by Dr Dafna Israel, Research manager and animal nutrition expert at Phibro Aqua

 

The quest for the best performing feed additive is a major mission in the aqua feed world. There is a variety of useful additives that raise the crucial debate regarding the selection of the best option. Acidifiers are functional feed additives with acidic properties. They first and most importantly improve growth performance and survival in intensive farming operations.

They do this through several modes of action that can benefit the cultivated animal in the following ways including providing an antimicrobial effect, activating digestive enzymes, increasing protein digestibility and improving feed hygiene by preventing spoilage and contamination.

An antimicrobial effect

Organic acids have the capacity to reduce bacterial loads in the proximal gastrointestinal tract of aquaculture species. As a result, fewer pathogens reach the gut from the stomach, leading to improved feed hygiene and minimising the risk of infections.

Organic acids inhibit the growth of micro-organisms, causing very extended lag phases. The inhibition of micro- organisms occurs by rapid diffusion of the undissociated molecule into the cell. Dissociation (release of the H+ Ion) of these molecules in the cell of the bacteria causes acidification of the cytoplasm, thereby preventing growth.

Most organic acids are effective in a strong or moderately acidic environment. Since each acid has a different pH optimum, depending on their pKa values, a blend of organic acids would significantly increase the efficacy against microorganisms, rather than using only one acidifier.

Composing a blend of different acidifiers comprises a broader spectrum of pH environments to match different applications (anti-mould, anti-yeast, anti-bacterial). Additionally, this also creates synergistic effects at the molecular level. For example, by combining formic and propionic acids is more effective than each of these acids on their own.

Activate digestive enzymes

Increasing nutrient utilisation is crucial in aquaculture due to the high costs of feed that comprises about 50 percent of the overall production costs. Protein is one of the most expensive macronutrients. The attributes of acidifiers on protein digestion have been well investigated on terrestrial animals and it is known that they stimulate the gastric and pancreatic enzymes.

The optimum pH level for pepsin activity is 1.5-2.0; though, the pH range level in the tilapia's intestine is higher (6.5-7.8). Consequently, lowering the pH level of the digesta in the stomach and intestine of the tilapia with the use of acidifiers can improve digestion by stimulating secretion and activation of digestive enzymes.

This was also demonstrated in shrimp as various acidifiers had the effect of elevating the activity of trypsin and chymotrypsin. These results aligned with a trial conducted with an acidifier blend product formulated by Phibro Aqua.

PAQ-TivateTM was strategically developed as a joint project with experts from Dr Eckel Germany and it was specifically designated for aquatic species.

The trial results showed increased enzyme activity in whiteleg shrimp (L. vannamei). In this trial three formulations were examined:1) highly digestible (nine percent fish meal) formulation 2) soybean base formulation and 3) soybean base formulation +0.5 percent of PAQ-TivateTM.

The enzyme activity in the hepatopancreas presented similar trends between the diet with the fish meal and with PAQ-TivateTM as the enzyme activities were significantly higher than soybean meal diet.

Also, trinitrobenzene sulfonic acid (TNBS) was significantly higher in the PAQ-TivateTM treatment indicating enhanced protein digestibility when it was added.

This study documented that soybean meal-based formulation supplemented with PAQ-TivateTM could significantly increase feed utilisation, proteolytic enzymes activity and in vitro protein digestibility of whiteleg shrimp (L. vannamei).

These results were supported with better performance: enhanced growth and higher survival.

Improving feed hygiene

Moulds in feed are causing economic and health problems in aquaculture operations 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.

Propionic acid and its salts are especially effective against mould development due to its pKa value of 4.8, propionic acid is also active at a less acidic pH, which more closely matches the native pH value in grain and feed.

The pKa value is one method used to indicate the strength of an acid. A lower pKa value indicates a stronger acid. That is, the lower value indicates the acid more fully dissociates in water.

To examine its effectiveness as a preservative agent against bacteria and fungi in formulated tilapia feed, two doses (0.2 and 0.5 percent) of PAQ-TivateTM were examined. The feed samples were moistened with 15 percent distilled water. A sample with no water and no PAQ-TivateTM served as a negative control, while a sample which included 15 percent distilled water (v/w) but without any PAQ-TivateTM served as the positive control.

All the samples were incubated at 28°C and the humidity level was maintained at 75-85 percent. Each treatment was sampled at days 1, 3, 7 and 14 from wetting. The results showed that at day 3, the levels of bacteria in the positive control samples were significantly higher compared to the PAQ-TivateTM treatments.

At day 14, both PAQ-TivateTM treatments of 0.2 and 0.5 percent feed had a significantly lower levels of bacteria compared to the positive control group (Table 1). There was no significant difference between the two doses treatments of PAQ-TivateTM.

With respect to fungal levels on day 14, the 0.5 percent feed treatment of PAQ-TivateTM had significantly lower count of fungi compared to the 0.2 percent feed treatment of PAQ-TivateTM and the positive control groups (Table 2). The 0.5% treatment of the product resulted in a significant reduction in the fungi/moulds count from day 7 to day 14.

At day 14, there was a visual growth of fungi on the feed samples of the positive control and the 0.2 percent feed treatments, whilst there was no visible growth of fungi in the 0.5 percent treatment group on day 14.

In addition to 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 terrestrial animals but there are relatively few studies in aquaculture systems.

The most important mycotoxicosis in fish and shrimp is caused by aflatoxins, primarily produced by Aspergillus spp. during storage of feed and raw materials. Propionates have been shown to prevent the formation of Ochratoxin A by Aspergillus sulphureus and Penicillium viridicatum.

Choosing the most beneficial blend

There are many organic acids available to the animal feed industry, including formic acid, acetic acid, propionic acid, lactic acid, fumaric acids, citric acid, sodium formate, butyric acids, sorbic acids and malic acids.

The question is, how do we choose the appropriate and most beneficial acid blend? In order to arrive at the correct solution, the consumer needs to consider the wanted benefits and the required technical traits (e.g., increase protein digestibility and high recovery following extruder).

In order to ensure that PAQ-TivateTM was suitable to the needs of the aquaculture feed industry, low leaching, thermal stability, small average particle size with a homogeneous distribution and easy handling (less corrosive) were characteristics considered to be of the highest priority.

The treatment was developed to contain fumaric acid in the form of its magnesium salt, formic and propionic acid and their calcium salts.

These organic acids function as feed attractants (propionate), antibacterial agents (propionate, formate), and growth promoters (fumarate) all of which are aligned with the required performance of this blend.

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