External factors of shelf life
The external factors of shelf life. All bakery products are stored for a short or longer period of time. The temperature at which this is being done varies from -21 ̊C to ambient temperatures (sometimes 10 ̊C and sometimes 55 ̊C). The effect this temperature has towards effects such as staling, is mostly topic of concern, where we will focus here on the microbiological part.
Microorganisms have each their own specific range of temperature where growth is accomplishable. The most important requirement is that water should be in liquid state and be available to support microbial growth. In general, microorganisms are not able to grow at temperatures below -8 ̊C and above 100 ̊C. No single organism is capable of growth of whole of this range. Bacteria are normally limited to a temperature span around 35 ̊C and moulds around 30 ̊C.
Baking & Cooling
Most bakery products are exposed to high temperatures during baking apart from e.g. crèmes, jams and fruits that are added after the baking process. During baking, almost all microorganisms will be eliminated. Only highly heat resistant toxins are possibly produced in an early stadium of the production process can survive. When the temperature is increased above the maximum for growth, cells are injured and killed as key cellular components are destroyed and cannot be replaced. This occurs at an increasing rate as the temperature increases.
Chilled storage implies temperatures near, but above the freezing point, approximately 0 – 5 ̊C. Chill storage can change both the nature of spoilage and the rate at which it occurs. Low temperatures exert a selective effect preventing the growth of mesophiles and leading to a microflora dominated by psychotrophs. As these psychotrophs are not in their optimum temperature range, the growth is slow, delaying further spoilage.
During freezing, formation of ice crystals within the product changes the availability of water to participate in reactions. As the temperature is reduced and more water is converted to a solid state, less water is available to support deteriorative reactions. Table 3 shows the effect of freezing on the water activity of pure water and water-ice. In most cases approximately 10% of the water remains liquid if the product is frozen. As far as microbial safety is concerned, the reduction of water activity during freezing has only significant influences on shelf life at freezing temperatures where microbial growth is possible, above – 8 ̊C. The temperatures used in frozen bakery storage are generally less than -18 ̊C. At these temperatures, no microbial growth is possible, although residual microbial or endogenous enzyme activity such as lipases can persist and eventually spoil a product.
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