Color Quality of Fresh and Processed Strawberries - ACS Symposium

Chapter 3

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Color Quality of Fresh and Processed Strawberries 1

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Ronald E. Wrolstad , Thao Ngo , Chad E. Finn , and Yanyun Zhao

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1

2

Department of Food Science and Technology, Oregon State University, Corvallis, OR 97331-6602 Horticultural Crops Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 3420 NW Orchard Avenue, Corvallis, OR 97330

Color and appearance are very important quality attributes of fresh and processed strawberries (Fragaria x ananassa Duch. ex Rozier). In evaluating new germplasm, the plant breeder considers size, shape, uniformity, along with hue and whether the berries are too light or too dark. The attractive color of fresh strawberries is dramatically affected by processing. While some pigment degradation occurs with freezing, physical changes, which are manifested by drip loss, are largely responsible for deterioration of appearance. Canning and manufacture into jam, juice, and wine results in marked color degradation. Anthocyanin pigment degradation and accompanying browning reactions cause their color deterioration. Reaction of anthocyanins with enzymes, ascorbic acid, acetaldehyde and other reactive compounds account for much of the pigment degradation. L*, hue angle and chroma are very useful indices for monitoring color change, and they are complementary to measurement of total anthocyanins, polymeric color and anthocyanin pigment profiles.

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© 2008 American Chemical Society

In Color Quality of Fresh and Processed Foods; Culver, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


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No one will deny that the attractive color and appearance of fresh strawberries are major factors contributing to their widespread appeal. Figure 1 is a color plate showing three cultivars and three experimental selections grown at the North Willamette Research and Extension Center, Aurora, OR. Most of Oregon's strawberry crop is sold for processing rather than fresh market. The color and appearance of strawberries are changed markedly by processing, and OSU's Food Science and Technology Department has had a long-term cooperative project with horticulturists and plant breeders with the objective of selecting cultivars with improved color, flavor, and textural quality. This chapter will give a historical perspective of what we have learned from our efforts to produce processed strawberry products with improved color quality and stability.

Color Quality of Fresh Strawberries Plant breeders when screening strawberry selections for potential release, need to give attention to a number of factors concerning color and appearance. Lightness, darkness, hue, brightness, color uniformity, pigment distribution within the fruit, shape, and size all need to be considered. Large-sized berries are preferable, not necessarily because consumers demand large berries, but rather because strawberries are hand-harvested, and picking small-sized berries is much more costly. In the field, the plant breeder in a medium sized program commonly evaluates 6-10,000 seedlings that represent 60-100 crosses (each cross is a single parental combination), and must make a decision whether to reject or save material for further study based on visual examination of a few fruit on a single plant. Typically a breeder keeps 0.5-1.0% of the seedlings evaluated. Promising selections are grown in subsequent seasons so that larger quantities of berries are available for objective evaluation of fresh and processed fruit. Instrumental measurement of color is very useful since humans have very poor color memory (7), making it very difficult to objectively compare visual color characteristics on fruit from different harvest days and different seasons. We recently (2) investigated the color properties of strawberries and the changes they undergo when processed into frozen and canned fruit and made into jam. Table I shows the CIEL*a*b* indices for the same six strawberry genotypes shown in Figure 1. These six genotypes were chosen for this study, because in a 2005 cutting of frozen strawberries from the 2004 season, they exhibited a wide range in hue and lightness and darkness. Close examination of Figure 1 reveals considerable berry-to-berry color variation in some samples. The color measurements in Table I were taken using a circular cell 130 mm in diameter and 50 mm deep. From a pool of approximately 50 berries, the cell was filled, measured, emptied and refilled so that the values are means of 3 subsequent measurements. Berry-to-berry variation will be a major contributor to



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Table I. CIE L*C*h Color Indices, Total Anthocyanin Pigment, and pH for Six Strawberry Genotypes. Selection

L*

C*

h°

Ovation

29.1

41.9

32.6

(±1.4) 22.5

(±1.5) 34.4

(±2.0)

Puget Reliance

(±0.9)

(±2.6)

(±1.2)

Totem

21.9

30.4

25.2

(1.3±)

(±2.2)

(±2.0)

1723-2

21.3

24.5

2273-1

(±1.2) 23.4

24.3 (±2.2) 32.6

30.0

(±3.6)

2384-1

(±1.6) 24.3 (±1.3)

Total ACN" 37.1 (±4.4)

26.3

(±2.0)

pH 3.28 (±0.07) 3.40

50.9 (± 2.8)

(±0.02) 3.58

76.0 (± 4.0)

(±0.01) 3.46

122.3 (±2.3)

(±0.02)

71.8 (±2.3)

(±0.01)

24.8

(±1.9) 28.8

(±2.6)

(±1.8)

62.1 (±0.4)

