[0:01]Welcome to Food Science and Technology 3. I am Dr. Vusi Mshayisa, and in today's session, we will discuss and explore the factors that influence the fish freshness and how spoilage of fish can be managed through various preservation techniques. So let's dive in. From what we've discussed in class, you may already know that the shelf life of fish is highly dependent on the temperature. So what does this mean? It means if we have fish that we store at 16 degrees Celsius, that fish is going to last for 24 hours until spoilage before spoilage sets in. But if we take the same fish and we store it at 1 degree Celsius, that fish can last up to 14 days. So this shows how vital the cold storage is in extending the freshness or the shelf life of fish. So, fish that are held in the hold of a vessel on ice undergo various changes. And their freshness or shelf life depends on various factors. So you ask yourself, what are these factors? Well, the factors that affect the freshness of fish um will firstly be, let's say, the method of procurement. How is the fish um procured? Uh, it will also be how the fish is stored after procurement. Um, of course, you may think about it, the species of the fish will determine its freshness and shelf life. Its composition, what we have discussed in class, will also determine the shelf life. And even the time of the year, uh, between summer, winter, or autumn, all of those things. And so, each of these factors play a significant role in the quality of the fish that we are going to process and the quality of the fish that is going to be consumed by the consumer. So, before refrigeration was widely available even in the shipping boats, so people used to preserve fish by placing it on ice and uh they also preserved it through salting and drying. So some of these methods are still widely used today, especially in regions where refrigeration is limited or where certain type of fish products are mostly um um consumed. So, Now, I want us then to think about what are the factors or the three main categories of fish spoilage? So we can categorize fish spoilage into three factors, which is microbiological, chemical, and physiological. So, we are going to look at these in detail, but let me just give you a synopsis of what we mean. So in terms of microbial spoilage, you're going to discuss this with Dr. Kasa in micro three. So this is a type of spoilage, which occurs when the bacteria that is present in the fish begins to multiply, especially after the fish has been killed. So what these bacteria do is that they can grow even at low temperatures, because remember, fish are cold-blooded animals. So and then when they multiply, they will begin to break down the tissue, causing it to spoil. And then the other one is the chemical spoilage. So the chemical spoilage, of course, uh, so let's think about the composition. So it's going to um, more or less involved the reactions in the fish fat. So we know that there's some fat that is present in the fish, because fish is a highly unsaturated. It contains highly unsaturated fatty acids, which means it reacts quickly with oxygen leading to rancidity. So this is why now fish has a strong unpleasant order when it starts to spoil. So we're going to look at these in detail. So finally the third factor is the physiological spoilage, which occurs because of these biological changes in fish after it has been caught. So remember that these fishes, they are fish, they are caught in nets, and uh they struggle during the catch. So what then happens is that they end up using their glycogen reserves, which means there is going to be little lactic acid um to slow down the bacterial growth. So then this makes the fish more vulnerable to spoilage um and then it also happens much faster than land animals like cows, uh which have more lactic acid um after they have been slaughtered. So, the question that I want to pose to you to think about is, "What are the challenges you think the fishing industry faces when trying to maintain fish freshness from the point of catch to the consumer's table?" So now we turn our attention to the first factor, which is the microbiological spoilage. To begin, let's talk about the fish itself. Did you know that while fish is alive, its flesh is actually sterile? So this means the muscle tissue of the live fish is free from bacteria, making it clean and safe. However, fish carry bacteria on their surface, particularly on the slime um on their skin and in the digestive system. And what these bacteria do is that they do not pose any problem while the fish is alive. Things begin to change once the fish dies. So after the fish has been killed, so what happens is that these microorganisms or bacteria, they quickly spread and begin to break down the tissue. So this is where the spoilage starts. As these bacterias begin to multiply, they begin to propagate, you know, what they do, they end up decomposing the proteins and the fats. So when they do that, the byproducts of these decomposition, they therefore renders the fish unsafe for consumption if it is not handled properly. So now that you might think, okay, but if we can just store the fish in the fridge, um on the ships, uh, it's going to slow down the process. No, it's important to understand that fish by their nature, I've already explained, they are cold-blooded. So what this means is that the bacteria that lives and grow in them um are adaptive to low temperatures. So which means they can continue to grow and thrive under refrigeration. So this explains why the spoilage still occurs, albeit at a lower temperature or at a lower rate, even when the fish is kept cold. So then what should we do? So to truly prevent the fish from spoilage, we need to use effective preservation techniques, such as freezing or the application of chemicals. So freezing at very low temperatures is going to hold the bacterial growth almost completely, then extending the fish um shelf life significantly.
[8:44]So what we need to understand here is that the fish spoilage is mainly caused by bacteria. And even though refrigeration can slow down the process, it doesn't really completely stop it. So therefore, we need to keep the fish fresh for longer. If we want to do that, we need to use chemicals or preservation methods such as um freezing. So now we turn our attention to the chemical spoilage of um fish. But before we do that, let's first take a closer look at the chemistry of fish. So as we've seen in the slides on the composition, we said that fish contains phospholipids. And within these phospholipids, there is a compound which is called trimethylamine. So why is this compound important? It's something that we need to know. This compound, trimethylamine, is primarily responsible for the strong fishy smell that develops when the fish starts to spoil. So, what basically happens is that when the fish dies, rest in peace, the bacteria and the fish's own enzyme, they begin to break down these phospholipids. And when these phospholipids are broken down, they release this compound, which is trimethylamine. So, once this compound is released, we then now have the characteristic fishy order which begins to develop. So here's an interesting fact. So if you take fish just a fish just out of the water, it doesn't really smell fishy. However, by the time the fish products reach, for example, the shore or before they reach, um, the supermarkets, they then have a much stronger order.
