WO2022077103A1

WO2022077103A1 - Method of making dried vegetable snack products

Info

Publication number: WO2022077103A1
Application number: PCT/CA2021/051434
Authority: WO
Inventors: Guopeng Zhang; Braden KNIGHTS; Shafique Ahmad; Erika Sandoval
Original assignee: Enwave Corporation
Priority date: 2020-10-14
Filing date: 2021-10-12
Publication date: 2022-04-21

Abstract

A method of making a porous, dehydrated vegetable product comprises increasing the rigidity of a surface layer of a vegetable by heat treating the vegetable, freezing the heat-treated vegetable to form ice crystals in it and exposing the frozen vegetable to microwave radiation in a vacuum microwave dehydrator at a pressure that is less than atmospheric and at which the boiling point of water is above 0°C. This causes the frozen vegetable to thaw and water to evaporate, leaving pores formed by the ice crystals, to produce the porous, dehydrated vegetable product. Subjecting vegetables to certain kinds of brief heat treatment prior to drying rigidifies their surface layer and, when combined with a drying process that includes freezing the heat-treated vegetable and vacuum microwave drying it under certain conditions, results in a product with an open and porous structure and a soft, crispy texture. The dried vegetable products are suitable as snack foods, superior to dried vegetable products that are made by freeze-drying or vacuum microwave drying without the step of rigidifying the surface layer.

Description

METHOD OF MAKING DRIED VEGETABLE SNACK PRODUCTS

Field of the Invention

The invention pertains to methods of making dried vegetable snack products having a porous structure, using vacuum microwave drying.

Background of the Invention

It is known in the food processing art to make dehydrated food products by means of vacuum microwave dehydration. Examples in the patent literature are WO 2014/085897, published June 12, 2014, which discloses the production of dehydrated cheese pieces, and US 6,312,745, issued November s, 2001 , which discloses the production of dehydrated berries. WO 2018/187851 , published October 18, 2018, discloses a vacuum microwave drying process in which a porous, crunchy, dehydrated food product is made by freezing a food product and exposing it to microwave radiation in a vacuum chamber at a vacuum pressure at which the boiling point of water is above 0°C, to thaw the frozen food product and evaporate liquid water from the thawed food product, resulting in a dehydrated food product with a porous structure.

It is also known in the food processing industry to dehydrate food products by freeze- drying, in which the process is conducted at very low pressures and temperatures and moisture is removed by sublimation. Freeze-drying can produce a high quality product for some types of foods, though it has the disadvantages of being slow and expensive.

Although vacuum microwave drying and freeze-drying produces dehydrated foods that are suitable as snack products for some kinds of foods, it has been found that using these processes for some vegetables, in particular green beans, cauliflower and some root vegetables, results in a dense or collapsed structure and texture, such that the product is not suitable as a snack food. There is a need for effective methods for preparing dried snack products from such vegetables, wherein the dried products have a porous structure and a desirable texture, colour, flavour and nutritional value. The invention is directed to methods for making such food products. Summary of the Invention

The present inventors have discovered that subjecting vegetables to certain kinds of brief heat treatment prior to drying rigidities their surface layer and, when combined with a drying process that includes freezing the heat-treated vegetable and vacuum microwave drying the vegetables under certain conditions, results in a product with an open and porous structure and a soft, crispy texture. The dried vegetable products are highly suitable as snack foods, superior to dried vegetable products that are made by means of freeze-drying or vacuum microwave drying without the step of rigidifying the surface layer.

The invention provides a method of making a porous, dehydrated vegetable product, comprising: (a) increasing the rigidity of a surface layer of a vegetable piece by heat treating the vegetable piece; (b) freezing the heat-treated vegetable piece and thereby forming ice crystals therein; (c) exposing the frozen vegetable piece to microwave radiation in a vacuum microwave dehydrator at a pressure that is less than atmospheric and at which the boiling point of water is above 0°C, causing the frozen vegetable piece to thaw and water to evaporate from the thawed vegetable piece, leaving pores formed by the ice crystals within the vegetable piece, to produce the porous, dehydrated vegetable product.

Further aspects of the invention and features of specific embodiments of the invention are described below.

Brief Description of the Drawings

Figure 1 is a photograph of a green bean processed in accordance with the procedures of Example 1 , with flash frying heat treatment at 176°C.

