Module 4. Fruits and vegetables juice processing
Lesson 16
CONCENTRATION AND DRYING OF FRUIT JUICES
16.1 Introduction to Concentration of Fruit Juices
Fruit juices are watery mixtures of most unstable volatile compounds. The solid content of most liquid food is 8-16% and is expensive to pack, store for long periods or to transport to distant places. Hence, it is desirable to remove a part or all of the water from such liquids.
Fruit juice concentrates are valuable semi-finished products for use in the production of: (a) fruit juice, (b) fruit juice beverage, (c) fruit juice powder.
16.1.1 Advantages of concentration
16.1.2 Methods of concentration
The methods used for concentration include the following:
16.1.2.1 Thermal evaporation under vacuum
This process is commonly adopted since it is economical method of fruit juice concentration. Use of multiple effect vacuum condensing plant is used for the purpose and use of high vacuum (i.e. 29 inches of Hg column) helps in evaporating water from fruit juice at much lower (i.e. 58-60°C) than its boiling temperature with steam economy too.
Table 16.1 The types of evaporators used for fruit juice concentration are
|
Concentration method |
Specific type |
|
Vacuum pan – Tubular |
Climbing
film Falling film |
|
Recirculation |
Single stage and one pass Five stages and single pass |
|
Plate |
Three stages and single pass |
|
Agitated film |
Single stage |
|
Centrifugal |
Single stage |
a) Disadvantages of conventional vacuum concentration
· Causes loss of much volatile flavouring compounds as well as nutritive value.
· Requires use of fining agents, enzymes and centrifugation for juice clarification
· High temperature promotes oxidation of compounds in the juice, which may result in chemical alteration of the aroma and flavor compounds.
b) Fouling of evaporators
When pulpy or cloudy juice is required to be concentrated, the deposition of burnt layer of organic matter on the hot surface of evaporator causes severe problem. The evaporation rate is retarded and it may become difficult to concentrate such fruit juices in a falling film and plate evaporators. In such cases use evaporators having agitators or use ‘Serum concentration process’ where the fruit juice is centrifuged to separate the solid phase (pulp) and the liquid phase (serum) is concentrated in an evaporator, before mixing with the pulp.
16.1.2.2 Freeze concentration
Freeze concentration (FC) of fruit juices is a cold, gentle and selective concentration procedure, in which two distinctive steps, viz., ice crystallization and ice separation from the concentrate phase are involved.
a) First stage
Fruit juice is supercooled below its freezing point to allow water to separate as ice crystals. This uses either (a) Direct contact crystallizer, or (b) Indirect contact crystallizer.
b) Second stage
The ice crystals are separated from the concentrated fruit juices. This takes help of Presses, Filtering centrifuges, Wash columns or a combination of these.
c) Advantages of FC over evaporative methods
· The energy needed to freeze a unit of water is much less.
· The low process temperature prevents undesirable chemical and biochemical reactions (minimum color change, non-enzymatic browning and vitamin losses).
· As vacuum is not involved, the losses of low-boiling flavor and aromatic esters are completely avoided
· The flavor profile is better.
d) Drawbacks of Freeze concentration
· Major problem is the loss of soluble solids of the juice in the separated ice.
· The final concentration of the concentrated juice is as low as 40-55% dry matter, due to steep increase in the viscosity of ice-concentrated mixture.
e) Multi-stage freeze concentration
As the juice concentrates, there is increase in viscosity which retards water crystallization. Multi-stage FC overcomes this to a great extent. In such process, the ice crystals are separated out at the end of each cycle and the remaining concentrate is fed to the succeeding crystallizing compartments. Here, the ice crystals are separated at different levels of concentration and viscosity.
16.1.2.3 Membrane processing
In the fruit industry, membrane technology is used mainly to clarify the by means of ultrafiltration and microfiltration and to concentrate it by means of nanofiltration and reverse osmosis.
a) Ultrafiltration (UF)
These membrane processes can perform clarification and fractionation over and above concentrating.
i) Advantages of UF
· Produces juice of desirable quality at low cost of operation and with greater speed.
