Peeling in food industry

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1. OBJECTIVE


The objective of peeling is to remove the skin/peel from raw fruit and vegetables. This improves the appearance and taste of the final product. During peeling, the losses need to be minimized by removing as little of the underlying food as possible but still achieving a clean peeled surface (BAT in the Food, Drink and Milk Industries, June 2005).


2. FIELD OF APPLICATION


Peeling is applied on an industrial scale to fruits, vegetables, roots, tubers and potatoes (BAT in the Food, Drink and Milk Industries, June 2005).


3. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT


Various methods for peeling are steam peeling, knife peeling, abrasion peeling, caustic peeling and flame peeling. The removal of unwanted or inedible material from raw fruit and vegetables, e.g. from leeks and artichokes, is considered to be trimming. Reportedly, the cleaning water from cans and jars can be re-used in peeling.


  • Steam peeling- Continuous process
    (BAT in the Food, Drink and Milk Industries, June 2005)

The continuous steam peeler is a drum with a screw inside. The steam is fed directly into the drum, generally at a lower pressure than the batch process and the product is heated during an adjustable residence time. Most of the peeled material is discharged with the steam. Any remaining traces are sprayed off with water. This water may be filtered and used to wash raw fruit and vegetables. If dry brushing is used to remove peel using a brush conveyor instead of water, it is reported that serious bacterial contamination of the brushes and the damage of the fruit or vegetable tissue will inevitably occur.

Steam peeling leads to reduced waste production compared to other peeling techniques, and the peel is often recovered and used as animal feed. Additionally, it uses less water than a combination of abrasion and knife peeling. Still, increased steam is used compared to both wet and dry caustic peeling, while high volumes of water are consumed and waste water is more contaminated. Steam peeling uses approximately five times more steam, e.g. for energy, than caustic peeling.

Steam peeling used large quantities of water up to five times more than the amount required for caustic peeling, but half of that for a combination of abrasion and knife peeling. It also produces waste water with high levels of product residue. When steam peeling is applied, cold water may be used to condense the steam. If water cooling is not used then less water us used, less waste water is produced and there are lower organic loads in the waste water. Product loss in the peeling process in 8-15%.

The steam peeling process in an example installation is shown below. This can be compared with the outputs if abrasion peeling followed by knife peeling is used.


Peeling food.jpg

Figure 1: Steam peeling process in an example installation in Finland Literature: BAT in Food, Drink and Milk Industries, June 2005


The above installation is applicable for all fruit and vegetables which are peeled, except where the peel is relatively hard compared with the fruit flesh, unless it is further processed to make stewed fruit or juice. Steam peeling is reported to be more economical than abrasion, knife and caustic peeling.

  • Steam peeling- Batch process
    (BAT in the Food, Drink and Milk Industries, June 2005)

Batch steam peeling is also called “flash steam peeling”. The raw materials such as roots and tubers are exposed to high pressure steam, 1500-2000 kPa in a rotating pressure vessel. The high temperature causes a rapid heating and cooking of the surface layer within 15-30 seconds. The pressure is then instantly released, which causes the cooked skin to flash off. Most of the peeled material is then discharged with the steam and results in a concentrated waste steam. Most of the peeled material is discharged with the steam, and water is only needed to remove any remaining traces. If dry brushing is used to remove peel using a brush conveyor instead of water it is reported that serious bacterial contamination of the brushes and the damage of the fruit or vegetable tissue will inevitably occur.

This process results in reduced water consumption and waste water production, when compared to continuous steam peeling, but with increased level of product residue. Additionally, it results in reduced waste production compared to other peeling techniques and the peel is often recovered and used as animal feed. Still, higher energy consumption is recordered comparing to caustic peeling.

This type of peeler is reportedly gaining in popularity comparing to continuous steam peelers, due to the lower water consumption, minimum product loss, good appearance of the peeled surfaces and the possible high throughput of up to 4500 kg/h, with automatic contol of the peeling cycle.

The above installation is applicable for all fruit and vegetables which are peeled, except where the peel is relatively hard compared with the fruit flesh, unless it is further processed to make stewed fruit or juice. Steam peeling is reported to be more economical than abrasion, knife and caustic peeling.


  • Wet caustic peeling
    (BAT in the Food, Drink and Milk Industries, June 2005)

The material to be peeled is either placed in or passed through a dilute solution, e.g. 1-2% but as high as 20% of caustic, heated to 80-120°C. This softens the skin which can then be sprayed off by high pressure water sprays. The caustic concentration and the temperature depend on the type of fruit or vegetable and the degree of peeling required. Although water is not used during the actual peeling operation, it is used for the continuous cleaning of rollers and blades, so contaminated waste water is produced. This process results in reduced water and energy consumption compared to steam peeling. Still, the use of chemicals results in higher contamination of waste water and solid waste, reduction of nutrients in the peel or product decolourization.

