The tensile test also showed significant (Pb0.05) differences in
strength between the films, with the 1% unheated whey protein film
having the numerically highest tensile strength (Table 1). Unlike the
puncture results, the tensile strength of the 1% unheated whey protein,
2% soy protein, 2% unheated whey protein and 1% heated whey
protein films were not significantly different from the control film.
In the dry alginate–gelatin–WPI composite films, Wang et al. (2010)
stated that heated whey protein had a larger effect on the tensile
strength of the films than gelatin or alginate. In fact, the film with
the highest proportion of heated WPI had the highest tensile strength
in their study. However, their results also suggested that dry films
with a high tensile strength could be obtained by mixing gelatin
and sodium alginate in the absence of heated WPI. They attributed
this to the random coil formation of gelatin which enabled it to
form a maximum number of interactions with the oppositely charged
groups on sodium alginate. Contrarily, Villagόmez-Zavala et al.
(2008) showed that sodium alginate had a greater reinforcing effect
on tensile strength than the heated whey protein concentrate
(WPC) in their composite dry films with their pure sodium alginate
film having the highest tensile strength. In both cases, the amount
of whey protein and alginate present in the films would be much
higher than the amounts present in the films produced in this study
(1–2%). This is due to the concentration of the hydrocolloids that
occurred with the removal of water during the drying of the films.
Additionally, neither of these studies used calcium to cross-link the
alginate and both used glycerol as a plasticizer in the films. All of
these factors may contribute to the differences observed between
the previous studies exploring ‘dry’ films and the current work
exploring ‘wet’ films.