Screw Conveyor Blancher – Vegetables, Blanching, Water-based
Industry: Fruit and Vegetable Preserving and Specialty Food Mfg (NAICS 3114)
Process Brief: Fruits and vegetables are scalded or briefly boiled prior to canning or freezing.
Energy source: Hot water
Energy Intensity: 300-400 Btu/lb of fruit or vegetable
Blanching is the rapid heating of fruits or vegetables to stop the enzyme action that causes deterioration of flavor, texture, and nutrients. Screw conveyor blanchers are long vessels with rounded bottoms in which the screw rotates, moving the product from the entry to exit in a continuous process. The blancher is usually made of stainless steel and the screw is rotated by a small (<5 hp) electric motor. A typical screw conveyor blancher is 20 feet long, four feet high and four feet wide. The product is added to water in a holding tank before entering the blancher and a sieve at the exit separates the product from the process water. Steam is injected into the bottom of the blancher under the screw conveyor. A manifold exhausts uncondensed steam. Water-based systems are commonly used for blanching fruits and vegetables and typical efficiencies are 30-40%. Screw conveyor blanchers are somewhat less efficient than tubular blanchers because less steam condenses in the process water and the excess steam is vented.
From the Key Technology web site:
Key’s Turbo-Flo® has an efficient product-to-steam ratio – about 7 to 9 pounds of product per pound of steam in most applications – that reduces cook/blanch/pasteurization times by up to 33 percent. Turbo-Flo as a blancher/cooker improves nutrient retention, taste, and appearance, and increases yields by 2 to 5 percent over conventional water blanchers. And Turbo-Flo takes up only about 60 percent as much space. Turbo-Flo as a pasteurization device retains the fresh quality of the product.
Hydrostatic Water Seals
Steam blanching with hydrostatic water seals takes the basic steam blancher and incorporates water seals at the ends of the unit. The conveyor dunks the product underwater as it passes through the seals, although the conveyor is not underwater in the main section of the blancher. The water seals condensed process steam and overflow water from the seals was cleaned and recycled.
A typical system used to blanch spinach incorporates a hood that is about 84 feet long, 5 feet wide and 3 feet high, which was somewhat longer than a blancher without end seals. A study of this type of blancher showed an efficiency of 27%, which is about twice the efficiency of a blancher without end seals. The main path for losses is the overflow of hot water from the condensed process steam.
No End Seals
Generally, steam blanching is less efficient that water blanching but some products cannot be blanched in water without deteriorating product quality. The most basic steam blancher is a conveyor covered with a stainless steel hood without any seals where the product enters and exits the hood. Steam is injected to the hood through a manifold system.
A typical system used to blanch spinach incorporates a hood that is 50 feet long, 4 feet wide, and 2 feet high. Steam is injected at specific points on the conveyor and a single blancher can be modified for different products. Blanching of spinach takes about 90 seconds with product temperatures rising from 77°F to 205°F. Although some energy is lost through radiation and heating of the conveyor, most thermal losses are from steam that vents out of the ends of the blancher. Measured efficiency of a steam blancher without end seals is only about 13%.
Steam blanching with hydrostatic water seals and Venturi nozzles incorporates the water troughs at the entry and exit and is similar to the system without Venturi nozzles in that respect. What makes this technology different is the addition of Venturi nozzles to the steam injection system. The Venturi design increases flow velocity which helps draw steam from the top of the blancher into a tube and into the nozzles. This recycling of steam helps boost the efficiency of the units to about 31%, which comparable to water-based blanching units. The advantage of steam blanchers is that minimizing the immersion of the product in water enhances the product quality and retention of nutrients.
Steam blanching with water curtains takes the basic steam blancher and adds water sprays at the ends of the unit to condense the steam before it escapes. The water from spray curtains stays inside the hood, which helps with energy recovery. Although the water sprays help save energy, their primary benefit is to keep excess steam from leaving the blancher where it can create unsanitary conditions. In addition, the water spray at the exit provides needed cooling of the product. A typical system used to blanch spinach incorporates a hood that is about 50 feet long, 5 feet wide and 2 feet high, which is similar to a blancher without end seals. The water sprays at the entry and exit reduce the effective blanching zone to about 40 feet in length. One study of blanchers puts the efficiency of this technology at 19%, which compares favorably to units without end seals (13%). Although the water curtains condense some process steam, considerable losses of steam occurred through vent ducts in the roof of the blancher housing.
P.O. Box 29505
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Web site: www.aeroglide.com
Key Technology, Inc
150 Avery Street
Walla Walla, WA 99362
Web site: www.keyww.com