· In a single step, it performs juice clarification and fining.
· Lower energy consumption (i.e. 20-30 BTU/lb. of water removed vs. 300 BTU for triple effect vacuum condensing plant)
· Increased flavor and aroma retention
b) Reverse osmosis (RO)
It is basically a concentration process. Pressure is applied to fruit juice that is greater than its osmotic pressure. This pressure forces the water out of the juice.
i) Advantages of RO
· Considerable amount of aroma retention at a cost competitive with evaporation, without undue loss of solids.
· Concentration without phase change or thermal damage.
ii) Drawback of RO
It limits the upper concentration level at about 28°Brix.
16.1.2.4 Combined UF and RO
Initially, the fruit juice is passed through UF system to remove suspended solids. The UF permeate is directed to an RO system to simultaneously concentrate the flavor and aroma compounds, sugars and amino acids for eventual reconstitution to single strength juice. This allows for concentration of orange juice to levels of ~ 42°Brix. On commercial scale up to 45-55°Brix can be achieved.
16.2 Drying of Fruit Juices
and vegetables are dried to enhance storage stability, minimize packaging requirements and reduce transport weight. Preservation of and vegetables using solar drying techniques can lead to poor quality and product contamination. Energy consumption and quality of dried products are critical parameters in the selection of drying processes. New drying technologies are being considered to have minimal drying time, economy with less nutrition loss; these includes osmotic drying, vacuum drying, freeze drying, superheated steam drying, heat pump drying and spray drying.
Fruit may be dried as a whole (e.g., grapes, various berries, apricot, plum, etc.), in sliced form (e.g., banana, mango, papaya, kiwi, etc.), in puree form (e.g., mango, apricot, etc.), as leather, or as a powder by spray or drum drying. Depending on the physical form of the fruit (e.g., whole, paste, slices), different types of dryers must be used for drying.
The advantages of fruit and vegetable drying are compensated by some negative changes that occur during drying, for example ‘heat damage’ of heat-sensitive constituents (vitamins, enzymes, etc.); browning, shrinkage, and ‘case hardening’; irreversible loss of ability to rehydrate; loss of volatile constituents; and changes in moisture distribution within the product.
16.2.1 Methods of drying fruit juices
There are several methods of drying the fruit and vegetables, solar drying being the oldest one. The type of dryers is listed below:
The fluidized bed dryers give good mass transfer due to enhanced air turbulence in such dryers. The description of some important drying techniques is given below:
16.2.1.1 Spray drying
Some fruit or vegetable powders are produced from juices, concentrates, or pulps by using a spray drying technique. Dry powders can be directly used as important constituents of dry soups, yogurt, etc. The drying is achieved by spraying of the slurry into an airstream at a temperature of 138°C to 150°C and introducing cold dry air either into the outlet end of the dryer or to the dryer walls to cool them to 38°C– 50°C. The most commonly used atomizers are rotary wheel and single-fluid pressure nozzle. A wide range of fruit and vegetable powders can be dried, agglomerated, and instantized in spray drying units, specially equipped with an internal static fluidized bed, integral filter, or external vibrofluidizer. Bananas, peaches, apricots, and to a lesser extent citrus powders are examples of products dried by such techniques.
Spray drying of soluble fruit powders and convective drying of fruit and vegetables reduces the thermo-plasticity of particles and product hygroscopicity. They also eliminate the need for adding stabilizers which may adversely affect the sensory properties of the final product.
16.2.1.2 Foam drying
Foam mat and foam spray drying are two foam drying methods. Foam mat dried fruit or vegetable powders have fewer heat-induced changes in color and flavor than conventional spray dried or drum dried products. It yields product with lower density than that of a conventional dryer. The product density is about equal to the density of instantized or agglomerated powder.
