Mustard: A Natural Antimicrobial Ingredient

Summary

• Mustard has natural antimicrobial properties, the bioactive compounds – glucosinolates in mustard, are converted to the antimicrobial isothiocyanates in the presence of water
• Natural preservative functionality of mustard can be very valuable to the food industry
• Mustard isothiocyanates can effect up to a 5-log reduction of E. coli 0157:H7 in fermented meats
• Mustard Essential Oils (MEO) can be added to bakery products to inhibit fungal growth and production of aflatoxins
• Glucosinolates from deheated / deodorized (bland) mustard can be converted into highly antimicrobial isothiocyanate by bacterial myrosinase-like enzyme action present in E. coli, 0157:H7, Staphylococcus carnosus and Pediococcus pentosaceus^11,12,13 and in L. monocytogenes, Enterococcus faecalis, Staphylococcus aureus and Salmonella typhimurium

Mustard’s inherent antimicrobial properties should fit well with the food industry’s growing interest and increasing consumer demand for the use of a natural preservative to enhance food safety and increase shelf-life of prepared packaged foods with a “clean label” claim.

Mustards in Foods

Mustards (Yellow and Brown) are commercially available as whole seeds, ground/cracked seeds, meals or flour forms and are widely used in the manufacture of condiments, salad dressings, pickles, sauces, processed meats and as substitutes for egg ingredients. While mainly used as a spice or for its functional properties, mustard can also provide raw and processed foods protection against pathogenic and spoilage microorganisms.

Antimicrobial Bioactives in Mustard

All mustards, Yellow (& White) (Sinapis alba) and Brown/Oriental (Brassica juncea), contain glucosinolates. It is these glucosinolates and their isothiocyanate (ITC) breakdown products which contribute to its natural antimicrobial activity and to the heat and pungency of mustard.

The main glucosinolate in Brown/Oriental mustards is sinigrin which when hydrolyzed by the inherent enzyme myrosinase in the presence of water, yields volatile allyl isothiocyanate (AITC) which has a sharp taste and pungent aroma. Hydrolysis of the glucosinolate sinalbin in Yellow/White mustard yields para–hydroxybenzyl isothiocyanate (ƿ-HBITC) which is responsible for the hot mouthfeel (Table 1).

Table 1. Type of glucosinolate and isothiocyanate present in Yellow and Brown/Oriental mustards

Mustard	Glucosinolate	Content in Seed	Isothiocyanate
Yellow (Sinapis alba)	Sinalbin	2.3%	p-HBITC
Brown and Oriental (Brassica juncea)	Sinigrin	0.8%	AITC

Thermal processing of mustard inactivates the myrosinase enzyme, thus preventing glucosinolate hydrolysis and isothiocyanate formation and results in bland flavoured mustards more suited for incorporation into many foods.

ITCs are present in mustard as part of the mustard essential oil (MEO) component in whole seed, ground seed, flours and meals. The MEO containing AITC can be extracted from Brown/Oriental mustard and used ‘as is’ in food products. AITC from natural sources is permitted as a food preservative in Japan and as a GRAS (Generally Recognized as Safe) flavouring agent in the United States.⁵ White mustard essential oil (WMEO) containing ƿ-HBITC also has the potential to be commercialized as an antimicrobial agent complementing, but not replacing, existing process controls.⁶

Isothiocyanates, and particularly purified AITC, have been widely studied as antimicrobial agents and show a wide antimicrobial spectrum inhibiting Gram-positive and Gram-negative bacteria, yeasts and mould (Table 2). AITC is antimicrobial in both liquid and gas forms, but gaseous AITC has greater antimicrobial potency.

Table 2. In vitro test of AITC on agar surfaces containing microorganisms ^3,4,17,20

AITC vapour tested on agar surfaces
Microorganism	Temperature (°C)	MIC* (µg/L)
Bacteria
Escherichia coli 0157:H7	37-40	34-1500
Listeria monocytogenes	40	1500
Salmonella spp.	37-40	54-1500
Pseudomonas spp.	5-37	54-1500
Mould
Alternaria spp.	25	22
Aspergillus spp.	5 - 25	37-100
Botrytis spp.	5 - 25	100
Fusarium spp.	25	16-34
Penicillium spp.	25	22-62
*Minimum inhibitory concentration (µL/L is ppm or parts per million)

