Abstract
Red raspberry (Rubus idaeus L.) has attracted attention due to its high nutritional
content and rich anthocyanins known for their health benefits, such as antioxidant and
anti-inflammatory properties, and these characteristics have made the fruit considered
to be a ‘superfruit’. Regardless of these advantages, its soft, juicy, and fleshy
characteristics make raspberry susceptible to rapid deterioration as it is easily
physically damaged, undergoes increased moisture loss, and rotting caused by
Botrytis cinerea (gray mold). These characteristics of raspberries cause them to have
a minimal postharvest and shelf life, contributing to major postharvest losses in the
raspberry farming industry and negatively affecting the industry’s economic aspects.
Numerous postharvest technologies have been used to extend postharvest life in fresh
produce, such as cold storage, physical and chemical treatments, controlled
atmosphere (CA) and modified atmosphere packaging (MAP), which minimise
moisture loss, gaseous exchange and delayed ripening. However, these technologies
have shown limited favourable outcomes in extending the shelf life of raspberry. To
increase the shelf-life of raspberry, the whole fruit can be dried to produce a valueadded
dried product. However, drying degrades quality attributes due to heat affecting
colour, texture, sensory and functional attributes. Novel technologies, such as edible
coatings (EC), used as pretreatment with a combination of drying, have shown
significantly increased shelf life, higher quality, and longer storage ability. Presented
in this study is the use of edible coatings (EC), which are considered a novel
postharvest preservation technique, used as pretreatments in combination with
conventional/hot air drying to minimise postharvest loss of raspberry, maintain quality
attributes and increase the shelf-life and storage ability of dried raspberry with an end
goal of developing a niche value-added dried raspberry, using simple yet scientific
methods. The initial experimental chapter (Chapter 3) determined the drying kinetics,
using mathematical modelling and machine learning algorithms to predict the drying
behaviour and patterns of Gum Arabic (GA) pretreated whole dried raspberry fruit and
perform a comparative analysis of eight mathematical models and 4 machine learning
algorithms. The second experimental chapter (Chapter 4) studied the effects of GA
pretreatments on the quality and functional properties of dried whole raspberry fruit.
The raspberries were pretreated in GA at concentrations of 3%, 5% and 10%, and the
process was performed by dipping the whole fruit for 2 min before oven drying at 60 ±
v
0.1 °C until a moisture content below 8% was achieved. The GA pretreatments had a
significant impact on the colour, hardness, water activity and moisture content of the
whole dried raspberries, resulting in increased redness (a*), increased hardness,
reduced water activity and moisture content with no suggested significant difference
in the total soluble sugar, titratable acidity and pH of the samples compared to
untreated samples. It was observed that GA pretreatments were able to minimise the
effects of quality degradation, such as colour and texture, and maintain an intact
surface of the dried fruit that has good rehydration capacity, antioxidant capacity, and
total phenolic content. Following a principal component analysis of the multivariate
data, it was observed that the 5% GA pretreatment indicated discrete qualities that
separated it from all other samples. In conclusion, pretreatment greatly minimised the
effects of drying and showed good quality and functional attributes of whole dried
raspberry.
The final experimental chapter (Chapter 5) investigated the effects of GA
pretreatments on the shelf-life and storage ability of dried raspberries using 5% GA
functionalised with 0.6% ascorbic acid. The pretreatment, which gave the optimum
results in chapter 4, was used and functionalised with ascorbic acid, an antioxidant,
which helped protect the dried whole raspberries against oxidation. By incorporating
ascorbic acid into the GA coating, the pretreatment helped slow oxidative deterioration,
maintaining colour and nutrient retention over time. The samples were stored in
polyethylene zip lock bags at 23-25 °C and 55% relative humidity (simulating retail
conditions). It was observed that the addition of ascorbic acid to the pretreatment
improved the colour attributes and maintained a low moisture content and water
activity, suggesting that pretreatment improved the shelf-life and storability of the
whole dried samples.
This study focuses on applying edible coating pretreatments prior to drying as a
method to minimise the degradative effects of heat, considering the safety and nontoxicity
of Gum Arabic and the cost-effective method of oven drying, aimed at
developing quality, shelf-stable, and storable value-added products that may be
utilised in the food industry.
Research on using edible coatings (EC) as pretreatments in thermal drying of berry
fruits is limited but shows promise for combining postharvest technology and
vi
agroprocessing methods that are cost-effective and accessible to small-scale
raspberry farmers. Further studies are recommended to explore storage and
packaging effects on GA pretreated dried raspberries, evaluate consumer acceptability
through sensory assessments, ensure product safety with microbial testing,
investigate quality impacts of various edible coatings, and assess applications for
these products as natural flavourings, colourants, or fortifying agents in the food
industry.