster “control group” would make this more bulletproof. But hey, we’re oyster people, not scientists. One thing we can say is that the oysters we selected each day were of average weight compared to everything else in the bag (see Figure 1).

Change in Smell and Flavor

The oysters evaluated towards the start of our test easily met our definition of fresh: ample liquor, great meat fill, and flavor on point. The oysters continued to pass our standards for a few weeks with a couple exceptions (May 8 & 10). There were no significant changes until we reached May 17, 20 days post-harvest. From that point on, the smell and flavor started to turn. A couple of dead oysters started to make the bag stink. Oysters still had full meats and liquor, but they no longer tasted clean or pleasant. The funky lingering finish clearly was not representative of its merroir anymore.

Conclusions & Considerations

So what did we conclude or prove from this test?

1. Oysters are safe to eat even 30 days after its harvest date *if it has been handled and stored properly.

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Here is a photo of me eating one of the oysters on May 31, 34 days post-harvest. Happy to report I did not get sick and am alive and well to write this! (I also ate them during week 5 to assess flavor.) The caveat about handling and storage is super important to mention, though. The oysters were safe to eat because they were properly handled and stored at temp. This is a must to ensure safe consumption regardless of its harvest date. Mishandling is one of the greatest risks for foodborne illnesses caused by oysters, so please do your part.

2. An oyster can maintain its “freshness” or quality up to 14 days after harvest.**

From our test, oyster quality started to decline 20+ days after harvest. We generally tell customers oysters stay fresh up to 14 days, but our results showed the period of freshness may actually be longer. We like to err on the side of caution, so 14 days from harvest is probably a good rule of thumb to follow.

**This is a general conclusion and may not apply to all oysters. We recognize that different species of oysters have different shelf lives. Atlantic oysters (virginica) tend to keep better than Pacific oysters (gigas). Performing this test with Pacific oysters could have yielded a shorter freshness window. We used a farmed oyster versus a wild oyster. We used a Massachusetts oyster that feeds longer than a Canadian oyster. We acknowledge these differences in oyster characteristics can affect the outcome. This was also only one test done on a small scale. Perhaps we will repeat this again to compare our results, and maybe on another variety!

3. Freshness is not determined by dates, weights, or visual indicators. It’s all in the taste.

Before we embarked on this test, we thought we could assess quality by looking at an oyster and its numbers, i.e. its dates and weights. But what this experiment showed us is those characteristics can be deceiving. It doesn’t matter if an oyster is live, plump, and full of liquor. It doesn’t matter if an oyster was harvested 24 days ago. What matters most is if an oyster tastes good, and to determine that, you’re just going to have to eat it.

Winter Effects On Oyster Quality

From February to April, we tend to see quality issues on certain varieties of oysters. It’s something that happens each year, so we put together this FAQ to help address your winter concerns.

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What are oysters doing during winter?

When we’re asked to think of animals that hibernate, bears, bats and other small mammals usually come to mind. But did you know, oysters also “hibernate” during the winter? Contrary to children’s books and fairy tales, animals in hibernation are not sleeping. Hibernation is a form of dormancy in which animals conserve energy to survive harsh environmental conditions. Oysters go dormant during winter because water temperatures can stay below freezing for months. Evolution has also taught them that there is no food in the water when everything is iced over.

How do oysters survive their winter dormancy?

To prepare for winter dormancy, oysters feed like crazy during the fall to build up glycogen, their sugar stores — which is why they are so sweet in autumn. When the water temperatures drop to 40° F, it signals the oyster to stop metabolizing and go dormant. The oyster will barely pump during this time and survive on its glycogen to get through the winter. When the waters warm up and food is back in the water, the oysters will become active again and start feeding. They will pump, filter, and eat from spring to fall until water temperatures drop again, triggering another cycle of dormancy.

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What is winter kill?

Inevitably, there are oysters that will die during winter dormancy. This phenomenon is called winter kill. Oysters run out of glycogen to stay alive or they are too weak to withstand the harsh conditions. The worst part is, some of these oysters die remaining shut. This makes them difficult to detect even when harvesters go through them by hand.

