Comparison of starch films and effect of different rice starch‐based coating formulations on physical properties of walnut during storage time at accelerated temperature

Abstract

Effects of different rice starch‐based coating formulations on physical properties of walnut were studied during 20 days storage at accelerated temperature. Results from different starch films indicated there were nonsignificant differences in water and oxygen barrier properties of the starch films. However, significant (p ≤ .05) differences were present in the elongation at break characteristics between the corn and rice films. Tensile strength varied significantly between edible films made from rice starch with that of wheat and corn. Melting temperature (Tm) of wheat and corn starch films was higher than rice starch films (152.77 °C). Thus, rice starch in combination with different additives such as chitosan, emulsifier, and red palm oil, which improve the properties of rice starches have been chosen to coat dried walnut kernels. The coated walnuts were significantly (p ≤ .05) lighter than the control group. Results on different functional ingredients also showed significant variations, for example; yellowness of walnut in the presence of chitosan was lower among the coating formulas (p ≥ .05). However, the variations in textural characteristics of walnuts with different coating formulas remained nonsignificant (p > .05). Practical applications Rice based edible coating added with chitosan can be used to extend the shelf life of walnut kernels. These coated walnuts can be safely distributed or sold in tropical countries where the surrounding relative humidity and temperature is high. Edible coating serves as a barrier which reduces the effect of oxygen, moisture and temperature (most important factors of rancidity) on nuts. The application of starch‐based coating for walnuts will also help to reduce storage space as removing the husk (green layer) and shell of walnuts (wooden layer) results in less space requirements. Furthermore, the coated walnuts can be stored at room temperature and requirement for refrigerated system can be eliminated, hence the cost of storage is minimized.

Starch‐Based Coatings for Food Preservation: A Review

“…[ 21,24,26 ] Another disadvantage of this method is related to the possibility of the coating solution diluting other layers of the food surface and degrade its functionality. [ 31 ] This technique has been widely used, mainly for coating of fresh products as Andean blueberries, [ 25 ] eggplants, [ 32 ] cucumbers, [ 33 ] mangoes, [ 34 ] black mulberries, [ 35 ] pomegranate fruit, [ 36 ] mandarins, [ 37 ] and pumpkins, [ 38 ] and to recover and protect chestnuts [ 39 ] and almonds. [ 40 ]…”

“…[21,24,26] Another disadvantage of this method is related to the possibility of the coating solution diluting other layers of the food surface and degrade its functionality. [31] This technique has been widely used, mainly for coating of fresh products as Andean blueberries, [25] eggplants, [32] cucumbers, [33] mangoes, [34] black mulberries, [35] pomegranate fruit, [36] mandarins, [37] and pumpkins, [38] and to recover and protect chestnuts [39] and almonds. [40] The spraying technique is one of the most used processes of coating materials deposition due to the wide possibility of macromolecules used and the great versatility regarding irregular shapes and different sizes (Figure 1d).…”

Functionality and Applicability of Starch-Based Films: An Eco-Friendly Approach

“…Adding essential oils into starch coatings is another approach to control pathogens and extend the shelf life of minimally processed fruits and vegetables. A 2-3% cassava starch coating containing carvacrol sufficiently inhibited pathogens in minimally processed pumpkins [151], and papaya [152] prevented weight loss and delay in fruit ripening.…”

In a nutshell: prospects and challenges on coatings for edible kernels

“…Another aspect to be considered is the adequacy of coatings to specific kernels. Among three sources of starch, rice was the most suitable for walnuts, since its smaller granules and higher flexibility allowed it to cover the walnut surface more homogeneously than the tougher coatings from larger wheat and corn starch granules …”