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Quotes - Engineering - SalesTomato processing
Heating tomato dice without turning it into sauce
Diced tomato is a different product from tomato sauce, and it is heated differently. The piece has to arrive at the can or the pouch as a piece: a 3/8 to 3/4 in. cube with its skin edge intact, firm enough to hold its shape through hot-fill or aseptic processing, in a juice that has been heated to 190–210°F for acidified canned product or to sterilization temperature for aseptic bulk. Anything in the line that shears, squeezes or over-cooks the dice sends the product to the sauce line at a lower price, and the heat exchanger is the first suspect when that happens.
Two things about the process bear directly on the stainless. The first is calcium chloride, added at a few hundred parts per million as a firming agent so that the dice survives the heat; that puts chloride into an already acidic juice at pH 4.0–4.4 and 200°F, which is the combination that pits 304 at the welds in a season. The second is that the line is cleaned with hot caustic and an acid step every day, and the dice itself scours the tube, so the surface is always fresh. 316L on the product tube is the standard answer, and the full-bore layout is the one that gets the dice through whole.
The rest of the design is about moving a soft piece gently.
Diced tomato runs in 316L because the calcium chloride firming agent and the acid at 200°F exceed what 304 tolerates; duplex is reserved for salted tomato brines. See Material & Construction for the grade comparison.
A 3 in. bore and a slow pump keep the dice whole
Tomato cubes up to 3/4 in. pass a 3 in. full-bore 316L tube at under 3 ft/s without bruising, and the calcium chloride that firms them is the reason the tube is 316L rather than 304 in a juice at pH 4 and 200°F.
What the dice does in the tube
Tomato dice is heavier than its juice but only just, so it settles slowly and suspends easily; the danger is not that it drops out but that it is crushed. A centrifugal pump will mash it, a reducer will jam it, and a short-radius return bend will split it against the far wall. The usual arrangement is a positive displacement pump or a piston filler feeding a full-bore tube at low velocity, with the dice-to-juice ratio controlled so that the pieces are carried rather than packed.
Thermally the dice lags the juice: the center of a 3/4 in. cube takes long enough to reach temperature that the heater is followed by a hold section sized on the center of the largest piece, not on the juice. Over-cooking is the other side of that coin, because a dice held too long at 210°F softens and loses its edges, so the hold is sized to the process authority's figure and no longer.
The 316L unit for it
The product tube is a single smooth 316L tube of 2-1/2 to 3 in. bore, smooth rather than corrugated because the corrugation would score the dice, with no internal step at the welds and long-radius returns of at least three tube diameters. Finish is 32 µin Ra inside, tri-clamp or flanged ends at the full bore, and a sloped layout that drains the juice and the dice out together at the end of the run. Product-side rating follows the positive displacement pump's dead-head pressure, often 150 psig, not just the hold-tube back-pressure.
Hot water or low-pressure steam runs in a 304L annulus; a hot-water loop is preferred where the juice has a high pulp content, because pulp burns onto a 250°F wall and the resulting film shows up as a rising pressure drop in the heater. A tomato brine salted above about 3% for a prepared product is reviewed for 2205 duplex or higher, since the chloride then passes 18,000 ppm at cooking temperature and 316L can pit at the gasket faces.
What to send for a diced tomato heater
The dice size, the solids ratio and the target temperature decide the bore, the velocity and the hold; the firming agent and any salt decide the grade. Send the following and we will size heater and hold tube together.
- Dice size and the ratio of dice to juice by weight
- Process: hot-fill, acidified canned or aseptic, and the target temperature and hold
- Calcium chloride dose and any added salt, with the juice pH
- Flow rate and inlet temperature from the dicer or the blend tank
- Pump type and the back-pressure at the hold tube
- Steam pressure or hot water temperature available
- Connection sizes at the pump discharge and the filler
Call and talk it through with an engineer: 1-805-484-2992
Questions from tomato processors
Because the chloride ion is what pits stainless, and the firming agent is a chloride salt dosed straight in to an acidic juice that is then heated to 200°F. The dose is small, a few hundred ppm, but 304 is already near its limit at 200 ppm chloride in neutral water at room temperature, and acid plus heat lower that limit further. 316L has the molybdenum to resist the combination and is the usual product tube for firmed, unsalted tomato lines.
The sauce heater is a corrugated tube chosen for viscosity and wall temperature; the dice heater is a larger, smooth, full-bore tube chosen for the piece. A sauce can run at 5 ft/s through a 1-1/2 in. corrugated tube and like it; a dice at that velocity in that tube arrives as pulp. The two products also carry different chloride loads, since sauces are salted and dice is only firmed.
Yes, and some lines do it: the juice is heated in a conventional exchanger and the dice is heated in the juice downstream, which lets the juice side run hotter and faster. The trade-off is that the dice then reaches temperature by contact with hot juice in a hold vessel, which is harder to validate than a tube with a known residence time. For aseptic dice the full-bore tube with a sized hold is the usual route.
Under about 3 ft/s in the tube and lower through the returns, with the pump speed set so that the dice is carried rather than driven. Faster flow does not bruise the dice in the straight run so much as at the bends and at the entry to the hold tube, where it is thrown against the wall. Long-radius returns and a hold tube at the same bore as the heater remove most of the damage.
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