Bringing Home the Bacon: Bio-protective Cultures Prevent Spoilage

Cooked bacon is a widely popular food with a rich Italian heritage. Following the cooking process, meat producers vacuum pack the bacon, pasteurize it and finally store it for a maximum of 90 days (the recommended shelf life). During storage, any microorganisms surviving the pasteurization process can seize the opportunity for growth, resulting in spoiled bacon.

To better understand which microorganisms are to blame for spoilage and if it can be prevented, the team examined bacon for signs of spoilage over time. Beginning with 200 cooked pieces of bacon from an artisan meat plant, the team stored bacon at 4 ± 2°C watching for signs of spoilage, including greening parts, a slimy texture, foul smell, and slightly inflated packaging. On day 30, the researchers randomly selected 10 out of 40 spoiled and 10 out of 160 unspoiled cooked bacon samples. At 90 days, they again analyzed 10 spoiled and 10 unspoiled samples. Interestingly, at just 30 days, the researchers saw signs of spoilage in 40 samples (20% of total) while the remaining 150 samples remained unspoiled until the end of the 90-day shelf-life.

Arming themselves with a list of likely microbial suspects, the team plated samples in triplicate on agar plates as follows:

Analysis	Plate Used	Culture Conditions
Total viable count (TVA)	Thermo Scientific Oxoid Plate Count Agar	30°C for 48 hours
LAB (Lactic acid bacteria)	Thermo Scientific Oxoid de Man-Rogosa-Sharpe (MRS) Agar, pH 6.2	30°C for 48-72 hours, in a microaerophilic environment
Clostridia	Differential Reinforced Clostridia Medium	37°C for 24-48 hours in an anaerobic jar

Additionally, the team also checked for presence of Salmonella species and Listeria monocytogenes. For Salmonella spp. they used the ISO 6579 method; following a pre-enrichment, selective enrichment, and selective isolation, the team confirmed any presumptive positive results using the Salmonella Latex Agglutination Test. For L. monocytogenes, they used the ISO 11290 method, which involves a primary selective enrichment, a secondary selective enrichment and finally a selective isolation obtained by streaking the secondary enriched culture onto Chromogenic Listeria Agar (according to Ottaviani and Agosti (ALOA)).

To analyze volatile compounds found in the inflated packages, the team used solid-phase microextraction (SPME) coupled to gas chromatography on a Finnigan Trace DSQ (Thermo Scientific).

From the cultures, lactic acid bacteria (LAB) had a significantly higher concentration (p < 0.05) in the spoiled bacon compared with unspoiled. After further molecular identifications, the researchers determined Leuconostoc mesenteroides is responsible for spoilage in bacon. It produces a greening color of the meat, slime and various compounds that result from the fermentation of sugars and the degradation of nitrogen compounds.

Switching gears, the team evaluated bio-protective cultures to both improve the quality of the product and prevent spoilage by L. mesenteroides. They divided new pieces of cooked bacon into three groups, and inoculated each group as follows: (1) bacon inoculated with a suspension of L. mesenteroides as a control group, (2) bacon inoculated with a (1/1) mixture of L. mesenteroides/Lactococcus lactis and (3) bacon inoculated with a (1/1) mixture of L. mesenteroides/ Lactococcus sakei. Following the inoculation, the team vacuum packed the bacon and again stored it for 90 days at 4°C.

As a result, the L. lactis and L. sakei bio-protective cultures prevented L. mesenteroides from growing, thereby eliminating spoilage and changes in color, smell, and texture. There is hope that this “fight fire with fire” type approach employing bio-protective cultures can also translate to other food matrices.

Learn more about culture media and molecular techniques for food microbiology in our food and beverage community.

Reference

Comi,G. et al. (2016) “Lactococcus lactis and Lactobacillus sakei as bio-protective culture to eliminate Leuconostoc mesenteroides spoilage and improve the shelf life and sensorial characteristics of commercial cooked bacon.” Food Microbiology, September 2016, (pp. 16–22)