Ketchup is made from strained, pulped tomatoes and additions such as spices, salt, sugar, vinegar, onions, and garlic, and it contains no less than 12% tomato solids.
The principles of manufacturing ketchup are based on the preparation of a tomato puree or paste. The percentage of tomato solids in the final formulation is related to the insoluble cellular materials released during pulping.
Raw materials for tomato processing
Desirable characteristics of a processing tomato include an intense, uniform red skin and pulp colour, a high total solids content with a good flavour and a firm, meaty texture. A round shaped tomato with a smooth skin is preferred since cracks and cavities complicate washing and peeling. If mechanical harvesting practices are used, it is further required that the tomato cultivar yields a uniformly ripened crop to minimise losses.
Harvesting of tomatoes for processing
Hand picking is the best option for small-scale operations. This will ensure that only the best-suited tomatoes with the minimum damage are selected. Mechanical harvesting is better for large-scale operations, but has the disadvantage of causing severe losses (> 30%) of ripe tomatoes.
Harvesters are sent into the field when there is a high percentage of ripe tomatoes available. Sorting and processing must follow as soon as possible after picking since further losses occur if the harvested fruits are stored for too long. The damage that results in disintegration and loss of soluble solids has a marked influence on the viscosity and flavour of the value-added end products.
Washing of tomatoes for pulping
Tomatoes are received at the processing site in crates that are dumped carefully in water tanks or flumes to minimise bruising. Washing is necessary to remove soil, dirt, spray residues and fruit fly eggs or larvae, especially in cases where the tomatoes are not peeled.
Soaking of the tomatoes prior to washing loosens dirt and fruit fly residues, making washing more effective. The soaking tank or flume should be equipped with inlets for air and steam. The injection of air under pressure agitates the water while the steam heats the water. A soaking period of
3 minutes in water at 54 °C is considered to be ideal. Detergents and lye may be added to the soaking water if necessary.
Washing is performed in a rotary drum washer or similar equipment that can dislodge dirt without causing damage to the tomatoes. The washed tomatoes are transferred to a roller conveyor with spray nozzles located directly above, which ensures thorough rinsing. The final rinsing water should contain a minimum of 5 parts per million residual chlorine.
Chlorinated water should take care of dirt, harmless extraneous materials, and microbial loads to ensure a safe product.
Heavy fruit fly contamination requires a lye washing treatment. This involves soaking and washing the tomatoes in a 0,5 to 1% NaOH (sodium hydroxide) solution at 49 to 54 °C, followed by a thorough water rinsing by 3,4 bar pressure jet sprays. This will not only remove 99% of the eggs but also most of the skin, and some of the surface tissue. Due to subsequent decrease in yields, lye washing is only recommended in cases of severe fruit fly infestation.
Washing may last 30 minutes to 2 hours, depending on the level of contamination.
Sorting of tomatoes for processing
This is done to select the best suitable raw materials for manufacturing the value-added end product. Sorting can be done at various stages namely in the field shortly after harvest, at a central storeroom close to the production area, and/or at the processing plant.
Field and/or central sorting are optional but have the advantage of providing the processing plant with more uniform loads of fresh tomatoes, thus minimising inspection and waste at the plant. The final sorting and trimming operations in the plant are, however, critical operations. Sorting is done manually in well-lighted areas on conveyor belts with a width of 45,7 to 50,8 cm moving at a speed of 7,6 m/min. All tomatoes affected with mould, rot, sunburn, insect damage, off-colour and mechanical damage should be removed. Minimal trimming may be done to save some lightly affected fruits.
Pulping of tomatoes for sauce
The washed and sorted tomatoes are pulped in a machine commonly referred to as a cyclone or pulper. Heavy paddles revolving at high speed crush the tomatoes. The pulp and juice pass through a screen at the bottom of the pulper, while the skin, seed and fibrous material pass through a different opening at the lower end of the pulper. The strained juice and pulp are collected and pumped through a continuous heat exchanger. Heating to at least 88 °C inactivates the pectic enzymes that destroy the pectin and thus damage the consistency of the sauce.
Refining of tomato sauce
The partially concentrated pulp is passed through a finisher, which consists of a horizontal cylinder with a fine stainless steel sieve and heavy paddles that rotate rapidly.
The holes in the sieve screen are approximately 0,83 mm in diameter and ensure the removal of any residual pieces of skin, seed, or fibre. This produces a pulp with a fine, smooth texture, which is required for ketchup.
Pre-concentration of tomato sauce
The pulp is pre-concentrated in vacuum pans to 1,060 specific gravity before seasoning and flavouring is added.
Additions made to tomato sauce
Various additions, namely spices, salts, sugar, chilli, onion, and garlic may be added in many different forms and blended into the pulp until thoroughly mixed. The spices may be added in a dried form or as spice oil extracts. Spice oleoresin is a highly concentrated extract and is preferred by some processors. Onion and garlic are usually added in dry powder form.
Distilled vinegar is preferred to cider vinegar because it is colourless. Sugar
may be added in the form of sucrose crystals, dextrose, glucose, or corn syrup. The sugar and salt are added to boiling tomato pulp to aid dissolvement and to prevent the formation of lumps. The addition of the spices and vinegar after pre-concentration ensures that the volatile oils and flavours will not be entirely lost during heating.
Concentration of tomato sauce
The concentration process is continued to a final specific gravity of 1,145 to 1,165. This density corresponds to about 32 to 36% total solids, of which a large percentage consists of sugar and salt.
Concentration may take place in open stainless steel kettles that heat the sauce with rotating steam coils known as “flash coils”. Under normal conditions these flash coils can reduce a charge of 1 900 litres of sauce to half its original volume in 35 to 45 minutes. Vacuum pans employ reduced pressure to reduce the boiling point of the sauce to 71 °C. This has the advantage of retaining the colour and flavour of fresh tomatoes. Multiple-stage vacuum pans can be used to increase the output and to conserve steam.
The finishing point can be determined with a refractometer. This is a rapid, dependable, and accurate method to determine the brix or total solids content.
Bottling of tomato sauce
The sauce is heated to nearly boiling point and de-aerated by passage through vacuum de-aerators before being filled at 88 °C into hot, sterilised bottles and sealed immediately after filling.
Sterilisation of tomato sauce
Sterilisation refers to the complete destruction of all micro-organisms in food. Most food products are, however, only commercially sterile. This means that the degree of sterilisation only destroys pathogenic and toxin-forming organisms as well as all other types of organisms which, if present, could grow on the product and produce spoilage under normal handling and storage conditions.
If the sauce is bottled at 85 °C and higher into hot sterilised bottles and sealed immediately, pasteurisation is not necessary. Should the temperature drop below 71 °C between final concentration and bottling, it may be necessary to heat the bottled product to 93 °C to ensure product safety and stability.
Cooling of tomato sauce
After filling and sealing, the bottles are first sprayed with hot water to remove any product adhering to the bottle and then cooled with cold water sprays. The bottles are labelled and should be stored at temperatures below 20 °C.
Published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.