Different methods for obtaining fruit pulp or puree, used to make nectars, jams, yogurts, and confectionery
By Michael Trommer
In Brazil, the special technical standard no. 21 of Decree no. 12448 states that pulp or puree is the product obtained from the crushing of the edible parts of the fruit.
According to the technical standard in question, the product must be prepared with fruit that are clean, healthy, free from parasites, etc. It cannot contain fragments of non-edible parts of the fruit (seeds, stones, and skin). It is allowed the addition of saccharide, in the proportion disclaimed on the label.
Fruit puree or pulp is a processed product that usually aims to substitute fresh fruit. Largely used in restaurants, vitamin bars, etc., it is incorrectly referred to as juice – the correct name for it being “fruit liquid product.”
Pulp or puree is used for making nectars, juices, jams, ice creams, yogurts, and confectionery. There are plenty of methods for pulp preservation, always very similar to the manufacturing method.
The main preservation methods are:
- hot-filling process;
- preservation via additives;
- preservation via freezing;
- aseptic process.
Main processing methods
No process is capable of improving the quality of a product – at best, it can maintain the product’s original characteristics. For most products, transformations begin even before they enter the processing line.
Mainly transformations occur due to the following types of reactions:
- Microbiological – promoting bacteria, fungi, and yeast;
- Chemical – oxidation reactions, hydrolysis;
- Biochemical – promoting especially the action of enzymes.
In fruit processing, certain operations have the objective of reducing or eliminating those reactions. The main processes performed to reduce or eliminate the reactions mentioned above are:
- enzymatic inactivation;
- concentration via freezing;
- addition of sugars;
- reverse osmosis;
- additional chemical preservatives;
- fermentative processes.
This type of processing involves the application of heat from condensed steam. Hot air coming from other methods is an alternative, but steam is more commonly used.
Pasteurization is a method that destroys part (not all) of the microorganisms found in foods and stored under conditions that minimize microbial growth.
In many cases, the main objective of the pasteurization process is to destroy pathogenic microorganisms. Some microorganisms may survive this treatment; in this case, more effective preservation techniques are necessary to be combined with pasteurization:
- chemical preservation;
- airtight packaging.
High-temperature short-time (HTST) pasteurization methods are preferred, since they do less damage to the product.
Cryoconcentration consists in cooling the water of a solution to crystalize it. With that, the water is precipitated, increasing the solute concentration in the system.
In this technique, the water is separated from a liquid medium with the formation of ice at a low temperature, followed by the removal of the ice from the concentrate.
Requiring temperatures above 100°C, this method is used with products that have pH level greater than 4.5.
Solvents – water dissolves the various components in the solution, allowing them to come into contact with each other, through convection or diffusion.
The concept of water activity (aw)
The amount of water contained in food can be found in the form of bound and unbound water. The relationship between the amount of unbound or free water is called water activity.
This parameter (represented by aw) has values between 0 and 1.
The relationship between aw, temperature, and nutrition at any temperature, and the possibility of microorganism growth is smaller with the reduction of the water activity.
Water activity is the main determinant of survival, growth, and development of a bacterial community. It depends on water availability and solute concentration.
Water is the predominant component of most food products: it makes up for 85% of fruit juices.
Frequently, water content is reduced for economical reasons on the package, in storage, and in transportation. However, the main reason for food stability lies in the reduction of water activity, resulting in microbiological stability, since microorganisms have water-activity limits for their development in food.
Water removal can be accomplished via evaporation, cryoconcentration, or reverse osmosis.
In this process, water is heated and separated in the form of steam until it reaches its boiling point.
Since the boiling point of water in food is above 100°C at standard atmospheric pressure, vacuum evaporators are used. With them, it is possible to achieve evaporation at lower temperatures (30 to 50°C). This is the most commonly used procedure when processing juice concentrates.
This method involves removing the water in the form of ice crystals from the product. After freezing of the (liquid) product, the crystals are separated by centrifugation.
It is a concentration method that uses a counterpressure higher than the osmotic pressure of the product to revert the liquid flow, pushing the water through a membrane from a more concentrated liquid to a less concentrated one.
Another key process is reverse osmosis, a separation process in which a solvent is separated from a low-molecular-mass solute by a membrane that is permeable to the solvent and impermeable to the solute. In order to accomplish that, it is necessary to apply high pressure on the mixture (solute and solvent).
Refrigeration is a conservation process that involves temperatures between 0 and 15°C, often used with fruits.
In Brazil, the use of chemical preservatives is a widespread practice, with the goal of preserving juices, sodas, and other non-alcoholic beverages. The more commonly used preservatives include:
- benzoic acid and its salts;
- sulfur compounds;
- sorbic acid;
Juices have water activity greater than 0.95 and pH below 4.0. Therefore, under these conditions, the greatest (90%) cause of deterioration in non-alcoholic beverages is due to the presence of Saccharomyces yeast.
Besides Saccharomyces, lactic acid and acetic acid bacteria, as well as mold, can also be found, in smaller amounts.
Commonly used preservatives
Preservatives can slow down undesired food alterations. Those alterations can be caused by microorganisms, food enzymes, or chemical reactions. The main reason for the use of preservatives, however, is the inhibition of microorganisms.
