How to control moulds in aquaculture
by Dr Elisabeth Holl and Tilman Wilke, Dr Eckel Animal Nutrition GmbH & Co., Germany
ne of the most critical factors in global feed and food production remains to be the prevention of post-harvest losses of raw materials. Efficient preservation methods for plant based components with organic acids are therefore of special interest also for the continually growing aquaculture sector and its requirement for industrially produced aquafeed. Aquafeed traditionally relies on fishmeal as the predominant protein source. Limited volumes and high prices make it economically and environmentally necessary to look for other sustainable options for aquafeed formulation. Therefore, plant components that are used as alternative protein sources in commercial fish feeds are soybean meal, rapeseed meal, sunflower meal and wheat. However, these components are subject to deterioration caused by moulds and yeasts, especially in warm climates. There are multiple factors leading to spoilage in feed and food such as moisture content, temperature, storage conditions and the naturally occurring microbial contamination.
Moulds in feed are a serious economic problem because they consume the main nutrients and affect the palatability of the feed. Losses of nutrients caused by moulds can occur in magnitudes up to 10 percent. Especially the crude fat content of grains is affected by mould growth during storage, even more so than proteins and carbohydrates. Losses in metabolisable energy from maize may even reach 25 percent.
aflatoxins, primarily produced by aspergillus ssp. during storage of feed and raw materials. For yellow catfish Gonรงalves (2010) described growth depression and decreasing feed conversion ratio with increasing levels of Aflatoxins in the diet. Trout, Tilapia and shrimp have also been shown to be susceptible to Aflatoxin contamination in the feed, reacting with weight loss, liver damage or increased mortalities (Tacon, 2002).
Feed preservation is key
Prevention is the recommended method to avoid the problems arising from nutrient losses and mycotoxin contamination due to mouldy feeds. This means minimising the microbial spoilage from time of harvest to the final use of the material. Several methods are possible to preserve raw materials and feed in practice. These methods include drying, oxygen deprivation, cooling and the application of chemical preservatives. The use of organic acids to prevent feed and food spoilage is widespread. To get the highest preservation success, the right choice for preservative and the correct application are imperative.
Choose the right preservative
Due to its lipophilic character, propionic acid and its salts is especially effective against moulds, making it the organic acid of choice when contemplating feed preservation. As a result of its pKa value of 4.8, propionic acid is also active at a neutral pH, which more closely matches the native pH value in grain and feed. Organic acids cause inhibition of micro-organisms by
The danger of mycotoxins
Apart from these nutrient losses, moulds also produce mycotoxins, threatening animal and human health. Aflatoxins for instance can be transferred into animal tissues and therefore pose a serious risk for the consumer (carry-through-effect). The detrimental effects of mycotoxins, on animal health and performance, have been comprehensively studied in land animals but there are relatively few studies in aquaculture systems. There is however evidence, that fungi and mycotoxins have a negative impact on health and performance of fish. The most important mycotoxicosis in fish and shrimp is caused by
Figure 1: Effect of Dr. Eckel MoldCid on CO2 formation in an in vitro preservation test
22 | March 2017 - International Aquafeed