3.25 3.80 (±0.09)

a

Color measurements were taken on a HunterLab ColorQUEST instrument with 45/0 geometry, using D65 illuminant, a 10° viewing angle, and a view port 88.9mm in diameter. Numerical values are the mean and standard deviation of 3 readings taken from a pool of c.a. 50 berries. Berries were placed in an optical glass cell (130 mm dia χ 50 mm ht), and emptied and refilled after each reading. Total monomeric anthocyanin pigment was determined by the pH differential method (27) and expressed as mg perlargonidin-3-glucoside per 100 g of fresh weight, using a molecular weight of 433.2 g/mol and a molar absorptivity of 22,400. Values are the mean and standard deviation for 2 replicate determinations. SOURCE: The table is derived from data presented in reference 2 by permission. Copyright 2007. b



Figure 1. Photographs of strawberries from Oregon State University's North Willamette Experiment Station, 2005 season. Varieties 'Totem', 'PugetReliance', Ovation', and selections ORUS 1723-2, ORUS2273-1, andORUS2384-1. Reproduced with permission from reference 2. Copyright 2007. (See page 3 of color inserts.)



22 the standard deviations of the measurements. The total anthocyanin pigment content of the six samples is also listed in Table I. The relation between pigment content and lightness (L*) is evident if one compares 'Ovation', an eastern U.S. fresh market cultivar, with ORUS 1723-2, a selection for the Northwest processing market. ORUS 1723-2 contains over three times as much pigment (122 vs. 37.1 mg/100g) and has a much lower L* (21.3 vs. 29.1. A higher correlation between L* and pigment content for all samples would be more likely if it was not for differences in pigment distribution (white-centered vs. pigmentation throughout) and pH. The relationship between hue angle and color is evident from the orange-red color of Ovation (h° = 32.6) and the bluish-red color of 1723-2 (h° = 24.5). Measurements on these same fruits were taken on two other instruments, the Minolta CR-300 having d/0 geometry and a 8.0 mm diameter viewing port and the HunterLab LabScan II with 0/45 geometry and a 6.35 mm diameter viewing port. For the two instruments with small viewing ports, readings were taken on the surface of an individual berry, readings being taken on from 40-50 berries and averaged. Figure 2 compares the L*C*h° data for the six selections with different instruments. Differences between instruments are evident, particularly for L*. The data from the two instruments with small viewing parts had larger standard deviations. Strawberries do not have a flat surface, and the "pillowing effect" can result in distorted values. Readings on individual berries necessitated multiple readings, whereas the larger cell used with the ColorQUEST instrument allowed for average readings from a large sample of fruits.

Impact of Processing on Color Quality of Strawberry Products Our laboratory has been involved with several projects concerned with the color quality of strawberries processed by freezing, canning, freeze-drying, and manufactured into preserves, juice, syrups and wines. The attractive appearance of fresh fruit is markedly changed by any of these processing methods. Furthermore, it is often difficult to know what fruit characteristics will be predictive of acceptable color in the processed product.

Color Quality of Frozen and Canned Strawberries Frozen sliced strawberries were an important commercial product in Oregon in the 1960's. Many strawberry selections had unacceptable color quality after processing, the appearance being described as faded, dull, bluish or purplish. With an objective of determining what compositional information could be predictive of color quality, we processed and analyzed 40 lots of strawberries,



23 consisting of 13 different selections and 5 cultivars (3). Compositional measurements of fresh fruit included total anthocyanin pigment, the amounts of pelargonidin-3-glucoside and cyanidin-3-glucoside (determined by densitometry of thin-layer chromatograms), ascorbic acid content, pH, titratable acidity, and °Brix. Gardner L, a, and b values were determined on fresh and thawed frozen berries. Detailed sensory evaluation of the processed berries was conducted. For acceptable color quality it was concluded that the total anthocyanin content should range between 45-70 mg/100g. A surprising finding was that pH had the highest correlation (r = 0.78) with overall color quality. The pH ranged from 3.21 -3.81 which is essentially the same as the range for the six samples in the 2005 study (Table I). Figure 3 is a visible spectral scan of strawberry juice adjusted to pH 3.2 and 3.8. The differences in absorbance intensity and slight shift in wavelength of maximum absorption will have an impact on color. This can be explained by the well-known equilibrium between the colored flavylium form that dominates at pH 1 and the colorless hemiketal form that dominates at pH 4.5 (Figure 4). The pK for this transformation with anthocyanin-3glycosides is approximately 3.0; this translates to having 37% of the pigment in the colored flavylium form at pH 3.2 but only 13% at pH 3.8. All strawberries in these two studies were grown in the same experimental plots under very similar environmental conditions. One would expect commercial strawberries from different geographic locations to show an even wider pH range. This project was re-visited 15 years later (4) where 45 lots of ripe strawberries representing 14 different selections and 14 cultivars were investigated. Measurements of total anthocyanin, which ranged from 13.3-57.3 mg/100g and pH (3.20-3.80) were helpful in predicting color quality, however, physical measurements were even more useful. Drip loss of thawed berries that ranged from 6.6-33.3 mL/lOOg of fruit and penetrometer readings (range = 185-490 g) correlated well with color acceptability. A general trend was the lower the drip loss and the firmer the fruit the more acceptable the color. A