Wet caustic peeling uses four times less water than steam peeling.

At an example installation, water use was compared for wet and dry caustic cleaning at a site processing 72 t/d of table beet. For the same quantity of product processed, dry caustic peeling reduced the water use by 75% compared to wet caustic peeling. Dry caustic peeling tends to have a lower caustic consumption than wet methods.

Wet caustic peeling is applicable in cases where the peel is relatively hard compared with the fruit flesh and where steam peeling cannot be applied.

Caustic peeling is reportedly more expensive than steam peeling.


  • Flame peeling
    (BAT in the Food, Drink and Milk Industries, June 2005)

This technique was developed for onions. Nowadays it’s used for peeling onions and peppers. A flame peeler consists of a conveyer belt which transports and rotates the material through a furnace heated to temperatures above 1000°C. The skin or root hairs are burned off and then removed by high pressure water sprays. Flame peeling requires heat, in contrast to other peeling operations which require electrical energy.


4. COMPETITIVE TECHNOLOGIES AND ENERGY SAVING POTENTIALS


a) Changes in the process
  • Abrasion peeling: (BAT in the Food, Drink and Milk Industries, June 2005)
In abrasion peeling, the material to be peeled is fed onto carborundum rollers or fed into a rotating bowl, which is then lined with carborundum. The abrasive carborundum surface removes the skin, which is then washed away with a copious supply of water. The process is normally carried out at ambient temperature.
This process results in reduced energy consumption, while the peel can be recovered and used as animal feed. In this case, energy is not required for heating water or producing steam, but is needed for the operation of the rollers or rotating bowl. Still, large quantities of water are consumed. There is a high product loss and high production of waste water.
The use of abrasion peeling can affect the hygiene quality of the product, since no peeling water is added, the cooling function of water is not fulfilled and there can be localised high temperatures.
This technique is used for peeling onions, potatoes, carrots and beets, as the skin is easily removed and the quality of the product can be maintained. Sometimes abrasion peeling is used as a pre-peeling step before knife peeling.
The capital and energy costs of the process are low, but steam peeling is reported to be more economical.


  • Knife peeling: (BAT in the Food, Drink and Milk Industries, June 2005)
In knife peeling, the material to be peeled is pressed to against rotating blades, or is itself rotated against stationary blades. Although water is not used during the actual peeling operation, it sis used for continuous cleaning of rollers and blades, so contaminated waste water is produced.
The process requires less energy consumption than steam peeling. The peel can be recovered and used directly as animal feed or for the recovery of its components.
Knife peeling is particularly used for citrus fruits where the skin is easily removed and little damage is caused to the fruits and for small quantities of, e.g. potatoes, carrots, beets and apples, or when vegetables are sued for catering or in institutional kitchens.
Knife peeling is reportedly more expensive than steam peeling.


  • Dry caustic peeling: (BAT in the Food, Drink and Milk Industries, June 2005)
In dry caustic peeling, the material is dipped in a 10% caustic solution heated to 80-120°C, to soften the skin, which is then removed by rubber discs or rollers. This reduces water consumption and produces a concentrated caustic paste for disposal. Peeling is followed by washing to remove the peel and any residual caustic.
In the case of peeling peaches and apricots, the skin is very fine and soft and not easily distinguishable from the fruit flesh, so it “clings” to the flesh. The skin clings to the flesh of less ripe fruit more strongly than it clings to ripe fruit. Peaches and apricots are immersed into the caustic solution and the skin is decomposed. The residue is then removed by spraying the fruit with water.
This process results in reduced water and energy consumption compared to steam peeling and wet caustic peeling. Additionally, it results in less caustic consumption than wet caustic peeling.
At an example installation, water use was compared for wet and dry caustic cleaning at a site processing 72 t/d of table beet. For the same quantity of product processed, dry caustic peeling reduced the water use by 75% compared to wet caustic peeling. Dry caustic peeling tends to have a lower caustic consumption than wet methods.
Dry caustic peeling is applicable in cases where the peel is relatively hard compared with the fruit flesh and where steam peeling cannot be applied.
Caustic peeling is reportedly more expensive than steam peeling.


b) Changes in the energy distribution system

No information is available.


c) Changes in the heat supply system

No information is available.


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