Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol84.Issue3-4.June2000:

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The effects of different freeze-drying processes

on the moisture content, color, and physical

strength of roses and carnations

Wei Chen

a,1

, Karen L.B. Gast

b,*

, Sheri Smithey

c,2

a

Graduate Student, Department of Biological and Agricultural Engineering, University of Missouri-Columbia, Columbia, MO 65211, USA b

Associate Professor, Department of Horticultural, Forestry and Recreation Resources, 2021 Throckmorton Hall, Kansas State University, Manhattan, KS 66506, USA

c

Senior Development Engineer, Friskies Research and Development Center, 3916 Pettis Road, St. Joseph, MO 64503, USA

Accepted 28 July 1999

Abstract

Freeze-drying is a relatively new preservation process for the preserved plant material industry. However, optimum freeze-drying procedures have been determined for very few ¯owers. This study evaluated the effects of different freezing times and vacuum-drying temperatures on color, moisture content, and stem and petal strengths of roses and carnations. Strength/stiffness was determined via force±deformation curves. The moisture content of freeze-dried ¯owers determines their longevity, and fewer changes in shape and color increases their aesthetic value. Vacuum-drying temperatures had more effect on the ¯owers than freezing time. Lower vacuum-drying temperatures resulted in ¯owers with color closer to fresh and control ¯owers, while higher vacuum-drying temperatures

resulted in lower moisture contents and stronger/stiffer petals but more changes in color.#2000

Keywords: Preserved plant material; Rosahybrida; Dianthus caryophyllus Scientia Horticulturae 84 (2000) 321±332

*

Corresponding author. Tel.:1-785-532-1439; fax:1-785-532-6949.

E-mail address: kgast@oz.oznet.ksu.edu (K.L.B. Gast). 1

Formerly Graduate Student, Department of Biological and Agricultural Engineering, Seaton Hall, Kansas State University, Manhattan, KS 66506, USA.

2

Formerly Assistant Professor, Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA.

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1. Introduction

Freeze-drying is a dehydration process that causes the vaporization of water directly from a solid ice crystal state to a vapor state without passing through the normal liquid state. The main advantage of freeze-drying is that it results in products that appear almost like the fresh originals. Physically, the original texture, structure, and highly volatile components including aroma, can be retained in many freeze-dried food products. Freeze-dried ¯owers are of suf®cient value such that they justify the substantial cost of freeze-drying. Although the shape, size, and color of freeze-dried ¯owers are similar to those of fresh ones, they are more fragile. The main disadvantages of freeze-drying are its high costs and precise processing techniques. The initial cost of equipment investment, electrical energy consumption, and equipment maintenance are relatively higher than those for other drying methods.

Freeze-drying is a relatively new preservation process for the preserved plant material industry. Other than those in trade journals and popular literature, few references de®ne the requirements of freeze-drying ¯owers. Wilkins and Desborough (1986) froze chemically sprayed ¯owers at ÿ808C for 12 h and then dried them under a vacuum of less than 100mm Hg for up to seven days. The results showed that the chemicals used (glycerin, clove oil, ethylene glycol, dimethylsulfoxide, and liquid detergent) failed to render the petal tissue ¯exible after freeze-drying. They found that the following freeze-drying times for baby's breath,Gypsophila paniculata,4±5 days; for carnations, Dianthus caryophyllus,7 days, and for roses, Rosahybrida and snapdragons, Antirrhinum andreanum, 8±9 days. They also noted that the effect of freeze-drying on color was dependent on the original color of the ¯ower. Red and purple ¯owers changed more in color than white, pink, or yellow ones.

For ¯owers, color is the one of the most important aesthetic characteristics. Color is measured asL,a, andb.Lrepresents the lightness of the sample, where a value of 100 represents white and 0, black;aindicates the redness when positive and greenness when negative; b designates yellowness when positive and blueness when negative. Hung (1993) used both instrumental and sensory methods to evaluate the color of apples under different storage conditions. This study showed that human perception of color and instrumental quanti®cation of color are highly correlated.