Figure 2 is a photograph of green beans processed in accordance with the procedures of Example 2, with flash frying heat treatment at 204°C.

Figure 3 is a photograph of green beans processed in accordance with the procedures of control Example 3.

Figure 4 is a photograph of a rainbow carrot processed in accordance with the procedures of Example 4, with flash frying heat treatment. Figure 5 is a photograph of a rainbow carrot processed in accordance with the procedures of control Example 5.

Figure 6 is a photograph of green beans processed in accordance with the procedures of Example 6, with grilling heat treatment.

Figure 7 is a photograph of green beans processed in accordance with the procedures of control Example 7, with hot air heat treatment.

Figures 8 and 9 are scanning electron microscope pictures of a carrot stick produced in accordance with Example 4.

Detailed Description

The method of the invention begins with a vegetable and produces from it a dried, porous, soft and crispy food product, intended as a shelf-stable snack food. Examples of suitable vegetables include green beans, cauliflower, and root vegetables such as carrots, beetroots, turnips and sweet potatoes.

Where the vegetable is substantially snack-sized, such as green beans, the whole vegetable would typically be processed in accordance with the invention. Where the vegetable is relatively large, such as cauliflower and some carrots, the vegetable would typically be sliced into smaller pieces before being processed. In the present disclosure, the term “vegetable piece” refers to both whole vegetables and smaller pieces of a vegetable.

Heat Treatment

The vegetable pieces are first subjected to heat treatment in a manner that rigidities their surface layer. The heat treatment is relatively brief and is done by means of either flash frying, grilling, or hot air drying.

The heat treatment step has several important effects. It rigidities the surface layer of the vegetable piece, by lowering the surface moisture content and forming a skin. It has the effect of reducing shrinkage that occurs during the vacuum microwave drying step. It results in more even pore distribution in the dried vegetable piece. It reduces the moisture content of the vegetable piece, primarily in the surface region, for example a reduction of up to about 50 wt.%, thus lowering the amount of moisture that has to be removed by vacuum microwave drying. Since the heat treatment is brief, nutritional damage to the product is reduced. Since the heat treatment does not involve immersing or boiling in water, loss of nutrients into hot water is avoided. Further, where the heat treatment is done by means of flash frying, the vegetable piece is left with an oil coating which makes a more pleasing snack product.

Heat treatment by flash frying is done by immersing the vegetable pieces in hot oil for a time period in the range of 3 seconds to 5 minutes, alternatively 1 to 2 minutes. The oil temperature is in the range of 150 to 350°C, or a range of about 150 to 300 °C, or about 176°C. Shorter times and lower temperatures are found to result in less blistering of the product, particularly for green beans. The oil may be any edible vegetable oil or animal fat.

In embodiments of the invention in which the heat treatment is done by flash frying, the method may optionally include the step of removing excess oil, depending upon whether a low fat product is preferred. This can be accomplished by centrifuging the vegetable piece after heat treatment, for example using a salad spinner at about 500 RPM.

Heat treatment by grilling the vegetable pieces is done using a suitable grill, for example an electric panini press, for a time period in the range of 10 seconds to 15 minutes, alternatively 3 to 8 minutes, alternatively about 5 minutes. The grill temperature may be in the range of 100 to 300°C.

Heat treatment by hot air drying is done using tray dryer for a time period in the range of 1 to 30 minutes, alternatively 3 to 10 minutes, alternatively about 5 minutes. The hot air temperature is in the range of 60 to 130°C, or a range of about 80 to 180°C, or about 100°C.

In some embodiments of the invention, an optional step of blanching the vegetable pieces is done before the heat treatment step. Blanching may be done by steam blanching or by water blanching. For example, green beans may be steam blanched for about 10 minutes, and carrots may be water blanched by immersion in boiling water for about 20 minutes. The blanching is useful in fixing the colour of the vegetable.

Freezing The heat-treated vegetable piece is next subjected to freezing. This is done using a low temperature freezer, for example at freezing temperatures in the range of minus 5 to minus 80°C, preferably lower than minus 20°C, until the vegetable piece is completely frozen. The freezing step forms ice crystals within the vegetable piece and these crystals result in the formation of pores.