A stable gas-liquid foam is a prerequisite. Glycerol monostearate, solubilized soya protein, and propylene glycol monostearate are the typical additives for the fruit and vegetable foam formulation from juice or pulp. Foam mat drying involves drying a thin layer (0.1–0.5 mm) of the stabilized foam in air at 65°C–70°C for a few minutes, as the foam structure decreases drying time to about one-third. The foam is spread on perforated floor craters as the airstream is forced through the bed. A continuous belt tray dryer or a modified spray dryer can be used. Good quality tomato, apple, grape, orange, and pineapple powders can be produced by this technique. Optimal initial concentration of feed solids is in the range of 30% for tomato and 55% for orange.
16.3 Other Methods of Fruit Dehydration
Though these methods cannot be employed for fruit juice dehydration, the fruit pieces can be subjected to following methods for dehydration as detailed below:
16.3.1 Osmotic dehydration of fruits
Osmotic dehydration is one of the processes used to reduce or avoid detrimental phenomena in fruit and vegetables without a sensorial and nutritional quality loss. Osmotic dehydration, consists of placing fruit pieces in contact with sugar syrup to remove 30-50% of by weight, before conventional drying methods, that inhibits the action of polyphenol oxidase and prevents loss of volatile flavour constituents during dehydration. The process involves immersion of the fruit and vegetables (reduced to 3-10 mm pieces), in a concentrated solution of sugar syrup and ascorbic acid to effect partial dehydration (from ~ 6-8 to 1.0-1.5 kg moisture/kg dry matter). Most are suitable for osmotic dehydration, except tomatoes and citrus . The technique is currently largely used in the production of semi-candied
The factors affecting the osmotic drying process include size and shape, type of osmotic agent, concentration of the osmotic solution, temperature, food to solution ratio, duration, pressure, agitation of the osmotic medium and food pieces, Ca fortification of and vegetables, etc. The osmotic agents used were a saturated glucose or sucrose solution, 60oBrix glucose or isomerized glucose-fructose syrups, sucrose (70%)-glycerol (65%) 1:1 and ethanol. The temperatures of about 25 -43oC have been successfully used.
The recent technologies that have been used to enhance osmotic drying include blanching, freeze-thawing, pulsed vacuum osmotic drying, ultrasound, pulsed electric fields, high pressures, supercritical CO2, etc.
Osmotic drying has been used as a pretreatment prior to another drying process while use of osmotic drying for production of intermediate moisture foods.
16.3.2 Freeze-drying of fruits
There are two main stages in the freeze drying process: (a) freezing of the food, when most of the water is converted into ice, and (b) sublimation, when the bulk or all of the ice is transferred into vapor under very low pressure or high vacuum. In some cases, additional final drying, in the same or other equipment, is necessary. Cabinet or tunnel batch-type dryers are typically used with pressures in the range 13.5–270 Pa. Bananas, oranges, strawberries, peaches, plums, tomato, fruit juices and flavors, asparagus, beans, cabbage, cauliflower, celery, mushrooms, onions, peas, parsley and chives are processed by freeze drying.
The advantage of freeze drying over other methods of drying is the superior quality of the product. Little or no shrinkage occurs. The dry product has a porous structure and a color almost as fresh as that of the raw material. The only disadvantage of this process is the high equipment and operational cost.
Freeze-drying includes fluidized bed processes, spray-drying, continuous processes, foam drying processes, slush freezing and the thermal shock processes.
16.4 Preservation of fruit juices
Especially for low pH fruit juices (like lime, lemon juices) mild pasteurization is sufficient to have the desired shelf life. However, for higher pH fruit juices over and above stringent pasteurization, chemical preservatives (i.e. salts of sorbic acid or benzoic acids) may be used, where permitted by laws. The main purpose is to prevent fermentation from occurring during the refrigerated or even ambient temperature storage.
Sterilization of the fruit juice by In-can (retort) or by Ultra high treatment (UHT) followed by aseptic packaging can help in extending the shelf life for months even under ambient storage conditions.