Development of Edible Coatings for Enhancing the Storage Stability of Fresh­Cut Lotus Root (Nelumbo nucifera)

概要

Minimal processed fruits and vegetables such as fresh-cut products have been gaining popularity amongconsumers because of convenience and freshness. An example of a fresh-cut product gaining interest fromresearchers and industries is the fresh-cut lotus root (Nelumbo nucifera). Its white color, crisp texture,pleasant aroma, and high nutritional content make this rhizome appealing to consumers [1]. In Japan, theMinistry of Agriculture, Forestry, and Fisheries (2017) reported that the Ibaraki region produces the highestamount of lotus root yearly (39 million kg), which accounts for approximately 49% of the total production.However, fresh-cut lotus root is prone to enzymatic browning and as a result, deteriorates its quality andshortens its shelf-life. Various strategies from researchers have been employed to improve the shelf-life offresh-cut lotus root. Most of these strategies are mainly focused on chemical methods such as the use ofantibrowning and preservatives and physical methods such as the application of gases and modifiedatmosphere packaging. But these methods have limited applications due to safety concerns of somechemicals, as well as the high-costs of equipment needed for modified atmosphere packaging (MAP). Inaddition, the author also reported that the use of acidic treatments led to the cell wall damage of fresh-cutlotus root, hence, resulted to tissue softening.

One of the alternative and promising approaches for the shelf life extension of fresh-cut lotus root is theapplication of edible coatings (Chapter 1). These coatings were reported to provide a semi-permeablebarrier to gases and water vapor which reduced respiration rates, enzymatic browning, and water loss. Dueto the limited information on the application of edible coating systems on fresh-cut lotus root, the authordeveloped an effective and low cost edible coatings from natural sources such as gums and marinepolysaccharides and evaluated its effects on quality changes of fresh-cut lotus root such as color, texture,enzymatic activity, total phenols, and microstructural changes during 16 days of storage at 5°C.In Chapter 2, the author targeted the formulation of polysaccharide-based edible coatings with differentionic charges from natural gum sources, such as guar gum, xanthan gum, and chitosan. The author alsocompared the effect of single layer coatings versus layer-by-layer (LbL) deposition as edible coatingtechniques for fresh-cut lotus root. Single layer coatings of xanthan gum, guar gum or chitosan separately,and LbL coatings of xanthan gum + chitosan, or guar gum + chitosan were applied to fresh-cut lotus root(10 slices per treatment) and stored for 16 days at 5°C. The changes in color, texture, weight loss,enzymatic activity, and morphological aspects were monitored during storage. The results have shown thatLbL coatings consisting of xanthan gum and chitosan were the most effective among all treatments,thereby reducing whiteness color (L*) changes and weight loss up to 60% and 86%, respectively.

Decreased polyphenol oxidase (PPO) enzymatic activity up to 70% was also observed in coated samples ascompared to the non-coated samples. The author also found that morphological analyses proved that ediblecoatings have maintained cell wall integrity of samples during storage. In this work, it had been proven thatthe LbL coating technique was more effective than single layer coating, which shows a promising strategyto increase the storage stability of fresh-cut lotus root especially during transport and distribution.Previously, the author reported in Chapter 2 that xanthan gum-chitosan edible coatings applied through theLbL electrostatic deposition of charges had the highest barrier properties in preventing oxidation duringstorage of fresh-cut lotus root. The author, then, evaluated the influence of xanthan gum concentration inimproving the barrier properties of xanthan gum-chitosan edible coatings on fresh-cut lotus root in Chapter3. In this study, LbL coatings of xanthan gum + chitosan was applied in fresh-cut lotus root with varyingconcentrations of xanthan gum concentration (0.5%, 1% and 1.5% w/w). Color and polyphenol oxidase(PPO) enzymatic activity of the fresh-cut lotus root were evaluated during storage at 5°C up to 16 days.The results have shown that layer-by-layer coatings of 1.5% xanthan gum and chitosan were the mosteffective in decreasing the total color changes and enzymatic activity of fresh-cut lotus root during storagefor 16 days at 5°C. In this part of the research work, the author concluded that increasing the xanthan gumconcentration in xanthan gum-chitosan edible coatings resulted to stronger barrier against oxidation andenzyme activity in fresh-cut lotus root.

Dipping, co-acervation and spraying are some reported techniques of edible coating application on foodproducts. In Chapter 2 and 3, the author developed polysaccharide-based edible coatings with differentionic charges from gum sources, such as guar gum, xanthan gum and chitosan and applied to the fresh-cutlotus root through dipping. However, due to the limitations of the dipping process such as uneven thicknessof coating layer, the author had been interested in an alternative application technique, which is thespraying method. In Chapter 4, the author evaluated the effect of the spraying method as an applicationtechnique for xanthan gum-based edible coatings and investigated its barrier and microbial properties onfresh-cut lotus root. Xanthan gum solutions (0.1%, 0.3%, and 0.5%) were prepared and incorporated withcitric acid as an antibrowning agent and 1% (w/w) glycerol as plasticizer. The coatings were sprayed usinga pilot spray system to 5 mm thick slices of fresh-cut lotus root for 20 s, packed in polyethylene bags,stored for 16 days at 5°C and analyzed for color, pH, morphology and microbial counts. It was found thatspray-coated fresh-cut lotus root samples had significant reduction in the total color changes as comparedto non-coated samples. These results suggest that the spray coating treatments were effective in decreasingthe enzymatic browning of fresh-cut lotus root during storage which could potentially increase its shelf-lifein the market. In addition, the author also found that the xanthan gum-based spray coated treatments werealso effective against inhibiting the growth of Bacillus subtilis during 24 hours of incubation which wereindicated by the lower microbial counts recorded as compared to non-coated fresh-cut lotus root samples.In this part of the work, the author highlighted the spray coating technique of xanthan gum-based ediblecoatings as a promising strategy in improving the storage stability of fresh-cut lotus root duringpost-harvest storage.

Dipping and spraying methods of coating applications were also investigated in this work and storagestability parameters were also evaluated. However, based on the obtained results, no significant differencewas found between the dipping and spraying method, in terms of the reduction of color changes duringstorage for 16 days at 5°C. Overall, the application of edible coatings is a promising strategy in extendingthe shelf life of fresh-cut lotus root. In the future, the author aims to widen the scope of the application ofthese coatings to other agricultural products which are prone to degradation during storage in the market.