Direct vs. Indirect UHT- What’s the Difference?

At Triplex, not only do we get our hands dirty and help support customer’s day-to-day pump and valve needs, but we also pride ourselves on being able to contribute to our customer’s overall product creation. We recently sat in on an SPXFlow Tech Talk focused on Ultra High Temperature (UHT) processing. UHT is a commercial sterilization process that eliminates most micro-organisms that are likely to grow in a product like juice or milk, extending shelf and product stability. As we see our partners bringing a variety of new products- like alternative meat proteins- to market, UHT is becoming increasingly prevalent in our market.  In today’s post, we wanted to share a little bit of what we learned about UHT in Jeff Timbro’s tech talk, and review specifically the difference between direct and indirect UHT systems.

While we most commonly see HTST (high temperature, short time) pasteurization of products requiring thermal processing (heating of milk to 160 F to 167 F for 15-20 seconds) in the US, these products are not processed and packaged under aseptic conditions and still require refrigeration following treatment. HTST dairy products typically have a shelf life of 10 days to 2 weeks depending on the quality of the raw product.

Increasingly, and especially in alternative products, such as nut milks and non-meat-based proteins, and other interesting things that come out of fermenters, we’ve seen customers adopting ultra-high temperature technology (UHT). In UHT, we raise the temperature of the product to 275 F or higher for just fractions of a second to achieve commercial sterilization.

When we look at UHT processes, there are two primary methods of processing- direct and indirect processing. With indirect UHT processing, we have a barrier between the heating media (steam) and the product. Types of traditional indirect UHT processing include tubular heat exchange, plate heat exchange, and scrape surface heat exchange for thick products prone to fouling. Good applications for indirect processing include commodity milk products (plate), stabilized desserts (SSHEX), and coffee creamers (tubular).

A more recent innovation in thermal processing has been direct UHT. In direct processing, we are physically mixing the product with the heating media (steam). Within direct UHT, we have two subcategories of processing, infusion and injection. With infusion, we physically put the product into the steam. In an infusion system, we start by preheating the milk with a traditional PHE. From there, we go into what’s called the “infusion” vessel, where product rains down through the steam filled vessel very rapidly (just fractions of a second), raising the product temperature to temperatures between 275- 320 F. Another cool note about the infusion vessel- the lower section of the conical tank is actually water cooled to prevent burn on during processing, increasing run time.

From the infusion vessel, we go into a short hold tube and then into the “flash” vessel. The flash vessel serves two purposes- first, to cool the product almost as rapidly as we heated it up and also “flash off” the steam we picked up in the infusion vessel. The product coming out of the infusion vessel is slightly watered down, so by processing through the flash vessel under vacuum, we’re able to pull that water off while cooling the product. Good products for infusion applications include premium milks, lower quality raw milk, baby food, and nutritional drinks.

Injection UHT systems are similar to infusion systems in that steam in introduced directly to the product. But instead of passing product through steam, injection systems eliminate the infusion vessel and instead use a special nozzle to inject steam directly into the product. Injection UHT systems still use a flash vessel to remove the steam we used to heat the product. Injection systems are less expensive than infusion systems, but provide slightly less temperature control. Injection systems are also good fits for premium milks, nutritional drinks, and baby food.

So there is the quick overview of UHT and direct vs. indirect processing. As always, if you have any questions about UHT or any of your pasteurization equipment needs, contact a Triplex Sales Engineer today!

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