Abstract

Solutions containing 7.5% (w/v) whey protein isolate (WPI) and sorbitol ( 2.5 w/v) were prepared in distilled water and pH was adjusted to 8 using 1N NaOH. Solutions were heated to 90^oC for 30 min while being stirred continuously. Next, melted carnauba wax or butterfat (1.5% w/v) was added to the warm mixtures. The mixtures were homogenized, filtered and degassed using a vacuum. Ida Red or Granny Smith apple slices were dipped in the coating solutions at room temperature and dried. The coatings were applied two times to ensure complete coating. Uncoated samples served as the controls in all the experiments. Samples were placed in LDPE bags and stored at 4^oC for 14 d. All experiments were replicated 3 times in a randomized block design. Browning of apple slices was evaluated using a HunterLab colorimeter and a trained sensory panel. Apples were also evaluated for moisture loss. Water vapor resistance provided by the coating was determined. Overall acceptability of the coated apple slices through the refrigerated storage study was confirmed by a trained sensory panel. Whey protein-based coatings were effective (p<0.05) oxygen and moisture barriers, thus they were effective in retarding browning and moisture loss. Whey protein based coatings were very effective in extending shelf life and quality of sliced apples up to 6 days of refrigerated storage.

Introduction

Increase in consumer demand for healthy, convenient, fresh and ready to eat produce has created a market for minimally processed (such as peeled and sliced) fruits and vegetables. Unfortunately, once fruits and vegetables are peeled and sliced their shelf life is very limited, thus, restricting their extensive distribution and use. Edible coatings have been used for centuries as a means of preserving food quality. Today, coatings are used commercially to coat pharmaceutical tablets. In foods they are applied to nuts, dried fruits and candies to preserve freshness and quality. Coatings function as moisture, gas and lipid barriers. In addition to preserving freshness and quality, coatings can enhance organoleptic, nutritional and mechanical properties of a food as well as serve as a carrier of flavor, aroma compounds and antimicrobial agents (Donhowe and Fennema, 1994). Functional properties of milk proteins, their cost and relative abundance make them excellent candidates for the production of edible coatings. The overall goal of this research is to develop edible coatings from milk proteins with good moisture and gas barrier properties to extend the shelf life of sliced apples. Development of edible coatings that could enhance the freshness and quality of sliced produce would greatly expand the market for minimally processed fresh produce.

Materials and methods

Coating process

Solutions containing 7.5% (w/v) whey protein isolate (WPI) and sorbitol (2.5 %w/v) were prepared in distilled water and pH was adjusted to 8 using 1 N NaOH. The solutions were heated to 90^oC for 30 min while being stirred continuously. Next, melted carnauba wax or butterfat (1.5 % w/v) was added to the warm mixtures. The mixtures were homogenized, filtered and degassed using a vacuum. Ida Red or Granny Smith apple slices were dipped in the coating solutions at room temperature (~23 C) and dried. The coatings were applied two times to ensure complete coating. Uncoated samples served as the controls in all the experiments. Samples were placed in LDPE bags with 4 small pin holes to prevent an anaerobic environment. They were stored at 4^oC at 80% RH for 14 d. Percent RH inside the LDPE bags was also monitored each day. Data for day 0 was obtained immediately after the coating was dry. Data was collected at 48 h intervals.

Thickness determination

Thickness of each coating applied was determined using a modified procedure of Park et al. (1994). A Bausch & Lomb DynaZoom microscope (Rochester, NY) with a flat field inclined monocular was used at a magnification of 100x. A red water based food dye was incorporated into the coating solutions and applied to apple slices as described in the previous section. Cross sections were cut lengthwise and widthwise and placed on a glass microscope slide for viewing. Thickness was measured using a micron scale. Twenty reading were obtained and averaged.

Oxygen permeability

The internal oxygen content of the coated and uncoated apple slices was evaluated by determining the percentage of internal oxygen in the coated apple slices at a steady state. A steady state was reached within 24 h. A vacuum system was developed using a desiccator jar with a HYVAC 7 vacuum pump (Central Scientific Co., Chicago, IL) connected through a septum in the lid of the desiccator. The coated apple slices were placed under an inverted funnel immersed in saturated calcium chloride solution with a silicone self-sealing septum placed at the opening of the stem. As the vacuum was applied the oxygen from the interior of the apple slice rose to the stem of the inverted funnel. A monoject _ cc gas tight syringe was inserted into the septum to extract 10 mL of air sample, which was then injected into a paramagnetic detection cell (Crowborough, Sussex, England), connected in series with N₂ as the carrier gas at a flow rate of 100 mL/min. Percentage of interior oxygen of the sample was recorded on a chart recorder and compared against a standard sample of air and a known standard of lower oxygen concentration.

