Dehydration of Fruit and Vegetable Juices and Purees and Liquid Extracts:
Dried food powders are used in nutritional supplements, prepared meals, powdered drinks, breakfast cereals, dessert mixes, soup mixes, and other food and nutritional products. Dehydration of liquids allows for long-term storage of fruit and vegetable powders thus allowing preservation of vitamins and other nutrients in fresh fruits and vegetables that are critical for human health. Extensive research on antioxidants and their importance in preventing cancer, heart disease, and aging is providing impetus for food processors to enhance the preservation of the quality of fresh foods (as reviewed in Cadenas 2002). Of developed countries, including the United States, between 11- 20% of total industrial energy consumption is from drying operations (Mujumdar 2001, Baker 2005a). With increasing demand for food preservation and dehydrated food products this number is expected to increase throughout the world due to population growth and increasing transportation costs. Radiant Zone Drying provides a low cost alternative to producing high quality food powders. Fresh food and discarded agricultural waste streams can be processed into value added nutritional ingredients and dehydrated for long-term food storage. Liquid drying methods with a rapid drying rate, low powder manufacture costs, low energy consumption, and capable of preserving initial fresh food quality by limiting additives and preventing oxidative and thermal degradation is paramount in food processing (Vega-Mercado 2001, Mujumdar 2001).
Radiant Zone Drying:
Radiant Zone Drying exceeds other drying methods in energy and production efficiency while preserving the nutrients and antioxidant activity of powders from fruit and vegetable purees and juice concentrates, and liquid extracts. Spray drying, drum drying, microwave assisted drying, freeze drying and refractance window drying produce powders at either a high throughput –OR- are capable of preserving quality. However, Radiant Zone Drying excels at both efficiency and quality. The graphs below compare Radiant Zone Dried fruit powders with Spray Dried (straw berry and blackberry) and Freeze Dried (Raspberry) powders sourced from commercial distributors.
Comparison of Radiant Zone Dried Berry Powders with Spray Dried (Strawberry and Blackberry) and Freeze Dried (Raspberry) Powders % Polyphenolics, % Anthocyanins, and ORAC |
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RZD Compensates for Change in Heat of Vaporization:
Spray, Drum, Air, Freeze, and Refractance Window drying do not directly monitor and modulate heat input, and therefore cannot adjust for the change in the initial latent heat of vaporization during drying. Radiant Zone Drying (RZD) technology has the unique ability to adjust for the change in latent heat of vaporization between initial constant rate drying period and the subsequent falling rate period allowing for higher evaporation rates without thermal damage in a controlled manner. This is achieved through a patented system that modulates heat based on the drying liquid product temperature. As a result the RZD technology retains quality while lowering production and energy costs.
Spray Drying:
The lowest cost option for food liquid dehydration is spray drying, however research demonstrates that spray drying is not a good alternative for heat sensitive fruit and vegetable juices due to the high processing temperatures (Bhandari 1997b), product losses on the walls of the dryer (Mani 2002, Bhandari 1993), and the requirement for a high concentration of maltodextrin carrier and low nutritive density of the resulting powders (Jaya 2006, Chegini 2005, Bhandari 1997a, Bhandari 1993).
Spray drying efficiently removes initial moisture resulting in a large throughput and low to medium operating costs (Mujumdar 2001), 30 to 50 times less than freeze drying (Desorby 1997). Through atomization by high pressure spray nozzles, a liquid is converted to droplets, or spray, which are transformed into dried particles by contacting hot air (Barbosa-Canovas 1996). Spray drying is well adapted to dry milk products, coffee, and tea, as well as proteins and enzymes. Spray dried products have set the price for dried powders for nutritional and food ingredients. This makes it difficult for the more expensive manufacturing methods, such as Refractance Window and freeze drying, to compete.
Fruit and vegetable powders which contain high amounts of low molecular weight sugars are more difficult to spray dry and often remain amorphous and sticky after dehydration (Dolinsky 2000) resulting in extensive losses on the walls of the spray dryer (Mani 2002, Bhandari 1993). Whereas the addition of aids such as maltodextrin during drying and anticaking agents to improve powder flowability help in the drying of sticky liquids and subsequent shelf-life, there can still be considerable losses and the addition of such agents reduces the purity and quality of the dried powder (Mani 2002, Bhandari 1997a, Bhandari 1993). Most high sugar liquids such as raspberry, mango, and orange require additions of at least 50% maltodextrin and 2% tricalcium phosphate (Jaya, 2006, Chegini, 2005, Bhandari, 1997a, Bhandari 1993). Losses can be as great as 80% with as much as 40% maltodextrin added to fruit juices (Bhandari 1997a).
The addition of a fluid bed dryer after the spray dryer has been investigated to remove final moisture from heat sensitive products so they can be dried at a lower temperature (Pilosof 2000). Also a fluid bed dryer is often added to preserve enzyme activity; to retain flavor volatiles in coffee and tea; and to improve appearance, color and redispersion of dried milk through crystallization of lactose monohydrate, but at the expense of loss of production capacity (King 1984). However, there are still significant losses drying sticky fruits and vegetable products on the walls of the spray dryer (Mani 2002) and in the fluid bed dryer (King 1984). Capital costs, energy costs, and production costs for spray dryers increases when an agglomerator or fluid bed dryer is necessary to improve rehydration characteristics of spray dried particles (Barbosa-Canovas 1996).
