Microencapsulated Sorbic Acid: The Winning Formula

    With modern consumer demands for high quality, long-life bread and global pressure to reduce food wastage, controlling the un-wanted growth of mold is an important factor when considering product formulation. The sorbic acid salts are one of the most widely used groups of preservatives in the world exhibiting highly effective anti-fungal properties. Despite sorbic acids potential in being a highly effective bakery preservative historically, it has not been used in yeast leavened products due to its efficacy against the leavening yeast itself, leading to a reduced proof and a product with an inadequate volume. This white paper explains how encapsulation technology allows sorbic acid to reach its full potential as a preservative for the bakery industry. It also outlines the evaluation of three commercially available sorbic acid products determining whether their production process and specifications have an impact on their efficacy as a mold inhibitor in bread.

    Sorbic acid & encapsulation technology

    Sorbic acid, an organic acid first isolated from rowan berries, is the most effective anti-fungal compound within the sorbic acid salts group. The acid shows efficacy against a broad range of spoilage microorganisms; molds, spoilage yeasts, and microorganisms such as Bacillus subtilis, which is the main cause of rope in bread, are all effectively inhibited by this compound. As mentioned, the main problem in relation to yeast-raised bakery products is that sorbic acid has such a profound negative effect on yeast activity that it cannot be used directly. Encapsulation solves this problem. Sorbic acid crystals are surrounded by a lipid barrier material, separating the sorbic acid from the yeast and allowing it to function with minimal inhibition. The encapsulation process provides a method for delivering the sorbic acid which maintains the proving times and the volumes expected from the modern bread maker. During the baking phase, when the yeast has performed its function, the lipid encapsulating material melts, releasing the sorbic acid to perform its function as an effective mold inhibitor. Encapsulation technology can be applied in various ways, two examples being a “core-shell” and a “matrix”. The core-shell approach covers the sorbic acid with a layer of encapsulating material. The core shell particles can be manufactured by processes such as fluidized bed, granulation, or pan coating. The matrix particles can be manufactured with processes such as spray drying, spray chilling, rotating disk atomization, or prilling. There are several approaches to encapsulation therefore there are a number of sorbic acid products currently available on the market. These products vary significantly in terms of their size, granulation and level of encapsulation.

    “The matrix particles can be manufactured with processes such as spray drying, spray chilling, rotating disk atomization, or prilling”

    Mold free shelf life trials

    Three commercially available sorbic acid products were chosen for the trial, each exhibiting different structural properties:
    1. SorbicPlus, a matrix of microencapsulated 50% sorbic acid (50:50, sorbic acid:fat)
    2. A core-shell coated granular 85% sorbic acid (85:15, sorbic acid:fat)
    3. An uncoated granular 100% sorbic acid.
    The level of sorbic acid was adjusted for each sample to ensure that all products had an equal level of preservative, and an intermediate dose of 0.1% flour weight (f/w) was chosen. This translated to 0.1% (f/w) of uncoated granular 100% sorbic acid, 0.118% (f/w) of coated granular 85% sorbic acid and 0.2% (f/w) SorbicPlus. A control was also produced without the addition of sorbic acid. All preservatives were added to a typical UK white pan bread formulation. The samples were observed daily for signs of mold growth. Samples were deemed to have expired when ≥2x2mm mold colonies were visible. Any samples which had not expired after 30 days were categorized into a “30+ days” category. This procedure was carried out in triplicate for all sorbic acid samples. The results of all three batches were compiled (n=45) to produce the data used for each sorbic acid sample in this study. For the purpose of this trial, the point at which the first individual loaf expires (n=1) marks the sample’s expiry, measured in P+ days. The mold free shelf life (MFSL) for the product is the sample expiry minus 1 day.

    MFSL trial results

    As illustrated, there is a high degree of variance between the performances of the three sorbic acid products:
    Samples expired from P+4 to P+10 days.
    The control had the shortest MFSL of 3 days.
    Granular 100% sorbic acid gave a MFSL of 7 days; this sample was the poorest performing sample of sorbic acid within the group.
    The coated granular 85% sorbic had a MFSL of 8 days.
    The preservative showing the highest performance was SorbicPlus, with a MFSL of 9 days.
    It should be noted that there were a considerable number (10 out of 45) of the SorbicPlus samples which achieved a MFSL of ≥30 days, indicating that the samples showed an extremely high resistance to the stresses of microbial growth. Achieving a guaranteed MFSL of ≥30 days in white pan bread could be of considerable benefit to the bakery industry, therefore further investigating this potential of SorbicPlus to give a guaranteed 30 day MFSL may be the focus of future work. This could involve the use of higher levels of SorbicPlus, or a combination of SorbicPlus and another preservative for a potential synergistic effect. 

    “…the size of sorbic acid particles in a sample affects both the number and distribution of particles in white pan bread”

    The impact of particle size on preservative coverage

    Considering the preservative used in all sorbic acid samples was chemically identical, and at the same % level, the results indicate that there may be another factor influencing the performance of sorbic acid products as mold inhibitors. As previously mentioned, the sorbic acid samples differed in size and granulation; it is likely that the size and granulation of particles may have an impact on the dispersion of particles in the end application. To investigate whether this is true in pan bread, a testing process was devised to illustrate the distribution of sorbic acid within the crumb structure of the bread. MFSL trials highlighted how different methods of encapsulation resulted in varied expiry times; coarser products had a shorter MFSL than the finer products, despite the same level of sorbic acid being used.7 This has been thought to be due to coarser microsphere distributions having less microspheres per unit of mass, The spheres labelled “B” have a diameter which is half that of the sphere labelled “A”, yet 8 of the “B” spheres have the same amount of material as 1 of the “A” spheres. This means fewer microspheres in each loaf of bread when using more granular products, which is believed to cause zones in which no sorbic acid is present. The theory behind these “sorbic zones” is that as the loaves are baked, the high temperature causes the sorbic acid to migrate out of the spheres and deposit throughout the loaf, expanding from each microsphere to ideally form a network of spherical zones of influence which overlap to provide continuous coverage of preservative throughout the loaf. With a coarser distribution of particle sizes, a lower number of particles are present in each loaf in comparison to a finer grade of microspheres. This means that there are fewer, but larger spheres of preservative coverage, which causes gaps in the loaf where there is no preservative coverage. The basis of the testing was to somehow show the difference in distribution based on three different delivery methods.

    ‘Hot-Spots’ caused by granular particles

    To be able to visualize these sorbic zones, pH sensitive dyes were used to illustrate how acidic or alkaline a product was, i.e. the dye changed color depending on whether or not there was sorbic acid present. If there is a lone zone of sorbic acid influence, it shows up as a red spot in an otherwise purple loaf of bread. The aforementioned “spots” are clearly visible. From the results of this study, it can be concluded that the size of sorbic acid particles in a sample affects both the number and distribution of particles in white pan bread. Larger particles give fewer particles per loaf and reduced distribution; finer particles give more particles per loaf and improved distribution.