Water vapor permeability

Moisture loss from the apple slices was determined according to Avena-Bustillos et al. (1994) and Water Vapor Resistance (WVR) was calculated using a modified Fick’s first law equation (Ben-Yehoshua et al., 1985).

Color determination

The browning of apples slices was determined using the HunterLab colorimeter by determining L-value from the color scale after calibrating with a standard white tile (Sapers et al., 1987). The difference in L-values between day 0 and sampling day was calculated and compared to the uncoated controls.

Sensory analysis

Consumer acceptance of the coated and uncoated apple slices through the duration of the storage study was evaluated using a 11-member trained sensory panel consisting of graduate students and research associates at Michigan State University. The panelists were selected through a screening process that determined their ability to distinguish between very slight changes in apple flesh. The selected panelists participated in a short orientation and three training session. They were trained to recognize color differences in two cultivars of apple slices over time and to recognize the extremes of color change within each apple cultivar. The panelists practiced using a structured intensity scale for the degree of color change and overall acceptability of the appearance of the apples and they were given feed back on their results. Panel session were held every 48 hr and all sessions were held in a climate-controlled sensory laboratory equipped with individual testing booths. Samples to be evaluated were placed on a styrofoam tray and labeled with a three digit number. The panelists were instructed to evaluate the samples visually for degree of brownness and for overall acceptability. Panelists evaluated brownness using structured 9-point intensity scale, where 1 indicated not brown (freshly cut) and 9 indicated very brown. Overall acceptability was evaluated using a 5-point hedonic scale, where 1 indicated unacceptable and 5 indicated acceptable. Sensory scores were averaged for 11 judges for each of the three replicates.

Statistical analysis

All experiements were replicated three times in a randomized block experiment. SigmaStat 1.0 (Jandel, SanRafael, CA) was used for the statistical analysis of the data. Appropriate comparisons were made using Student-Newman-Keuls method for multiple comparisons. The student t-test was used for paired comparisons.

Results and discussion

Thickness

Table 1 shows the thickness of the whey protein based coatings applied to Ida Red and Granny Smith apple slices in this study. The coating thickness ranged 31.4 to 26.8 mm. The coatings applied to the apple slices in this study were thicker than Sempfresh^TM coatings applied to whole apples (Park, 1994) which ranged from 4.5 mm to 13.2 mm depending on the concentration of Sempfresh^TM used in the coating (the higher the percentage the thicker was the coating). Mate and Krochta (1996) reported average coating thickness for WPI/glycerol coatings applied to peanuts ranged 111mm to 144mm for coatings applied (dipped) twice, which was nearly three times as thick as the coatings in this study.

Oxygen determination

Once an apple is picked it respires through the skin. Disruption of this skin (ie. cutting) will affect the rate of respiration. A coating on the surface of the cut apple will act as a barrier to oxygen and decrease the rate of respiration thus reducing the rate of enzymic browning. The coatings on the apple slices were effective oxygen barriers (p<0.5) compared to the uncoated controls (Table 2). The differences between the two coating treatments however were not significant. Although the level of internal oxygen was significantly lowered with the coatings they were not low enough to conclude that this decrease resulted in the reduction in the browning rate of the apple slices. Grand and Beaudry (1993) reported that an internal oxygen content of 2 kPa would be needed to retard color change on the surface. In this study lower internal oxygen contents were achieved with thicker coatings (data not shown).

Moisture loss and water vapor resistance

All treatment were effective (p<0.5) in decreasing moisture loss from the apple slices compared to the uncoated controls. The differences between the two treatments and apple varieties were not significant (Table 3). As expected the coated apple slices had higher (p<0.5) WVR compared to the uncoated controls (Table 4).

Color determination

Table 5 shows the browning of the coated and uncoated apple slices over the 14 days of refrigerated storage. Both coatings were effective in retarding browning of the apple slices up to approximately 6 days of storage. At approximately six days of storage the degree of browning of coated apple slices were similar to the uncoated controls.

Sensory analysis

The visual panel was discontinued after 6 days due to unacceptability of the samples after this point. Table 6 shows the color scores for the apple slices by the sensory panel. As expected color scores increased with increase in storage time. The coated apple slices consistently scored lower in degree of browning compared to the uncoated controls although these differences were not apparent statistically. After six days of storage the apple slices were similar to the uncoated controls and they were all unacceptable to the sensory panel (Table 7).

Conclusions

Whey protein-based edible coatings were very effective in extending shelf life and enhancing quality of sliced apples by preventing browning and moisture loss up to six days of refrigerated storage. Application of edible coatings would be suitable for salad bars and for catering purposes to preserve the appearance and quality of minimally processed apple slices. However, further studies are needed and the coatings need to be refined further to determine their effectiveness in longer storage studies. Studies on their perception and overall acceptance by consumers are also needed.