Freeze Drying:
The introduction of freeze dried powders has increased the quality of dried whole foods and powders available in the market. Several studies illustrate nutrient retention in freeze dried powders and whole fruit/vegetables as compared to other drying methods (Abonyi 2002, Lin 1998, Grabowski 2002). The liquid is initially frozen than dried by direct sublimation of the ice under reduced pressure (Vega-Mercado 2001). Because products are dried in the absence of air and with low processing temperatures deterioration due to oxidation or chemical modification/decomposition is prevented (Vega-Mercado 2001). Quality issues are the requirements of drying aids for high sugar liquids due to a phenomenon of collapse that occurs during storage of freeze dried products high in sugar resulting in caking or clumping of powder (Bellows 1973, Slade 1991), and it is also often difficult to remove the final moisture due to a resistive layer on the frozen material (Vega-Mercado 2001).
Freeze drying has an average batch time of 3 days (Desobry 1997; Abonyi 2001; Lin 1998; Barbosa-Canovas 1996), resulting in high manufacturing costs and low production rates. Oregon Freeze Dry processes over 20M kg of fresh foods. If this is annual capacity (not stated), for whole fruits at 15% starting moisture, then the annual throughput for the entire operation is approximately 3M kg of powder. One commercial RZD is estimated to have an annual capacity of 2.3M kg of powder. The recently installed 20 ft drying length (1/5 sized) commercial RZD replaced six freeze dryers with a total monthly capacity of approximately 2,000 kg at a 3 day batch time. The 1/5 sized commercial RZD has a monthly capacity of approximately 20,000 kg.
Drum Drying:
If the liquid to be dried is thermally stable and can withstand a relatively high temperature for short periods, then drum drying can achieve relatively high throughput. Drum drying is a common dryer used to produce a flaky product from a liquid (Mujumdar 2001). A drum dryer consists of hollow metal cylinders that rotate on horizontals axes and are heated internally by steam hot water, or heated oil. A film is applied to the drum surface and the dried product is removed with a scraper. Temperatures during drum drying of fruit powders can exceed 140ºC (Abonyi 2001, Desobry 1997). Refractance Window and drum drying operate at one drying temperature and therefore do not actively compensate for the change in latent heat of vaporization in the product as it dries and therefore exhibit slow drying rates for temperature sensitive fruits and vegetables.
Air Drying:
The high temperatures (Bhandari 1997b) used during spray drying (130ºC – 200ºC) and air drying (60º-100ºC) cause loss of vitamins, antioxidant activity, color and volatilization of important flavor compounds (Desorby 1997, Bakker 1992, Lin 1998, Ratti 2001, Barbosa-Canovas 1996). Air drying is not capable of drying liquids, but it is used to produce powders from whole foods which are then powderized with fiber.
Refractance Window Drying:
Refractance Window drying developed and patented over 25 years ago is a belt drying system whereby products are spread on a transparent belt that moves over 95-97ºC circulating water (Abonyi 2001). Radiant and conductive heat pass through the membrane and into the product. The low heat and continuous throughput make this a good alternative to low temperature batch drying such as freeze drying. Refractance window drying is limited in the amount of initial heat available to quickly vaporize moisture in the constant rate drying period, because the heat source cannot exceed 97ºC.
The final drying temperatures in Refractance Window drying cannot be lowered to avoid thermal degradation of quality during falling rate drying, which may affect the preservation of nutrient content. Additionally, there are two quality concerns regarding the refractance window drying belt riding on a processing water interface. First, because the process water and resulting steam are not sealed from the product side there is the possibility of process water contamination of the product (Current Good Manufacturing Practices 21 CFRCh.1, Part 110.37,(b) (3)) and second, the belt riding on the water can result in peaks and valleys across the transverse direction of the belt which may result in uneven drying.
Microwave Assist:
Radiant Zone Drying (RZD) is unique because it has the ability to direct a large amount of heat into the liquid product, in a controlled manner, during the constant drying period when the heat of vaporization is high to increase the drying rate, while reducing heat in the falling rate period to protect against thermal degradation. Recent advances in liquid dehydration have tried to increase the drying rate by adding heat in the constant rate period using microwave and radio frequency technology, or by combining drying methods to reduce thermal product degradation in the falling rate period.
Relative to other methods, microwave and radio frequency assisted drying is faster, but energy costs are also significantly higher and scale-up for large production capacities is more difficult (Sanga 2000, Mujumdar 2001, Vega-Mercado 2001). There is evidence that dried juice powder produced with this method has evidence of aroma loss and unacceptable color (Sanga 2000) most likely due to non uniform heating (Khraisheh 1997).
Energy Efficiency:
Initial calculation of RZD energy efficiency is very close to 100% by measuring power consumption and steam evaporation. Direct comparison of energy efficiency of spray, freeze, drum, and Refractance Window drying can be problematic due to differences in the types of products processed and quality factors that can affect operating conditions and therefore energy consumption. Energy efficiency, defined as the ratio of energy required for evaporation of the moisture (based on a mass balance) to the total energy required to run the drying operation (Baker 2005a) will be addressed in this research.
The majority of spray dryers in operation do not operate on a closed system so ambient air is continually heated to high temperatures (Vega-Mercado 2001), increasing energy consumption, and in a survey of 23 spray dryers 29% of the total heat input was not used for drying (Baker 2005b).
The energy efficiency of freeze dryers is notably low, with the sublimation of the water vapor consuming only 45% of the total energy, the other 55% of energy is used to freeze the product, pull a vacuum and condense moisture from the vacuum pumps (Ratti 2001). Freeze drying is the most expensive drying processes in regards to energy consumption and operation costs (Ratti 2001, Grabowski 2002) and is 4-8 times higher than conventional air-drying.
The energy efficiency of Refractance Window drying is 30% to 40%, which is reported as comparable to other drying methods. However, the products tested for this unpublished study contained as much as 9.9% moisture, which is too high to produce a flowable powder or to prevent bacterial growth during storage.
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