Stem and ¯ower strength are important in the handling of dried ¯owers because it determines whether they will withstand packaging and distribution. The Instron Universal Testing Machine is used in analysis of product strength/ stiffness or mechanical force±deformation. Labuza and Katz (1981) used it to test and quantify the strength of snack food products. Simonton (1992) used it in bending and compression tests of main stems of zonal geranium cuttings.

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When considering freeze-drying of any product, one must determine the proper dehydration conditions, including parameters such as freezing time and temperature, vacuum-drying time and temperature, and preprocessing treatments. Unfortunately, optimum freeze-drying procedures have been determined for very few products, including ¯owers. Processors of freeze-dried ¯owers often consider speci®c processing information proprietary because shortened processing times translate into more ef®cient use of equipment and greater pro®tability. The objective of this study was to evaluate the effects of different freezing times and vacuum-drying temperatures on moisture content, color, and stem and petal strengths of roses and carnations.

2. Materials and methods

Flowers were purchased from a local wholesale ¯orist. Cultivars were red roses, `Dallas'; pink roses, `Bridal Lorena'; red carnations, `Tanga'; and pink carnations, `Nora'. Flowers were processed immediately upon receipt and held in a 1.6% solution of ¯oral preservative. Flowers were allowed to open at room temperature for no more than two days. They were then either used immediately or held in cold storage (58C) with 80% relative humidity for up to seven days.

A commercial-sized freeze dryer, model 36DX66 Freeze Dryer (The Virtis, Gardiner, NY) was used. The freeze-drying process consists of two phases, (1) the freezing-phase where the product is frozen at atmospheric pressure, and (2) the vacuum-drying phase where a vacuum is placed on the product and the temperature regulated to ``dry'' the product. The freezing temperature in the chamber was ÿ358C, and the vacuum pressure was 30±50mm Hg for all treatments and the control. The treatments were a factorial with two levels for freezing time (2 and 4 h) and three levels for vacuum-drying temperature (278C, 378C, and 478C). Each treatment combination was replicated three times. These speci®c parameters were selected after preliminary studies showed that they offered the most promising results. The prescribed time for the chamber temperature to rise from the freezing-phase temperature ofÿ358C to the speci®c vacuum-drying treatment temperature was 4 h, and the time at the vacuum-drying temperature was 20 h. A standard manufacturer's recommended protocol was used as the control treatment. Freezing time for the control atÿ358C was 24 h. During the vacuum-drying phase, the temperature was raised in 58 increments to 208C over a period of seven days. Moisture content, color, and stem and petal strength were measured immediately after each freeze-drying treatment.

The moisture content of two ¯owers of each type from each treatment replication and the control were measured using a Stabil-Therm Laboratory Oven (Blue M Electric, Blue Island, IL) before and after freeze-drying. Each sample was dried at 1058C for 24 h (Anon., 1993a,b).

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Petal color was measured before and after freeze-drying on six ¯owers of each type from each treatment replication and the control using the Miniscan Color Measurement System, model MS (Hunter Associates Laboratory, Reston, VA). Tristimulus color parametersL, a, andb were measured three times at the same location on a ¯ower. For roses, one petal was labeled for color measurement. For carnations, the top center of a ¯ower was measured.

An Instron Universal Testing Machine Model 4502 (Instron, Canton, MA) was used to measure the stem and petal strength/stiffness via compression tests. An Instron 1 kN load cell was used for the measurements. Three stems of each ¯ower type from each treatment replication and the control were compressed immediately following the freeze-drying process. The maximum load that could be applied to the stem before it broke was determined. Stems were cut 6 cm long from the base of the ¯ower. The cross-head speed for stem compression was 25 mm/min, and the sampling rate was 5 points/s.