WMEO is more effective against pathogenic and spoilage Gram-negative bacteria (Escherichia coli, Salmonella enterica, Enterobacter aerogenes) than Gram-positive bacteria (Listeria monocytogenes, Bacillus cereus and Lactobacillus fermentum) with its activity being strain specific.¹⁴ In addition, ƿ-HBITC also showed significant bactericidal activity against other bacteria including Staphylococcus aureus, Campylobacter jejuni, Pseudomonas aeruginosa, Shigella boydii and Clostridium perfringens in media⁷ and poultry.¹⁵

Mode of Action

AITC has a multi-targeted mechanism of action that causes membrane damage and enzymatic inhibition.^2,10,11 Little, as yet, is known about the specific mode of action of ƿ-HBITC from Yellow mustard.

Supporting Research

Recent research has shown that Yellow mustard flour can be used as a natural alternative to eliminate E. coli O157:H7 from dry fermented sausage or other meats (Table 3). The goal is a 5-log reduction of E. coli viability during manufacture of fermented sausages as required by North American regulatory agencies. Cooking of fermented meat products is effective but reduces the value and alters the sensory characteristics of the sausage.⁸

Table 3. Reduction of E. coli 0157: H7 in meat formulations incorporating processed mustard powders (flours)

Food	Mustard flour processing treatment	% Mustard preparation in formulation	CFU/g reduction	No. days to achieve reduction
Dry cured Westphalian ham16	De-heated (cold) Yellow mustard flour	6%	5 log10	45
Ground beef under nitrogen-flushed packaging15	Regular (hot) Brown mustard flour	5%	0.5 log10	21 days
		10%	3.0 log10	21 days
		20%	5.1 log10	21 days
Dry fermented sausage12 (all Yellow mustard)	De-heated (cold) mustard flour	6%	5 log10	38
	Regular (hot) mustard flour	6%	5 log10	31
	Autoclaved	6%	5 log10	18
Dry fermented sausage8	Regular (hot) Yellow mustard flour	2%	3.4 log10	30 d
		4%	4.4 log10	30 d
		6%	6.9 log10	30 d
Dry fermented sausage8	De-heated (cold) mustard flour	6%	Undetectable	24d

Although AITC has been more extensively researched it has recently been found that because Yellow mustard has a greater glucosinolate content, it was a more potent antimicrobial agent than Oriental mustard. In addition, de-oiled mustard meal at 2% was as effective in eliminating live E. coli O157:H7 from dry fermented sausages during ripening as twice the concentration of mustard flour.^2,12 Deheated or deodorized (bland) mustard glucosinolates can be converted into highly antimicrobial isothiocyanate by bacterial myrosinase-like enzyme action present in E. coli, 0157:H7, Staphylococcus carnosus and Pediococcus pentosaceus^11,12,13 and in L. monocytogenes, Enterococcus faecalis, Staphylococcus aureus and Salmonella typhimurium.⁹

Use in Antimicrobial Films

Incorporation of AITC or mustard flours into food formulations at too high a concentration (>4%) may adversely affect the sensory characteristics of foods. Thus, the incorporation of AITC or mustard extracts into antimicrobial films or coatings is an alternative way to capitalize on the antimicrobial properties of mustard and control food pathogens. Researchers are investigating ways to incorporate AITC into packaging such that the continuous release of minimal amounts will prolong the effective treatment time, enhance efficacy of AITC and reduce the impact of its odour. Research examples include:

• In edible coatings consisting of k-carrageenan/chitosan coatings, the addition of AITC, mustard extracts (from B. jejuni) alone or combined with EDTA, malic or acetic acid significantly reduced C. jejuni and Salmonella on fresh, refrigerated, vacuum-packed chicken breasts and L. monocytogenes on refrigerated, cured roast chicken^18,19
• Incorporation of AITC into packaging labels inhibited the growth of fungi on cheese products²²
• AITC was incorporated into high density polyethylene (HDPE) films and when combined with modified atmosphere packaging, inhibited the growth of Salmonella typhimurium and Listeria monocytogenes in fresh chicken samples²¹

 Mustard Use Opportunities

• De-heated Yellow mustard flour could be used as an added natural antimicrobial to fermented meat to produce pathogen-free products
• Addition of AITC to edible packaging could be used to inhibit the growth of pathogens and spoilage organisms in processed meats, cheeses, nuts and fruits
• Incorporation of mustard essential oils in bakery products may be able to inhibit fungal growth and production of aflatoxins

References

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