[10:43]So this is a clear sign that the fish has started to deteriorate uh due to the release of this compound, which is trimethylamine. So as this trimethylamine is released, it begins to amalgamate, big weight. It begins to combine with other chemicals resulting from the breakdown of fish uh fat. And so these release or these reactions, they intensify the smell, making it more unpleasant as the time goes on. That's why you then have this rancid smell that is developed. So another important factor that we need to consider is the fact that fish is highly unsaturated. Hey, your chemistry comes into play now. Unsaturated, we need to think of double bonds and triple bonds. So this means it reacts quickly with oxygen, and then it is going to cause, um, the fat to spoil or to become rancid. So once we then have the rancidity, um, it only worsens the smell, but it also leads to undesirable um flavors. So from a chemical point of view, what I think is important for us to grasp, we then need to associate the fishy smell, um, with spoiled fish, which is caused by chemical reactions within the fat. And an important way to underline is that chemical compound, which is trimethylamine. Right. So, the question now that arises that we need to think about is, considering the rapid oxidation of unsaturated fish fat, what preservation methods would you prioritize to maintain the freshness and flavor of the fish products?
[12:53]So now let's turn our attention to the physiological factors that contribute to fish spoilage. And how do these differ from land animals such as cows? So when fish are caught, they typically struggle and fight. So these guys, they put up a fight. And in this process, they use their energy reserves, which are stored in their muscles as glycogen. So we know from the meat chapter that glycogen is a type of sugar. So as the fish continues this fight, trying to escape out of the net, bumping into each other, this glycogen is rapidly consumed. So after the fish dies, very little glycogen remains in the muscles. And because of this, there isn't much glycogen to be converted to into lactic acid.
[14:03]So lactic acid, remember, we spoke about it also in the chapter on meat. So, why is lactic acid important? Lactic acid is important because it plays a crucial role in preserving meat by slowing down the bacterial growth. So when we have got cows, sheep and pigs and whatnot, lactic acid is produced in higher quantities after death. And this then helps to keep the meat from spoiling quickly. So in the case of fish, this is different, it's a totally different ball game, because fish, unlike cows or pigs, which are rested before slaughter. You remember those ones, they need to be rested before the slaughter. And so during the resting period, what happens is that their glycogen levels are replenished. And so, when you replenish the glycogen levels, you allow for higher lactic acid production after death. And this is the lactic acid, uh, that is going to act as a chemical preservative, delaying the bacterial spoilage. So we know that with fish, uh, everything happens multaneously there. They are caught off guard, you know, they don't have the time to rest.
[15:34]You know, they are not being rested after they have been caught. And since they use up most of their glycogen during the catch, they are not going to have enough lactic acid to preserve their flesh. And so as a result, that's why fish are more, much more vulnerable to bacterial growth, and this is why they tend to spoil even faster than land animals. So now if we look at these three types of spoilage, we've spoken about the microbiological, we've spoken about the chemical and the physiological. There are some interlinks that are there. I hope you are catching that. So if we summarize now this section, we would say that the physiological differences between fish and then land animals plays a major role in the spoilage. Fish don't produce enough or much lactic acid after death, leaving them to be more susceptible to rapid bacterial growth. And this is one of the reasons why fish spoils faster and require immediate preservation methods. So it must be preserved immediately. And so now the question that arises is, knowing that the fish spoils faster due to lower lactic acid levels, how would you approach preserving freshly caught fish to minimize the spoilage? So this is a question that we need to ponder on.
[17:16]So, when it comes to preserving fish, the most common methods includes salting, drying, smoking, and canning. These are methods that we are all familiar with by now in the course. And so these methods, of course, they've been used for centuries to extend the shelf life of fish, making them more available, um, after they've been, um, caught. However, not all of these preservation methods are acceptable in every culture or society. For example, in South Africa, in the Western Cape, uh, and in the west coast of South Africa, dry snoek is a delicacy. So, while in other regions, uh, they may not favor this method of preservation. So in addition to the traditional methods, so we also have chemical methods or chemical preservation that can be used. And so this often includes the use of chemicals. So think about it, what kind of chemicals could be used to preserve uh fish? Sodium benzoate and sorbic acid is, that's correct. So these are going to help um prevent bacterial growth and also extend the shelf life of fish. So interestingly, in the United States, uh it was acceptable to use antibiotics like aureomycin in the ice during the transport of fish to processing plants. However, this practice is not used in South Africa. And also, this just highlights the differences in terms of the regulatory, um, environments between the countries. So another example that can be cited for chemical preservation of fish here is the use of sodium nitrate or sodium nitrite. We spoke about these, uh, in the chapter on meat. Uh, these are also permitted in some fish processing applications. And why? It's because these compounds are primarily used in cured and processed fish, uh, products. But an interesting application, which is coming more and more, which is a modern approach to fish preservation, is none other than gamma radiation. Yeah, gamma radiation. So, at pasteurization temperatures or levels, gamma radiation can extend the shelf life of fish, um, by two or three weeks, two to three weeks, making it a powerful tool in reducing spoilage and ensuring the food safety of these marine or fish products. So, let us summarize. There are several ways to preserve, uh, fish. Each has its own advantage and cultural acceptance. From traditional methods like smoking, salting, to modern techniques like chemical preservatives and gamma radiation, um, the right approach depends on the type of fish. The market and, of course, the regulatory, um, factors. Before we conclude, I'd like to leave you with a question. So if you think about it, considering the different preservation methods, you now as a food scientist, how would you choose the most appropriate method for a specific type of fish, considering both cultural and regulatory, um, factors? So, yeah, thank you very much for listening. Uh, let's see each other in the next video. Thank you.