Strawberry anthocyanins degrade during long-term frozen storage. A common industrial practice has been to "cap" barrels of strawberries with sugar to preserve color and flavor. We (5) analyzed samples of strawberries that had been packed with 0, 10, 20 and 40% by weight added sucrose and stored at 15°C for 3 years. Sucrose had a small but statistically significant protective effect on anthocyanin pigment content and also retarded browning and polymeric color formation. Possible mechanisms are competitive inhibition of anthocyanin degrading enzymes, steric interference with condensation reactions, and provision of a partial oxygen barrier. A commercial lot of 'Totem' strawberries were processed into frozen and canned berries, and also processed into jam (2). Table II compares total anthocyanin pigment, % polymeric color, and L*, C* and h° values for the samples. Total anthocyanin pigment content is actually higher in the frozen sample than in the fresh fruit. This apparent increase may partially be due to



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Figure 2. CIE L*C*h° color indices for six strawberry genotypes measured by three different instruments. The HunterLab ColorQUEST has 45/0 geometry and a viewing port with 88.9 mm diameter. The HunterLab LabScanll has 0/45 geometry and a 6.35 mm diameter viewing port. The Minolta CR-300 has 0/0 geometry and an 8.00 mm diameter viewing port. Reproduced by permission from reference 2. Copyright 2007.



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15 +Ovation

1

1

1

2273-1

1 2384-1

1

1

1

Pug et

1

1——» Totem

1723-2

Reliance Figure 2. Continued.



400

500

λ (nm)

550

600

Figure 3. Visible spectrum of strawberry juice adjusted to pH 3.20 and 3.82.

450

650


700


Total Berries Liquid

Puree Berries Drip

20.6

21.7 8.7 13.0

69.7 20.7 49.0

65.1

Total ACht %

2.1 1.9 5.0

27.7

1.1

1

Browning Index

33.2 27.4

12

Polymeria

3.47 (±0.3)

24.0 (±0.3) 80.9 (±2.8)

21.6 (±0.4) 30.6 (±0.6) 66.5 (±0.2)

23.3 (±2.2)

L*

12.5 (±0.6)

28.6 (±0.5) 38.4 (±4.7)

44.7 (±0.2) 30.6 (±0.1) 89.0 (±0.1)

31.1 (±0.7)

C*

12.5 (±0.9)

31.7 (±0.1) 35.8 (±0.6)

33.7 (±0.4) 24.8 (±0.1) 49.4 (±0.1)

21A (±0.2)

h°

d

% P o l y m e r i c c o l o r is a spectral determination o f the proportion o f the p i g m e n t in the extract that is resistant to b l e a c h i n g by s o d i u m metabisulfite. B r o w n i n g Index= absorbance u n i t s / 1 0 0 g o f initial fruit in product. SOURCE: T h e table is derived from data presented in reference 2 by p e r m i s s i o n . C o p y r i g h t 2 0 0 7 .

c

b

L i g h t n e s s ( L * ) , C h r o m a (C*) and h u e angle ( h ° ) v a l u e s are m e a n and standard d e v i a t i o n s o f t w o replicate measurements. T o t a l m o n o m e r i c anthocyanin p i g m e n t w a s determined the p H differential m e t h o d , and m e a s u r e d as m g p e l a r g o n i d i n - 3 - g l u c o s i d e / l O O g fruit. N o t e : For c a n n e d berries and j a m , results are b a s e d o n fruit content rather than w e i g h t o f final product.

a

Jam

Canned

Frozen

Fresh

Sample

Table IL Total Anthocyanin Pigment Content, % Polymeric Color, and CIE L*a*b* Color Indices of Fresh, Frozen and Canned 'Totem' Strawberries, and Strawberry Jam.


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Figure 4. Reversible transformation

of anthocyanin flavylium and hemiketal

forms.


00


29 sampling differences due to berry-to-berry variation, but may also be because of increased extraction efficiency due to cellular rupture during freezing and thawing. The drip loss was measured to be 19.7 g/100 g fruit with 70% of the anthocyanins being in the drip. Much of the color change is due to pigment transfer rather than pigment degradation. L*, C* and h° measurements do not adequately describe the change in appearance, since cellular breakdown with pigment transfer, loss of gloss, and decreased brightness are better explanations for the observed appearance change. Canned strawberries show marked change in color and appearance, which partially explains their declining popularity as food item. Breakdown of cell structure occurs, with pigment transferring to the draining liquid. In contrast to frozen berries, there was a 69% loss in total anthocyanins, with 19% being in the liquid and 13% remaining in the berries. The canned fruit was analyzed after 70 days of storage at 25°C, so pigment degradation can be attributed to both processing and storage. Adams and Ongley (

Color Quality of Fresh and Processed Strawberries - ACS Symposium

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