What are some signs of winter kill?

Because some oysters die shut, they cannot be detected until they are shaken up or shucked open to reveal dry, shriveled, smelly meats. Dormant oysters are weaker and have difficulty healing themselves, so any chipping during harvest, culling, packing, or transit can lead to liquor loss.

Once we hit late February and into March, we are basically asking the oysters to do the most impossible journey possible. Out of the water, through the packing house and shipped across the [Canadian] border by refrigerated truck. If at any point in that journey, the oyster [attempts] to feed or gets jostled in a harsh manner that loosens the abductor muscle, the oyster will spill some of its precious liquor and there’s no opportunity to replace the liquor… The oyster will not likely open again until spring when he’s either bone dry from survival or is sufficiently convinced that the water temperatures are steady again.
— Jacob Dockendorff, PEI Producer

Why does winter kill affect certain varieties more than others?

If you are a Pangea Shellfish customer, you will notice that certain oyster varieties are unavailable from March to May like wild-harvested Malpeques. These are the months when winter kill is most apparent, and Canadian supply is a great example.

Canadian waters get colder earlier in the year than locations down south. This also means the oysters enter dormancy earlier than its southern brethren. If a Canadian oyster enters dormancy in early November, it may not start feeding again until May when waters warm. It basically needs to survive on its glycogen that whole time! It’s impossible to ask the oysters to be in tip-top shape from March through May when they have not eaten for six to seven months.

You will also see signs of winter kill in New England oysters as we approach March, but it’s not as noticeable because their dormancy period is shorter. New England oysters are likely feeding until the end of November and start feeding again in late March. They have to survive about four months compared to the six months Canadian oysters endure.

Average Water temperatures in Summerside, PEI.  Water temps drop to 40F by early november and do not warm back up until late May. Oysters are dormant that entire period.  Source: NOAA

Average Water temperatures in Summerside, PEI. Water temps drop to 40F by early november and do not warm back up until late May. Oysters are dormant that entire period. Source: NOAA

Average Water temperatures in Barnstable, MA.  New England water temps do not drop to 40F until late November / December, which allows the oysters to feed longer and remain in dormancy for a shorter amount of time compared to Canadian oysters.  Source: NOAA

Average Water temperatures in Barnstable, MA. New England water temps do not drop to 40F until late November / December, which allows the oysters to feed longer and remain in dormancy for a shorter amount of time compared to Canadian oysters. Source: NOAA

Which varieties are less susceptible to winter kill?

Winter kill is less of an issue for southern varieties because of warmer water conditions, but generally, farmed oysters have a better chance of survival than wild oysters. Growers make sure their oysters have the best access to food throughout the year and condition them to have stronger abductor muscles. Farms also have different wintering and culling methods to tackle winter kill and limit its effects on product quality. So, despite Canadian oysters being more susceptible, many of the cultured Canadian varieties look nice throughout the season.

When will quality improve?

When the oysters finally start pumping and feeding again, it will take a couple of weeks for their quality to improve as they replenish themselves. Timing will vary by area because water temperatures differ by location, but we will see most varieties in better shape by late May.

Is there anything I can do to limit the effects of winter kill?

Yes! Be gentle. The oysters are weak, so any tough handling will result in dry or dead oysters. Give them extra attention and protection, especially in transit.

If you encounter a smelly bag, make sure to go through the bag or box because it could just be one dead oyster responsible for the smell. Discard the dead and rinse the rest. The remaining live oysters are just fine!

If there are any issues, contact your supplier. Dead oysters can go unnoticed until shucked, so giving your supplier feedback can help them assess the situation.

Duxbury Bay, January 2019, Mike Cesarini.

Duxbury Bay, January 2019, Mike Cesarini.

Special thanks to our Canadian producers for contributing to this piece.

Have a winter quality question not listed here? Ask it in the comments section below.