Compression tests for ¯ower petals were also conducted after freeze-drying. Young's modula were calculated and used as a standard comparison parameter (Anon., 1995). This is a modulus of elasticity and is usually expressed as the ratio of strain to stress, or the slope of the linear portion of a stress±strain diagram. Petals collected from three ¯owers of each type from each treatment replication and the control were compressed after freeze-drying. The cross-head speed for petal compression was 300 mm/min and the sampling rate was 5 points/s. A ¯at wood block was used to hold the roses horizontally. Carnations were held vertically by a wood block with a blind hole, 17 mm diameter13 mm. Young's modula were obtained between 10 and 20 mm starting from the zero point of the deformation (Chen, 1995).

The general linear model procedure (GLM) of the statistical analysis package SAS (Anon., 1992) was used to analyze the data via the analysis of variance. Contrasts were performed to determine means separation.

3. Results and discussion

When evaluating the effects the different treatment combinations had on the different parameters measured, results similar to or better than the control procedure, are considered acceptable alternatives. Visually, the processed ¯owers were darker, more shriveled, and smaller than the fresh ¯owers. Freeze-dried pink carnations appeared to be the most shriveled. Improvements over the control were seen with the highest vacuum-drying temperature at both freezing times, where pink rose stem strength was greater, and with both higher vacuum-drying temperatures for pink carnation stem strength and a lesser change in b. No differences occurred between the treatments and the control for red carnations in stem strength and changes inLand bvalues; for red roses in stem strength; for

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pink carnations in changes ofLanda; and for pink roses in changes of aandb

values after freeze-drying.

The moisture contents after freeze-drying of the four types of ¯owers are shown in Table 1. Except for red carnations, the only treatment not different from the control procedure was the 478C vacuum-drying temperature. The 478C vacuum-drying temperature reduced the moisture content of all ¯ower types signi®cantly more than the other treatments.

One of the main bene®ts of freeze-drying ¯owers is preservation of fresh colors, but the process does change color (Tables 2±5). Most changes were to more negative values,Lbecoming darker, a becoming greener, andb becoming bluer. The only exception was the pink control roses where the ¯owers became lighter (L) and all ¯owers became redder (b) (Table 3). The control procedure resulted in the least amount of change from the original fresh ¯ower color values, except for pink carnations where the higher vacuum-drying temperatures resulted in the least change in which bwas the only value affected (Table 2).

Table 1

Means of moisture contents (%) of roses and carnations after freeze-drying under various freezing times (FT) and drying temperatures (DT)

Treatment Color/species moisture content (%)

FT (h) DT

2 27 50.84 61.87 19.35 46.13

2 37 32.09 46.37 9.06 44.48

2 47 21.88 27.58 4.87 31.78

4 27 45.52 63.51 14.17 47.71

4 37 31.54 44.89 7.53 43.36

4 47 24.56 23.48 4.53 20.81

Control 4.68 5.57 3.74 5.09

Significance df

Signi®cant atP0.05.

**_{Signi®cant at}

P0.01.

***

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Table 2

Color values after freeze-drying and changes in color values from fresh to after freeze-drying treatments for pink carnations after freeze-drying under various freezing times (FT) and drying temperatures (DT)

Color/species Treatments Color value before

freeze-drying

Control 56.56 24.71 6.46 46.14 24.24 4.65 ÿ10.43 ÿ0.44 ÿ1.82

Significance df

**_{Signi®cant at}

P0.01.

***

Signi®cant atP 0.001.

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Table 3

Color values after freeze-drying and changes in color values from fresh to after freeze-drying treatments for pink roses after freeze-drying under various freezing times (FT) and drying temperatures (DT)

freeze-drying

Control 70.50 18.73 9.87 79.04 5.43 11.98 ÿ8.54 ÿ11.63 2.11

Significance df

**_{Signi®cant at}

P 0.01.