Drying

The frozen vegetable pieces are subjected to drying by means of microwave radiation and reduced pressure in a vacuum microwave dehydrator. The frozen vegetable pieces are not allowed to thaw prior to vacuum microwave treatment. The reduced pressure in the vacuum chamber is set at a pressure at which the boiling point of water is above 0°C, for example an absolute pressure in the range of 5 to 100 Torr (0.67 to 13.3 kPa), alternatively 20 to 40 Torr (2.67 to 5.33 kPa). The boiling point of water at these pressures is 1 °C at 5 Torr (0.67 kPa), 22°C at 20 Torr (2.67 kPa), 34°C at 40 Torr (5.33 kPa), and 51 °C at 100 Torr (13.3 kPa). The vegetable pieces rapidly thaw in the dehydrator under the vacuum microwave treatment, and evaporation of water causes steam pressure to be created in the pores formed by the ice crystals, preventing the pores from collapsing. The dried vegetable product is thus highly porous. It has a texture that is superior to that of product prepared by vacuum microwave treatment of vegetable pieces that are unfrozen or that are thawed prior to treatment.

There is no freeze-drying of the vegetable pieces in the drying step of the invention.

The step of drying may be conducted in two stages having different conditions in order to optimize the drying conditions and quality of the product. For example, in some embodiments, in the first stage, the microwave power level may be higher than in the second stage. In the first stage, higher power is used to achieve faster drying. Lower power is used in the second stage to avoid over-drying and excessive temperatures in dry portions of the vegetable pieces that may lead to dark or burned portions. Or, in the different stages, the drying time or the speed of rotation of the product basket (where a rotating basket is employed to tumble the vegetable pieces during drying) may be different. Likewise, more than two drying stages may be employed. The vegetable pieces are dried to the desired moisture level, for example to a moisture level less than 5 wt.%, alternatively less than 3 wt.%. The radiation is then stopped, the pressure in the vacuum chamber is equalized with the atmosphere, and the porous, dehydrated vegetable piece product is removed from the vacuum microwave dehydrator. It will be understood that the term “drying” in this disclosure means that the moisture level is reduced to a desired level, not necessarily to zero.

An example of a vacuum microwave dehydrator that is suitable for drying the frozen vegetable pieces is a resonant cavity-type microwave apparatus, as disclosed in WO 2009/049409, published April 23, 2009, commercially available from EnWave Corporation of Delta, BC, Canada, under the trademark NutraREV. Using this type of apparatus, the frozen vegetable pieces are placed for drying in a cylindrical basket that is transparent to microwave radiation and has openings to permit the escape of moisture. The loaded basket is placed in the vacuum chamber with its longitudinal axis oriented horizontally. The pressure in the chamber is reduced. The microwave generator is actuated to radiate microwaves in the vacuum chamber and the basket is rotated within the vacuum chamber, about a horizontal axis, so as to slowly and gently tumble the vegetable pieces. The rotation of the basket may be effected, for example, by means of rollers on which the basket is supported, or by means of a rotatable cage in which the basket is placed.

Another example of a vacuum microwave dehydrator suitable for carrying out the step of drying is a travelling wave-type apparatus, as disclosed in WO 2011/085467, published July 21 , 2011 , commercially available from EnWave Corporation under the trademark QuantaREV. The frozen vegetable pieces are fed into the vacuum chamber and conveyed across a microwave-transparent window on a conveyor belt while being subjected to drying by means of low pressure and microwave radiation. With this type of apparatus, the vegetable pieces are dried while resting on a tray or the conveyor belt, and are not subjected to tumbling.

Examples

Example 1 - Flash frying of green beans

Fresh green beans were subjected to heat treatment by flash frying. A 600 gram sample of green beans was immersed in hot canola oil at a temperature of 176°C for 60 seconds. The heat-treated green beans were then frozen at -20°C in a freezer for 24 hours. The solidly frozen green beans were placed in a perforated polypropylene drying basket. The basket was loaded into a vacuum microwave dehydrator manufactured by EnWave Corporation, model mpqREV™, having a turntable to rotate the basket. The drying was done at 3600 W of power for a processing time of 3600 seconds, at an absolute pressure in the range of 23 to 27 Torr (3.06 to 3.60 kPa). The final weight of the dried, porous, green beans was 72 grams (a 12 wt.% yield) and the final moisture level was 2 to 3 wt.%. The product is shown in Figure 1.

