How to store and preserve the quality of Nuts and Dried Fruit

How to store and preserve the quality of Nuts and Dried Fruit

How to store Nuts?

You can properly store nuts and dried fruit by following a few simple rules, that take into account the products' nutritional characteristics and their storage conditions. In this article, we're going to explain step by step how to properly store your favorite nuts and dried fruit.

  • What happens during processing:

The shelf-life (i.e. the period of time during which the product maintains its qualitative characteristics under normal conditions of storage and use) depends on several factors that concern both the characteristics of the fruit - such as its nutritional characteristics and the water content inside it (AW) - and external variables - such as temperature, relative humidity (RH%) of the storage environment, exposure to light and oxygen, types of processing that the fruit has undergone (drying, roasting, etc.), any infestations (insects, mites, cockroaches, Lepidoptera).

These two types of factors work together, so they both must be taken into consideration for optimal food storage.  Estimating the shelf-life of food is often difficult because it mainly depends on how a product is stored.

  • Nutritional characteristics

There are differences between nuts (e.g. hazelnuts, pistachios, pine nuts, etc.) and dried fruit or dry fruits (pineapple, coconut, dates, berries, etc.):

Nuts are mainly made up of kernels, that are rich in oils (called lipids) acting as an energy reserve for the future seedling. These lipids are mostly made up of unsaturated fatty acids which are easily affected by acidification and rancidity. The rate of oxidation, which gives rise to rancidity, increases as the quantity of polyunsaturated fatty acids increases: for example, walnuts, which contain more of these, are more likely to be subject to oxidation than hazelnuts, which contain far less polyunsaturated fatty acids compared to monounsaturated fatty acids and therefore are far more oxidation-resistant.

Dried fruit, on the other hand, has had most of its original water content removed and consists mainly of simple sugars and fibre. These sugars can undergo caramelization and Maillard's reaction, which give them a dark colour (browning). Even proteins can turn brown and become harder due to the loss of the ability to reabsorb lost moisture from the environment.

  • Relative humidity

The relative humidity (% RH) in the storage environment influences the water content available inside the fruit. If it exceeds 65%, the kernels tend to reabsorb the water from the atmosphere thereby leading to enzymatic hydrolytic acidification through lipases (enzymes that breakdown the lipids). The resulting release of fatty acids facilitates self-oxidation phenomena. High % RH levels also modify the mechanical strength of the kernel, which loses turgidity and becomes rubbery. By contrast, % RH levels that are too low (around 20% or less) cause excessive weight loss in the kernels, which then become fragile and easily damaged during handling, quite apart from the fact that excessive dehydration increases lipid oxidation.

Excessive humidity also encourages the growth of moulds (especially of the genus Aspergillus spp.), which produce aflatoxins (AF) that are hepatotoxic and immunosuppressive substances. Among all aflatoxins, the most dangerous is B1, the most powerful known natural carcinogen.

  • Temperature
  1. This conditions the growth of moulds, which grow best at a temperature between 6 and 46°C.
  2. It is a useful tool for mitigating infestation risk. Many infestations are subtropical in origin and therefore do not tolerate very low temperatures very well.
  3. It keeps the bacterial load under control since most bacteria prefer environments with temperatures between 20 and 40°C.
  4. By raising the temperature, the speed of a chemical reaction increases, so lowering the temperature means decreasing the rate at which food decomposes.
  • Exposure to light and oxygen

For the growth of obligate aerobic bacteria, oxygen is vital. To overcome this problem, the fruit is packaged in a modified atmosphere, from which oxygen has been removed and inert gases, e.g. N2, are used instead. Furthermore, under normal conditions oxygen would react with some components of the food, thereby changing its organoleptic characteristic, so using N2 slows down browning and rancidity, keeping the aroma of the fat fraction unchanged. Then there are certain substances that are photosensitive and degrade in light. Moreover, light is one of the main causes of oxidative alteration. It emerged that by storing in the dark the level of lipid oxidation is drastically reduced, despite the ambient atmosphere.

To store Nuts and Dried Fruit at home, just follow these simple rules:

After buying Nuts and Dried Fruit that you intend to keep for a long time, the overriding rule is to place the fruit in a cool, dry, dark place, away from sources of heat and humidity.

The best storage condition is a refrigerated environment, whether the product is packaged or not. Indeed, refrigeration can make nuts' and dried-fruits' shelf-life last longer, up to several years. The product can also be stored at room temperature during the winter season given the low temperaturesDuring summer, however, it is advisable to store the product in the refrigerator or in the coolest possible place, as increased temperatures could encourage decay.

The best storage container for nuts and dried fruit is glass. Because of its composition, glass is impenetrable to chemical and gaseous agents; furthermore, since it has excellent thermal insulating properties, it maintains the initial temperature far longer than other materials. It is also made up only of mineral salts, so there is no danger of transfer of organic materials. It is even better if the glass is coloured: using coloured glass blocks the entry of certain wavelengths of light (including ultraviolet), and thus certain nutritional and organoleptic characteristics remain unaltered.  They help us even if the product is in non-ideal conditions, such as exposure to heat or sun.

The type of closure of the container is also important as an airtight cap ensures that the food is protected from excessive contact with oxygen that can lead to lipid oxidation and encourage the growth of aerobic bacteria.

The best possible storage environment is a well-ventilated place, because ventilating the premises keeps internal humidity under control, thus guaranteeing the right conditions to prevent mould growth. 

Source:

Contini M., Cecchini M., Massantini R., Monarca D., Moscetti, R., Proprietà nutrizionali-salutistiche e conservazione della frutta secca in guscio, Italus Hortus 17 (4), 2010.

http://www.federica.unina.it/agraria/igiene/deterioramento-alimenti

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