***

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Table 4

Color values after freeze-drying and changes in color values from fresh to after freeze-drying treatments for red carnations after freeze-drying under various freezing times (FT) and drying temperatures (DT)

freeze-drying

Color value after freeze-drying

Change in color value

FT (h) DT (8C) L a b L a b L a b

Red carnations 2 27 20.49 33.59 9.56 15.48 13.97 3.14 ÿ5.01 ÿ19.63 ÿ6.15

2 37 20.98 34.07 9.54 12.19 13.38 2.75 ÿ8.79 ÿ20.69 ÿ6.79

2 47 20.06 32.55 9.14 11.31 10.45 1.93 ÿ8.75 ÿ22.10 ÿ7.21

4 27 21.3 34.06 9.52 14.34 17.53 3.89 ÿ6.96 ÿ16.53 ÿ5.63

4 37 21.12 34.32 9.48 12.18 13.22 2.45 ÿ8.94 ÿ21.11 ÿ7.02

4 47 20.59 33.26 9.24 12.64 14.18 2.65 ÿ7.95 ÿ19.08 ÿ6.59

Control 21.62 36.11 10.22 15.01 22.43 3.64 ÿ6.56 ÿ13.68 ÿ6.60

Significance df

**_{Signi®cant at}

P 0.01.

***

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Table 5

Color values after freeze-drying and changes in color values from fresh to after freeze-drying treatments for red roses after freeze-drying under various freezing times (FT) and drying temperatures (DT)

freeze-drying

Control 29.58 40.68 7.19 25.85 21.81 1.21 ÿ3.70 ÿ18.91 ÿ5.96

Significance df

**_{Signi®cant at}

P 0.01.

***

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Freeze-drying had a greater effect on the color values of the red ¯owers (Tables 4 and 5). The reds often become unattractively dark and ``muddy'', so identifying process parameters that incur the least amount of color change is important. The higher vacuum-drying temperatures resulted in darker, bluer, and greener ¯owers for both red species.

Stem and petal strength measurements were not taken on fresh materials because the durability requirements of a preserved ¯ower cannot be reasonably compared to a fresh ¯ower since their respective use and handling is different. The stem strength values for the different treatments are shown in Table 6. Treatment effects were different for each type of ¯ower. Although freezing time was that only factor affecting carnations, red carnation stems were stronger after only 2 h of freezing while pink ones were stronger after 4 h. No effects were seen for red roses, but higher temperatures made pink rose stems stronger.

The petal strength values are shown in Table 7. Higher drying temperatures tended to make petals stiffer/stronger. For both red and pink roses, the highest

Table 6

Means of stem strengths (MPa) of roses and carnations after freeze-drying under various freezing times (FT) and drying temperatures (DT)

Treatment Color/species stem strength (MPa)

FT (h) DT (8C) Red

Control 1.35 0.57 0.75 0.41

Significance df

***_{Signi®cant at}

P0.001.

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vacuum-drying temperature produced the strongest petals, but not as strong as those of the control. As with stem strength, pink carnation petals were stronger after 4 h of freezing and red carnation petals were stronger after only 2 h.

4. Conclusions

An overall recommendation for the best freeze-drying conditions all ¯ower types studied based on the results in the study is not possible. There were no consistent results among the different types of ¯owers. The recommendation for pink ¯owers would be the higher drying temperature with either freezing time. For red roses and carnations, there is no clear choice, because higher drying temperatures resulted in stiffer/stronger ¯owers but a greater color change.

Table 7

Means of young's modulus of petal strengths/compression (MPa) of roses and carnations after freeze-drying under various freezing times (FT) and drying temperatures (DT)

Treatment Color/species petal strength (MPa)

FT (h) DT (8C) Red

Control 2.72 1.73 1.68 0.79

Significance df

**

***

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Acknowledgements

This research was made possible by a grant from the Kansas Value-Added Center. Contribution no. 97-345-J from the Kansas Agricultural Experiment Station.

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