Significance to the Michigan apple industry

Increase in consumer demand for healthy, convenient, fresh and ready to eat produce has created a market for minimally processed (such as peeled and sliced) fruits and vegetables. Unfortunately, once fruits and vegetables are peeled and sliced their shelf life is very limited, thus, restricting their extensive distribution and use. Development of an edible coating that enhances the freshness and quality of sliced apples would greatly expand the market for minimally processed fresh apples. This should greatly benefit the apple industry.

Literature Cited

Avena-Bustillos, R.J., L.A. Cisneros-Zevallos, J.M. Krochta and M.E. Saltveit. 1994. Application of casein-lipid edible film emulsions to reduce white blush of minimally processed carrots. Postharvest Biology and Technol. 4:319.

Ben-Yehoshua, S., S.P. Burg and R. Young. 1985. Resistance of citrus fruit to mass transport of water vapor and other gases. Plant Physiol. 79:1048.

Donhowe, I.G. and O.R. Fennema. 1994. Edible films and coatings: Characteristics, formation, definitions and testing methods. In Edible Films and Coatings to Improve Food Quality . J. Krochta, E. Baldwin and Nisperos_Carriedo ed. Technomic Publ. Co., Lancaster, PA.

Gran, C.D. and R.M. Beaudry. 1993. Determination of the low oxygen limit for several commercial apple cultivars by respiratory quotient breakpoint. Postharvest Biol. Technol. 3:259-267.

Mate, J.I. and J.M. Krochta. 1996. Whey protein coating effect on the oxygen uptake of dry roasted peanuts. J. Food Sci. 61:1202-1206.

Park, H.J., J.M. Bunn, P.J. Vergano and R.F. Testin. 1994. Gas permeation and thickness of sucrose polyester, Semperfresh^TM coatings on apples. J. Food Preserv. 18:359-368.

Sapers, G.M. And F.W. Douglas. 1987. Measurement of enzymic browning of cut surfaces and in juice of raw apple and pear fruit. J. Food Sci. 53:1258-1262.

Table 1. Thickness of whey protein based edible coatings

	Average thickness (mm)
Treatments (Protein:plasticizer:lipid)	Ida Red	Granny Smith
1. WPI/BF (7.5:2.5:1.5)	31.4	28.2
2. WPI/CW(7.5:2.5:1.5)	27.4	26.8

_{WPI = whey protein isolate; BF= butterfat; CW=candelilla wax}

_{Table 2. Interior oxygen content of apple slices coated with whey protein based edible coatings}

	Interior oxygen content (kPa)
Treatments (Protein:plasticizer:lipid)	Ida Red	Granny Smith
1. WPI/BF (7.5:2.5:1.5)	15.79+0.19a	16.87+0.48a
2. WPI/CW(7.5:2.5:1.5)	16.79+0.75a	17.81+1.07a
3. Uncoated control	19.88+0.38b	19.79+0.31b

WPI = whey protein isolate; BF= butterfat; CW=candelilla wax

^a-b Means with different superscripts are significantly different (p<0.5), comparisons are made only within the same column, n=3 for all treatments.

Table 3. Moisture loss of apple slices coated with whey protein based edible coatings over 14 days of refrigerated storage.

	Total moisture loss (g)
Treatments (Protein:plasticizer:lipid)	Ida Red	Granny Smith
1. WPI/BF (7.5:2.5:1.5)	0.42+0.25a	0.62+0.44a
2. WPI/CW(7.5:2.5:1.5)	0.48+0.29a	0.51+0.40a
3. Uncoated control	1.48+0.31b	1.64+0.32b

WPI = whey protein isolate; BF= butterfat; CW=candelilla wax

^a-b Means with different superscripts are significantly different (p<0.5), comparisons are made only within the same column, n=3 for all treatments.

Table 4. Water Vapor Resistance (WVR) of apple slices coated with whey protein based edible coatings over 14 days of refrigerated storage

	WVR(s/cm)
Treatments (Protein:plasticizer:lipid)	Ida Red	Granny Smith
1. WPI/BF (7.5:2.5:1.5)	20.84+1.25a	19.64+2.80a
2. WPI/CW(7.5:2.5:1.5)	18.69+2.29a	21.69+2.90a
3. Uncoated control	11.38+3.36b	12.50+3.62b

WPI = whey protein isolate; BF= butterfat; CW=candelilla wax

^a-b Means with different superscripts are significantly different (p<0.5), comparisons are made only within the same column, n=3 for all treatments.