Color and gloss of whey-protein coated paperboard.

Application: Whey-protein-isolate formulation coated on paperboard is not as glossy as clay-coated paperboard, but it is as glossy as commercial polyvinyl alcohol coating and glossier than fluorocarbon coating.

Natural coatings have the potential to replace current synthetic paper and paperboard coatings such as polyethylene, polyvinyl alcohol (PVOH), rubber latex, and fluorocarbon (FC) in food packaging applications. Considerable research has studied edible and biodegradable films and coatings from corn, wheat, soy, whey protein, and other natural materials.

Color and gloss characteristics can describe the appearance of an object. Whiteness is desirable in commercial paperboard (PB) because it indicates high quality. Whiteness gives the impression that an object is fresh, clean, and contaminant free. Conversely, consumers perceive yellowness in PB as a characteristic of low quality. As a result, barrier coatings applied onto white-colored PB basestock should produce no yellowing. Gloss is another PB attribute that is important. Increased gloss in PB relates to increased smoothness.

Whey-protein films and coatings have excellent grease and oxygen barrier properties. Recent studies showed that coating whey protein on PB imparts excellent grease and oxygen resistance. Whey-protein-isolate (WPI) coatings have low yellowing rates. Gloss studies showed that WPI coatings produce high, stable gloss values. WPI therefore might replace commercial paperboard coatings such as PVOH and FC as grease and oxygen barriers while maintaining desirable color and gloss. This study determined color and gloss characteristics of WPI-coated PB.

Results and discussion

Initial yellow index (YI) comparisons showed that heat-denatured WPI (DWPI), native WPI (NWPI), PVOH, FC, clay coating (CC), and uncoated (UN) paper did not significantly differ, as Fig. 1 shows. This indicates that both WPI coatings were not more yellow than PVOH, FC and UN before storage. In addition, no significant change in YI occurred for DWPI, PVOH, FC, and CC during two months of storage. Previous work showed that DWPI coatings have a low yellowing rate. UN also showed no change in YI during two months of storage. NWPI coating showed an increase in YI at one and two months. A visual inspection confirmed noticeable yellowing in the NWPI-coated samples compared with other samples.

[FIGURE 1 OMITTED]

For better differentiation, gloss measurements used an 85[degrees] angle for all coated and UN samples. Initial gloss measurements for DWPI, NWPI, FC, PVOH, CC, and UN were 18.94, 49.80, 6.32, 17.91, 57.85, and 7.07, respectively. Initial NWPI and CC were significantly different from all other samples but were not significantly different from each other at zero storage time. DWPI and PVOH were significantly different from the other samples but not significantly different from each other. Initial CC and UN were significantly different from the other samples but were not significantly different from each other. CC and NWPI coatings had the highest specular gloss (SG) values. The high CC gloss value is consistent with the the common use of clay in commercial PB to increase gloss. UN and FC coatings had the lowest SG values. DWPI and PVOH coatings had higher SG values than FC and UN, but lower values than CC and NWPI coatings. These results indicate that WPI coated on PB increases gloss.

Conclusions

DWPI coating on PB was as colorless as commercial PVOH and FC coatings and showed no yellowing over two months of storage. NWPI coating showed yellowing after two months--an undesirable packaging characteristic. DWPI and NWPI increase gloss on PB--a desirable attribute in PB packaging. WPI coated on PB is not as glossy as CC PB, but it is as glossy as commercial PVOH coating and glossier than FC coating. Cracking occurred after two months of storage for all WPI coatings. Further research is necessary to investigate this.

Chan is at the Department of Biological and Agricultural Engineering, University of California, Davis, CA 95616, USA. Krochta is at the Departments of Food Science and Technology and Biological and Agricultural Engineering at the same school. Address correspondence to Krochta at [e-mail].