There is an increasing trend toward delivering ‘fresh’ and ‘healthy’ food to the consumer. Yet these demands are frequently offset by lengthy processing and transport times for ingredients, intermediate products and final packaged goods.
Food producers are looking for ways to increase shelf-life without increasing the amount of preservatives that could impact the health of the customer. Shelf-life and spoilage are complex problems that need to be considered throughout the production cycle. In general, food producers are looking to replace subjective end-of-shelf-life information by objective, data-driven facts.
Factors Affecting Microbial Spoilage
Considerations need to be made for package design in order
to ensure the ingress of oxygen is limited so as to slow down
growth of spoilage organisms.
As expected, high-quality food packaging meets the needs of the consumer while controlling the overall cost. This can be viewed as the ‘optimum’ package design. The choice of materials and closures are critical.
Generally, the goal is to limit the ingress of excess oxygen into a package, since this accelerates the growth of spoilage organisms. Also, precise control of water vapour loss avoids drying and browning of the food while limiting the potential growth of fungi (yeasts and moulds).
Although there are a host of parameters to be considered when assessing the fitness of a packaged food, including texture, colour and sensory feedback, controlling microbial spoilage can reward the food producer with significant reduction in wastage and cost at the retail and consumer level.
Testing for microbial spoilage should seek to eliminate problem areas throughout the production process. This includes the grading of ingredients, the control of the microbial environment in the plant, the quality of the packaging and the assessment of the finished packaged product.
The enemy of the food quality professional is time. Traditional microbiology is time-intensive. It relies on the culturing of food samples in order to identify potential contaminants. The sampling and testing process is cumbersome, mainly manual and prone to error.
Microbial QC Issues
It is quite alarming to consider the actual amount of food that is tested in a typical QC protocol. Culture techniques including plate counts on agar might deposit only 0.1 mg of product from many kilograms in a batch. Although laboratory test protocols call for replicate testing and strict adherence to statistical sampling plans, in actuality the microbial environment might be assessed from a very small fraction of a lot.
Traditionally, microbial testing has had to balance cost with depth of analysis, particularly as the consumer has become increasingly vigilant concerning the presence of food-borne pathogens. Simple screening methods, such as the Total Viable Count (TVC) are used to make predictions as to microbial quality.
Unfortunately, these methods rarely model typical conditions for the food when it is packaged and presented to the consumer. In order to get as close to a real-world environment for testing, it is wise to look at spoilage parameters that can be viewed over time and in large enough samples to mirror actual package conditions. This is why the measurement of oxygen use by resident micro-organisms has gained acceptance as a method in determining the condition, growth and potential of aerobic microbes.
Time is critical for foods that cannot experience a heat treatment step due to their natural condition. Fresh vegetables and greens, meats for fresh processing, certain dairy foods and seafood must be processed quickly and kept in the cold chain.
Processors face a roadblock by using traditional testing methods. They normally take from 48-72 hours to produce results. In-process control becomes impossible and the final product might be released before predictive results are obtained. The risk to producers, retailers and the public is increased when the probability of a recall is high.
In a search for new methods, the food processing community has sought rapid methods in order to cut down the waiting time for objective results. Looking specifically at spoilage and shelf-life, the accurate monitoring of microbial spoilage sometimes takes such a long time and uses so many resources that it is often not checked regularly and may never be calculated at all.
In summary, the prediction of shelf-life should be quantitative, objective in terms of its assessment of actual conditions, timely and easily inserted into online production processes.
Advancing Method: Oxygen Depletion
Oxygen sensors in vials can better predict numbers of actual
live microbial cells.
Among an increasing number of rapid microbial test methods, systems that sense cellular respiration are gaining ground. For aerobic organisms, the consumption of oxygen as viable microbes breathe, or the measurement of carbon dioxide as they exhale are parameters that can be tracked continuously and automatically with high repeatability.
Respiration measurements work at the cellular level and avoid the confusion and inaccuracy that arise from counting microbial colonies on agar plates. Numbers of live colonies have poor correlation to actual live microbial cells.