It was observed that the structure of the dried product had very even pore size and distribution under a microscope and the texture was very soft compared to a control with no flash frying heat pre-treatment.

Example 2 - Flash frying of green beans

Fresh green beans were subjected to heat treatment by flash frying . A 600 gram sample of green beans was flash fried at a temperature of 204°C for 5 seconds, by submerging the fresh beans into the hot oil. The heat-treated green beans were then centrifuged for 1 minute in a salad spinner at 500 RPM to remove free fat. They were then frozen at -20°C in a freezer for 24 hours. The solidly frozen green beans were placed in a perforated polypropylene drying basket and were dried in the same manner as described in Example 1. The final weight of the dried, porous, green beans was 72 grams (a 12 wt.% yield) and the final moisture level was 2 to 3 wt.%.

The product is shown in Figure 2. It was observed that the product had little blisters but was otherwise similar to that of Example 1 .

Example 3 - Control done without flash frying heat treatment.

Green beans were steam blanched to fix the colour, for 10 minutes. The procedures of Example 2 were then repeated, except that no heat treatment and centrifuging was done before the freezing and vacuum microwave drying. The product is shown in Figure 3. The dried product of Example 3 was less porous, had a harder texture, a darker colour and showed greater shrinkage that the dried product of Example 2. The product showed a high degree of collapse.

Example 4 - Flash frying of rainbow carrot sticks

Fresh rainbow carrot sticks were subjected to heat treatment by flash frying . An 894 gram sample of carrot sticks was flash fried at a temperature of 176°C for 2 minutes. The product weight after frying was 559 grams and the moisture content was 77.6 wt.%. They were then frozen at -20°C in a freezer for 24 hours. The solidly frozen carrot sticks were placed in a tray and were dried in the same manner as described in Example 1 . The final weight of the dried, porous, carrot sticks was 125 grams (a 14 wt.% yield) and the final moisture level was 2 to 3 wt.%.

The product is shown in Figure 4. Figures 8 and 9 are scanning electron microscope pictures of the dried product, showing its high porosity. It was observed that that the flash fried carrot stick of Example 4 had a softer texture than the control product of Example 5.

Example 5 - Control done without flash frying pre-heat treatment

Rainbow carrots were hot water blanched by boiling in water for 20 minutes. The procedures of Example 4 were then repeated, except that no flash frying pre-heat treatment was done before the freezing and vacuum microwave drying. The product is shown in Figure 5. The dried product of Example 5 was less porous, had a harder texture, a darker colour and showed greater shrinkage that the dried product of Example 4.

Example 6 - Grilling of green beans

Fresh green beans were subjected to heat treatment by grilling. A 612.3 gram sample of green beans was grilled with an electric panini press for 5 minutes. The grill temperature was approximately 200°C. A single layer of green beans was put on the grilling surface. The weight after grilling was 545.6 grams and the moisture content was 89.9 wt.%. The heat-treated green beans were then frozen at -20°C in a freezer for 24 hours. The solidly frozen green beans were placed in a tray and were dried in the same manner as described in Example 1. The final weight of the dried, porous, green beans was 55 grams (a 9.0 wt.% yield) and the final moisture level was 2 to 3 wt.%. The product is shown in Figure 6.

Example 7 - Hot air treatment of green beans

Green beans were subjected to heat treatment by hot air drying. A 600 gram sample of green beans was hot air dried for 5 minutes at 100°C on air drying trays. They were then removed from the drying chamber. The heat-treated green beans were then frozen at -20°C in a freezer for 24 hours. The solidly frozen green beans were placed in a perforated polypropylene drying basket and were dried in the same manner as described in Example 1. The final weight of the dried, porous, green beans was 61.7 grams (a 10.3 wt.% yield) and the final moisture level was 2 to 3 wt.%. The product is shown in Figure 7. It was observed that the product had very similar porosity and texture to the flash fried samples but it did not contain any oil.

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the following claims.

Claims

1. A method of making a porous, dehydrated vegetable product, comprising:

(a) increasing the rigidity of a surface layer of a vegetable piece by heat treating the vegetable piece;

(b) freezing the heat-treated vegetable piece and thereby forming ice crystals therein;

(c) exposing the frozen vegetable piece to microwave radiation in a vacuum microwave dehydrator at a pressure that is less than atmospheric and at which the boiling point of water is above 0°C, causing the frozen vegetable piece to thaw and water to evaporate from the thawed vegetable piece, leaving pores formed by the ice crystals within the vegetable piece, to produce the porous, dehydrated vegetable product.

