Chapter 56

Fishing for Freshness: Using NIR to analyze TVN to ensure freshness

📂 Workshop overview: The detectives head off the land for the first time to assist a client who wants to ensure that the salmon they sell remains as fresh as possible. Salmon from the ocean is generally caught, stored, and frozen before being sold at the market. The temperature and time of the storage can affect the composition, and the Total Volatile Nitrogen (TVN) amount will change as the meat begins to rot and the freshness decreases. The detectives need to find a way to analyze freshness to ensure the quality of the client’s produce.

The detectives are back together, discussing the fun time they had helping their last client determine the fat content of their ice cream using the Weibull-Stoldt method . Holmes congratulates Cornlumbo for his hard work solving the case as fast as possible and then brings up the details of the next case. “Who can tell me what Sockeye, Chinook, Chum, Pink, and Coho are?” asks Holmes. The detectives all look very confused. “Well, I know that the Chinookan people are indigenous to the Pacific Northwest in the United States,” says Nancy Beef. “Ah, well done, Nancy; the things I mentioned are also indigenous to the Pacific Northwest,” says Holmes. However, the detectives are none the wiser about what Holmes is talking about, so he has to tell them. “They are all species of Pacific Salmon and also the focus of our latest case,” says Holmes before explaining the details of the case to the detectives.

A salmon wholesaler is keen to provide the highest quality fish to high-end restaurants. To keep the wild fish as fresh as possible on its journey from the ocean to the kitchen, it must be cooled and stored appropriately. Recently, a batch of the client’s salmon had partly rotten, and luckily, the client noticed before delivering the salmon to the restaurant, potentially ruining their reputation. The next few batches received were fine, and then another rotten batch arrived. As far as the client was aware, nothing about the supply chain had changed, and they couldn’t determine why certain batches were not fresh. Keen to ensure it doesn’t happen again, the client has sought out the assistance of the detectives to optimize their operation so that they can charge a decent price and maintain their reputation as a provider of the highest quality salmon.

Holmes asks the detectives if they have any idea how the fisherman could determine the freshness of his salmon beyond merely looking at it and smelling it. Again, Nancy was the first to respond. “The client needs to determine the Total Volatile Nitrogen of the fish and assess the impact of storage temperature and time upon the freshness of the fish,” says Nancy. “Total Volatile Nitrogen? What’s that? We’ve done plenty of work before analyzing protein, fat, and other such things, but never Total Volatile Nitrogen,” asks Eggcule. Nancy explains that proteins and amino acids in the fish break down due to enzymatic activities and microbial actions, which leads to the formation of volatile nitrogen-containing compounds that give spoiled fish its unpleasant odor. Measuring the quantity of these nitrogenous compounds offers an indication of freshness beyond what is possible with the senses. “Fantastic work Nancy – and seeing as you appear to know the most about the Pacific Northwest and the details of the case, I’d like you to visit the client in person and see if you can work out what happened.”

“The client needs to determine the Total Volatile Nitrogen of the fish and assess the impact of storage temperature and time upon the freshness of the fish”

Nancy is delighted to have been chosen and immediately prepares for her adventure. On arriving at the client’s distribution center, Nancy is immediately impressed by the quality of the operation. The place where the fish arrives and is stored is immaculate, with polished stainless-steel surfaces everywhere. All refrigeration units have huge temperature gauges, continually monitoring the temperature. Looking around the place, Nancy cannot comprehend how anything but the freshest and finest produce could ever pass through such an establishment. The client is keen to hear Nancy’s thoughts on how to determine the freshness of their fish, and Nancy tells the client about Total Volatile Nitrogen (TVN) determination that can be used to pick up volatile nitrogen-containing compounds like ammonia and trimethylamine. “This sounds exactly like what we need to do, so how do we go about performing the determination?” asks the client. Nancy explains how to analyze TVN using the Kjeldahl method, but as soon as the client hears how long the analysis will take, they immediately begin to shake their heads “That won’t do; by the time we get the results of the analysis the fish will have been rotting in the meantime; therefore we still won’t have an indication of the freshness of the fish we are delivering to the client.” Thinking on her feet, Nancy immediately thinks of a solution that would give the client the speed of analysis they require. NIR spectroscopy. Nancy tells the client they could install NIR sensors at critical points in their facility and throughout the supply chain to optimize each process step.

“Proteins and amino acids in the fish break down due to enzymatic activities and microbial actions, which leads to the formation of volatile nitrogen-containing compounds that give spoiled fish its unpleasant odor”

By installing NIR , the facility would be able to analyze the TVN content rapidly and assess what parts of the process can be optimized. Not only that, but whole batches could be monitored instead of single spot samples as the process is fast and requires only very little sample preparation and other things could also be monitored to ensure quality, such as moisture, acidity, and protein content. The client is ecstatic and immediately sets about installing NIR sensors throughout the facility. They monitor the incoming goods, the stored fish, and, finally, before the fish leaves the facility to be delivered to the restaurants. Nancy oversees the installation and helps the client to calibrate the equipment. Finally, the day comes when the facility is ready to analyze the incoming goods. After a few days, Nancy begins to notice a pattern. The majority of the produce that arrives is very fresh with very low TVN determined; however, a batch with higher values arrives on certain days and at certain times. Nancy investigates further and realizes that the out-of-spec batches come from the same truck. It turned out that the refrigeration unit in the truck was cutting out, causing temperature oscillations that led to the fish rotting en route to the facility. The client was thrilled that Nancy was able to solve the case, and with her advice, they also managed to optimize the temperature and storage conditions and managed to reduce further the TVN content of all the fish they sold. To thank Nancy, the client took her to a two-Michelin-starred restaurant to sample the fish that she helped to keep so fresh. Nancy couldn’t believe her luck and enjoyed the greatest meal of her life – she also took plenty of photos as she knew it would make Cornlumbo jealous and Miss Mapple smile.