The measurement of oxygen consumption (depletion) as a rapid method has benefited from the development of precise oxygen sensors that reside within the sample food test vial. These polymer sensors produce repeatable fluorescent light signals as oxygen level changes. While measuring the aggregate of cells that are breathing while under incubation, the sensors can be read by optical means from outside the vial.
Another advantage of real-time oxygen sensors is the range of information that can be gathered from one cell to 100 million cells without the need for dilution of the samples. Such a dynamic range decreases the preparation time and cost for each sample. Each vial is typically up to 15 ml in volume, which increases the amount of actual product compared to the lot.
The use of optical technology means that all the incubation and measurement is continuous and computer controlled with each result being displayed to the lab scientist when it available, without waiting for the end of a fixed incubation cycle.
Case Study Examples
Extended Shelf-Life Milk (ESL)
There is a trend toward producing liquid milk with a taste superior to those ultra-high temperature processed. The products are only partially heat treated and then kept in the cold chain for distribution. Dairy experts think that potential spoilage bacteria are most likely psychrotrophic, which means they grow well at cold temperatures.
The ESL dairy technologies produce milk with low bacterial counts, but not zero. The organisms are hard to detect and count on traditional agar plates and the tests run for 72 hours. This causes cold chain storage at high costs but sometimes the product is still shipped after receiving clearance from the microbial QC.
False negatives occur due to the inability of agar plate testing at psychrotrophic temperatures to detect certain spoilage organisms. Therefore the spoilage occurs much later when the product is on the retail shelf.
In an industrial study, an oxygen depletion technology was used to improve the detection of spoilage bacteria while reducing the test time to below 24 hours. Results from extensive QC samples showed that the actual rate of positive detection for spoilage doubled and the average positive reading was achieved in 11 hours. This provided the dairy company with much reduced cold chain storage costs and an increased confidence that retail spoilage and recall also would be reduced.
Grading Of Raw Meats
Sometimes the final customer expects totally freshly produced foods that are to be made raw with no heat processing until they are consumed. The burden of safety is placed on the processor. In the case of certain sausage products, there are several suppliers of raw meat and so the lowest bacterial level ensures the least spoilage throughout the process. Even if there are heat treatment processes, these are not absolute and it is possible to retain bacterial presence in the meat that will accelerate spoilage.
In an industry study, five pork meat suppliers were graded on the bacterial load in samples from their deliveries. An oxygen depletion technology was used to test the samples when they arrived at the processor, very early in the morning.
Within five hours, and with a pre-assigned pass/fail level, the rapid oxygen depletion method gave acceptance or rejection results before the lots were taken from the cold storage and used for sausage processing. This resulted in an objective, statistics-driven assessment of each meat supplier over many weeks and was a reason for continuous improvement.
Yeast/Mould In Yoghurts
In a university study, it was proven that an oxygen-depletion rapid method could reduce the testing time for spoilage in yoghurts by 80 percent.
Yoghurts are popular, individually packed foods that families enjoy because of the variety and healthy contents. However, the food and the packaging are highly susceptible to fungal contamination that gets into the products by multiple paths.
Excess moisture in the process can condense under the foil caps of containers and the fruit ingredients can carry mould contamination because of inadequate cleaning. Yeasts and moulds have a very slow respiration cycle and can cause bad taste, discolouration and sometimes illness.
In order to assess a new rapid method for yeast and mould detection, a European university deliberately contaminated retail yoghurts and determined the typical time to detect very small amounts of contamination. The rapid method successfully detected yeasts and moulds to a much lower level than the traditional testing techniques, all in 24 hours when compared to five days of testing with the alternate method.
New rapid methods such as oxygen-depletion can be used to measure shelf-life and produce objective data for food processor to use. They reduce costs of recalls and can return results before excessive costs are incurred for cold storage or return to the manufacturer.
These technologies feature reduced laboratory costs and improved sensitivity to low level contamination. Rapid methods, when used in a rigorous analysis of packaging and food process, can improve quality and reduce wastage.