2. The method according to claim 1 , wherein the heat treating comprises flash frying in oil.

3. The method according to claim 1 , wherein the heat treating comprises grilling.

4. The method according to claim 1 , wherein the heat treating comprises hot air heating.

5. The method according to claim 2, wherein the flash frying is done for a time period in the range of 3 seconds to 5 minutes.

6. The method according to claim 2, wherein the flash frying is done for a time period in the range of 10 seconds to 10 minutes,

7. The method according to claim 2, wherein the flash frying is done for a time period in the range of 1 to 2 minutes.

8. The method according to claim 3, wherein the grilling is done for a time period in the range of 5 seconds to 15 minutes.

9. The method according to claim 3, wherein the grilling is done for a time period in the range of 10 seconds to 10 minutes.

10. The method according to claim 3, wherein the grilling is done for a time period in the range of 3 to 8 minutes.

11 . The method according to claim 3, wherein the grilling is done for a time period of about 5 minutes.

12. The method according to claim 4, wherein the hot air heating is done for a time period in the range of 1 to 30 minutes.

13. The method according to claim 4, wherein the hot air heating is done for a time period in the range of 3 to 10 minutes.

14. The method according to claim 4, wherein the hot air heating is done for about 5 minutes.

15. The method according to claim 2 or 5-7, wherein the flash frying is done at a temperature in the range of 150 to 350°C.

16. The method according to claim 2 or 5-7, wherein the flash frying is done at a temperature in the range of 150 to 300°C.

17. The method according to claim 2 or 5-7, wherein the flash frying is done at a temperature of about 176°C.

18. The method according to claim 3 or 8-11 , wherein the grilling is done at a temperature in the range of 150 to 350°C.

19. The method according to claim 3 or 8-11 , wherein the grilling is done at a temperature in the range of 100 to 300°C.

20. The method according to claim 4 or 12-14, wherein the hot air heating is done at a temperature in the range of 60-150°C.

21 . The method according to claim 4 or 12-14, wherein the hot air heating is done at a temperature in the range of 80 to 120°C.

22. The method according to claim 4 or 12-14, wherein the hot air heating is done at a temperature of about 100°C.

23. The method according to claim 2, further comprising removing some of the oil from the flash fried vegetable piece by centrifugation.

24. The method according to any one of claims 1-23, wherein the vegetable piece comprises green beans,

25. The method according to any one of claims 1-23, wherein the vegetable piece comprises cauliflower.

26. The method according to any one of claims 1-23, wherein the vegetable piece comprises a root vegetable.

27. The method according to claim 26, wherein the root vegetable comprises carrot.

28. The method according to any one of claims 1-27, wherein step (b) is done at a temperature in the range of -80°C to -5°C.

29. The method according to any one of claims 1-26, wherein step (b) is done at a temperature of -20°C or less.

30. The method according to any one of claims 1-29, wherein step (c) is done at an absolute pressure in the range of 5 to 100 Torr (0.67 to 13.3 kPa).

31 . The method according to any one of claims 1-30, wherein step (c) is done at a pressure in the range of 20 to 40 Torr (2.67 to 5.33 kPa).

32. The method according to any one of claims 1-31 , wherein step (c) is done in the absence of any freeze-drying of the vegetable piece.

33. The method according to any one of claims 1-32, wherein the porous, dehydrated vegetable product has a moisture content less than 5 wt.%.

34. The method according to any one of claims 1-33, wherein the porous, dehydrated vegetable product has a moisture content less than 3 wt.%.

35. The method according to any one of claims 1-34, wherein step (c) causes the formation of steam in the pores, preventing collapse of the pores.

36. The method according to any one of claims 1-35, wherein step (a) reduces moisture content of the surface layer of the vegetable piece.

37. The method according to any one of claims 1-36, wherein step (a) forms a skin on the vegetable piece.

38. The method according to any one of claims 1-37, further comprising a step of blanching the vegetable piece before step (a).

39. The dried, porous, vegetable product made by the method of any one of claims 1-38.