Benzoic acid and benzoate
In a solution, the salt converts to its acid form, which is the active form.
Sodium benzoate is normally deemed to have strong activity against yeast and bacteria, while being less active in fungus control.
Efficiency of benzoate as a preservative shows a close dependency with the pH level of the medium. With pH at neutrality, it is mostly inefficient.
Sorbic acids and sorbates
Knowledge about the fungistatic activity of monocarboxylic fatty acids
( – C = O )
is not recent. Particularly, unsaturated fatty acids have a lot less intense antimicrobial activity than saturated fatty acids with the same chain length.
CH3CH = CHCH = CHCOONA
Sorbic acid and sorbates have activity against yeast and fungi, and are less efficient against bacteria.
The raw material for pulp elaboration can be whole fruits or waste from a fruit preserve processing line, as in the case of pineapples, mangoes, and peaches.
As for transportation, it must be performed using shallow boxes, in order to protect the fruits on the bottom layers from being crushed. Processing must occur as fast as possible.
In case of storing fruit for the next day, they must be kept in a cold chamber at proper temperatures – namely 5 to 7°C for tropical fruits and 0 to 2°C for temperate fruits.
Fruit washing is normally performed using mechanical washers, which combine immersion bathing, to remove gross impurities, and a system of sprays, to complement the washing.
For disinfection of the material, free chlorine (at least 5 ppm) must always be used. With free chlorine from 20 to 50 ppm, bacterial load drops by 96% (guava).
Selection is manual, in order to remove fruit that is green, bruised, or infected with fungus.
For many fruits, such as pineapple, the peeling process is mechanized; for others, it is continuously performed using a chemical solution. In this case, boiling soda is used (it varies according to the type of fruit), for example, peach and guava.
For other fruits, however, peeling is still normal (e.g. banana).
The peel must be always removed, because, if it is crushed along with the edible part of the fruit (pulp), certain organic components will be incorporated, inevitably giving the final product a strange taste.
Disintegration or grinding can be performed in a blade hammer mill, always with a sieve with variable mesh sizes, according to the fruit being processed, in order to reduce it to small fragments.
For fruits such as banana, apple, peach, and avocado, the grinding must be performed after the application of an antioxidant compound, such as ascorbic acid, directly on the product.
Certain machines allow the employment of a method called hot-break processing, using live steam or even by indirect heating. They are called thermal pulpers, capable of disintegrating the fruit while oxidant enzymes are inactivated.
For this specific case, temperatures of around 90°C for 2 minutes are sufficient to stabilize the pulp, while inactivating those enzymes.
The seeds must not be disintegrated in any of the disintegration processes.
This stage is usually performed in a horizontal pulper with stainless steel arms and a coupled sieve. The objective here is to eliminate seeds, floral waste, and fibers. The sieve openings vary between 0.060 and 0.045 inches. Whole seeds must be removed.
One of the more important stages in pulp processing, it is normally performed in a horizontal pulper, similarly as described in the previous stage.
A fine mesh sieve with openings of about 0.020/in is used, ensuring the elimination of small portions of fibers, pieces of seed, etc. In addition to that, the pulp is given such a consistency that a previous stage of processing, known as homogenization, can be skipped.
The process can be performed with the absence of air. This is essential for certain fruits like acerola, very rich in ascorbic acid, which degrades even in the cold. Air can be eliminated by using live-steam jet or even through the application of an inert gas, such as nitrogen, inside of the machine, during the operation.
After finishing, the product is pumped to a balance tank with the use of a positive displacement pump. This is done through the bottom of the tank, in order not to incorporate more oxygen into the product.
Brix and pH corrections are performed in the tank, which will define the physicochemical properties of the puree. A good agitation system is necessary for the homogenization of the product.
The pulping and finishing stages incorporate air into the product. It is a fact that oxygen causes color, aroma and taste alterations in the puree, as well as the degradation of ascorbic acid.
Eliminating the air from the product is recommended. This can be performed in a centrifugal or flash deaerator.
From the deaerator, a positive displacement pump sends the product to pasteurization.
Pulp pasteurization is usually performed inside tubular heat exchangers with scraped surface, due to the product’s viscosity and consistency.
The use of a scraped-surface heat exchanger in the pasteurization of several types of pulp, such as guava, banana, papaya, etc., yields excellent results for temperature (90°C ± 2°C), taken at the product output.
After the pasteurization process, the pulp can continue to different forms of pasteurization.
Most fruits have an acidic nature (pH below 4.5) or accept acidification, such as banana, papaya, etc. For that reason, they are suitable for the use of a preservation method known as hot filling.
In this process, the pulp, after being properly pasteurized, is immediately sent to a filling system. Then it is packed at a pasteurization temperature close to its own. Allowing it to cool off exposed to air leads to overcooking.
Preservation via chemical process
In this process, addition of chemical preservatives is performed after cooling off the pasteurized pulp. The maximum content for these preservatives is 0.1% in weight.
Various fruit pulps that have a pH level between 3.9 and 4.0 remain perfect for 6 months after the addition of preservatives.
For pulps, this dosage can be increased, since it will occur dilution afterwards, as it is the case with nectars. Therefore, the content can reach around 0.2 to 0.3% in weight. The final product cannot exceed preservative content of 0.1% in weight.