The development of a dehydration model to prevent damage during the mechanical drying of high moisture almond fruit in bulk

Michael Coates is an engineer and PhD candidate at the University of South Australia whose research is looking at strategies to take almond harvesting into the 21st century. In a traditional almond harvest, as the fruit ripens on the tree, the shell containing the kernel is exposed as the hull splits and dries back. The fruit is then shaken from the tree, where it dries to an equilibrium moisture content on the ground. It is eventually collected from the orchard floor where it is stored on farm or immediately sent to a processing facility where the hull and shell are removed from the kernel. While this has been the practice for many years, harvesting relies heavily on a Mediterranean style warm, dry climate from hull split through to the processing of the kernel. Rains during this period will increase the chances of kernel damage as a result of moulds and bacteria, brought about by the high moisture environment and the transfer of soil. Cool temperatures slow down the drying process and, combined with high relative humidity, produce a kernel with a higher moisture content and a less appealing mouth feel. Twenty first century harvesting is likely to deliver greater control of the almond harvest, making it possible to sustainably harvest under any conditions and reduce potential hazards from pests and pathogens. In addition, new processes are looking to return the by-products of the kernel back to the environment or to create a secondary market that is commercially viable. Michael's PhD work is looking into ways of mimicking natural dehydration through mechanical means, teasing out any potential hazards that may come about through the process. This started in the orchards looking at how early the fruit could be removed from the trees. The process started 8 weeks before harvest (for that year) and looked at the progression of hull split, the accumulation of dry weight and the nutritional content of the fruit to determine the optimum time to harvest, based on the six hull-split categories previously established by the University of California. This showed that the fruit is very inconsistent and collectively can be in many states of hull-split at any given time, which reflects multiple stages of moisture content. The results showed that when each of the split categories (B-F) were equally represented on the tree, shaking could begin. The biggest obstacle was that early categories (B and C) could potentially remove spurs with the fruit, affecting the following year's harvest. Rates of moisture movement were then established for almond fruit at its highest moisture content when hull-split was just beginning (category B). This was used to determine the constant and falling rates of dehydration. This showed that moisture comes out of the hull and shell relatively quickly (8-24 h) but much slower from the kernel (80-120 h), indicating a potential 2-stage drying process. This also showed that drying at rates that exist above 40°C can produce cavities within the fruit that may cause splitting to occur later during the hulling/shelling process. Almond fruit were then examined in bulk to look at air resistance and the necessary temperature and air flow rates to allow high moisture fruit to dry below 65% relative humidity, to prevent the onset of moulds. This required an understanding of psychrometrics and the dynamic relationship between air, relative humidity, pressure and temperature. This showed that fruit sitting in a state of saturation where the air is essentially sitting at 100% relative humidity creates a condition for concealed damage that affects the flavour and shelf life. The final stage of Michael's PhD was to establish a model to predict the amount of moisture being produced from a cubic meter of fruit and the time required to dehydrate it without producing damage to the kernel. This model can be used to extrapolate the required airflow rates for large volumes of fruit under different configurations, which will benefit the development of future almond dryers. The ability to dry almonds mechanically is the first step towards 21st century harvesting and paves the way for new technologies such as catch frame shaking and in-field hulling. These technologies can be adapted to new orchard designs and tree architectures and have the potential to open up new regions with differing terrains and environments that are not traditionally suited to growing almonds.

Michael Coates won an ISHS Young Minds award for the best poster at the VII International Symposium on Almonds and Pistachios in Australia in November 2017.

Michael Coates, University of South Australia, Building J, RM 208, Mawson Lakes SA 5095, Australia, e-mail: michael.coates@mymail.unisa.edu.au