Cold Chain Begins Before Processing Starts

The fresh-cut fruit and vegetable cold chain is all about quality.

As the food distribution system has developed over the years – with most of the food that Americans eat grown far away from where they live – maintaining cooler temperatures during transit has become important for shelf life and quality. Temperature fluctuation in trailers is now quite rare, so decreases in shelf life can often be attributed to handling procedures prior to packaging – and even prior to fresh-cut processing.

Cooling Down

Cold chain management really begins in the field – the sooner a fruit or vegetable gets into cold storage, the better.

“Everything needs to be cooled, so the question is, when?” said Jim Thompson, Extension specialist for the University of California, Davis.

The first step is to prevent produce from heating up and losing water immediately after harvest. Two simple measures that can reduce heat stress on fruits and vegetables are to provide shade, either under tarps or trees, and schedule frequent pickups so produce doesn’t sit in the field.

“In general, removing field heat is important in maximizing postharvest quality and shelf life,” said David Slaughter, a professor at the University of California, Davis, College of Biological and Agricultural Engineering.

Measuring the core temperature of harvested fruits or vegetables in the field will provide a baseline for cooling the produce. To effectively cool product, workers in the field have to continually monitor the pulp temperatures of the harvested product, Thompson said. Because every fruit or vegetable has a different freezing temperature, the time in and temperature of the storage facility is critical.

For initial cool-down, there are a variety of ways to lower the core temperature. The first is hydrocooling, a water-based process. Produce is either immersed in cold water or it is sprayed onto the product, with spraying the fastest and most common practice – although it requires plastic or wax-covered boxes. The size of the product determines how long it needs to stay in the water, with small products like carrots and asparagus requiring less than 10 minutes and produce larger than 3 inches in diameter requiring anywhere from 30 to 60 minutes. Mushrooms and berries are commonly cooled using water because the heat transfer cools them quickly and can add water to the produce.

Vacuum coolers are the most common application for produce that can lose water quickly, including cauliflower, lettuce and leafy greens. The vacuum coolers work by lowering the temperature that water boils at, creating an evaporative cooling process.

“Leafy vegetables in California are normally cooled using vacuum cooling, which is capable of cooling large quantities of lettuce fairly quickly,” Slaughter said.

The disadvantage to vacuum coolers is their tendency to cause water loss. For every 11º F the temperature is reduced, the product loses about 1 percent of its total moisture. For most produce, that amounts to a 2 percent to 4 percent moisture loss.

Vacuum coolers also use a large amount of energy and are expensive, but their ability to cool large volumes of produce – up to 10 pallets at a time – makes up for the added cost.

The third cool-down method is ice, but it’s used primarily for broccoli, which is extremely sensitive to moisture loss and temperature swings, Thompson said. Ice packing is very inefficient and expensive, so growers are looking for alternatives to cooling broccoli.

The final, and most applicable, cooling process is forced air cooling, Thompson said. With properly vented containers, forced air can effectively cool a wide range of produce, making it the most adaptable cooling process.

“Most everything can be exposed to cold air,” Thompson said.

It’s important to consider first the type of produce that is to be cooled when choosing a cooling method. If a wide range of produce is going through the cool-down process, then a forced air cooler may be the way to go, but if it’s primarily for leafy greens, then a vacuum cooler may be more efficient.

Because fresh-cut products are going to be more perishable after they’ve been processed, starting the cold chain earlier in production can extend the shelf life and improve quality.

“It’s very important to start the process for maximum life,” Thompson said.

Reducing the time between picking and the start of cooling is more important than the speed of cooling, he said.

“Even when you get a product half cooled, you’ve slowed the decay process considerably.” Thompson said. “You do a tremendous amount of good even by slow cooling.”

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