Many reasons are given to explain why mechanised methods of processing food products have replaced labour intensive systems in numerous food processing and packaging industries.

Cost efficiency is certainly high on the list along with reliability, consistency, food hygiene, quality, security and customer preferences. An innovative technology developed in South Africa now adds humanitarian reasons to this list.

The familiar cashew nut is the seed of the plant that is related to poison ivy and thrives as a fast growing tree in tropical and sub-tropical climates. The raw kernel is protected by Cashew Nut Shell Liquid (CNSL) contained between an inner and outer shell.

Allergenics such as phenolic resin and urushiol, together with the potent skin irritant anacardic acid, are components of this rather unpleasant fluid that does, however, preserve the kernel from the ingress of water and deters insects very effectively.

Traditional methods of extracting cashew kernels involve immersion in water followed by air drying to an optimum moisture content of 9%. Each nut is then hand massaged and cracked via a manual process that entails putting the nut against one sharp blade and bringing another blade, which is operated by a foot powered lever, through the outer shell – separating it from the nut so that it can then be carefully picked from the outer shell.

The efficiency of experienced workers in terms of kernel material yield and the proportion of whole kernels extracted have proved impossible to match by the many automated mechanical systems that have been developed. The problem is that manual technology as practiced in India, the world’s largest producer of cashew kernels, exposes large numbers of workers to unhealthy levels of CNSL, resulting in sensitisation and allergic dermatitis.

Cashew Technology International (Pty) Ltd holds the patent to a process that reliably extracts whole cashew kernels at up to 500kg per hour and simultaneously minimises the risk of human exposure to CNSL. Shells are embrittled by immersion in liquid nitrogen, then fed automatically at the optimum temperature into an impact chamber where they are fractured.

A multi-step preparation process precedes immersion that is critical to its effectiveness in achieving up to 99% yield of whole cashew nut kernels. Mike McChesney explains that although the nitrogen consumption ratio of 1:1 has been demonstrated, the process is economically feasible even at twice this rate.

Although the cleaning of raw materials and sizing and kernel grading have remained labour intensive manual operations, this cryogenic process has the potential to significantly reduce the cost of processing cashews and could be the basis for the recovery of the African cashew business. One obvious challenge though, is that liquid nitrogen is not readily available in many of the tropical zones where cashew trees are grown and is expensive to transport over long distances.

Our daily bread
It is common knowledge that yeast causes bread dough to rise by generating carbon dioxide gas bubbles, which creates the sponge-like texture required. Less well understood is the interesting interaction of three atmospheric gases in the process of baking bread.

The fact is that as carbon dioxide evolves within the body of dough, being readily soluble in water it simply dissolves into the aqueous component and does not form a single new gas bubble.

The correct mixing of dough not only ensures that ingredients are completely dispersed to form a homogenous mixture, but profoundly affects the leavening process by entraining air bubbles in the dough. The oxygen in these trapped air bubbles is rapidly consumed by the yeast and typically will be reduced to undetectable levels by the end of a three to five minute mixing period.

The remaining trapped gas is of course nitrogen and it is critical to the process by providing bubble nuclei into which carbon dioxide diffuses as it is expelled from the soon saturated solution.

Flexibility to control the population of bubbles in dough is provided by dough mixers that can manipulate the atmospheric pressure in the headspace. At pressure above atmospheric, more air is occluded resulting in larger nitrogen bubbles and an open cell structure suitable for baguettes is the result, whereas mixing under conditions of partial vacuum will produce the fine and uniform cell structure desired in sandwich bread.

The Chorleywood Bread Process is almost universally used by modern commercial bakeries, because bread dough is developed during high speed mixing by intense mechanical working of the dough in just a few minutes. The dough temperature must be carefully controlled at 28ºC to enable optimum yeast growth consistent with good dough elasticity.

High ambient temperatures and warm ingredients can cause the dough temperature to overshoot this limit and it is common to employ a range of cooling techniques, including the injection of carbon dioxide.

Additional carbon dioxide is not harmful to the dough, because it is produced naturally by the fermenting yeast, but as a precaution it is normally blown in as a cryogen snow to avoid the damage that direct contact with colder liquid carbon dioxide might cause to the yeast.

Preserving the fruit of the vine
Modern winemaking embodies a scientific understanding of all of the traditional processes and techniques of the trade, but many of these are now applied in completely new and unconventional ways.

The fundamental challenge through the many steps between picking grapes and ageing wine in the bottle is to preserve the quality of the wine by preventing premature or uncontrolled oxidation. The threat of oxidation, of course, is ever present from the 21% oxygen component of Earth’s atmosphere.

Carbon dioxide applied in the form of ‘snow’ chills the grapes directly after picking and by slowing down chemical activity prevents premature fermentation. As the gas sublimes it completely surrounds the grapes, replacing air in the container, protecting them while in the crusher and during transfer to maceration tanks, where the juice is allowed to absorb colour and flavour compounds from the skins.

Far colder liquid nitrogen is sometimes preferred, because it can reduce the temperature sufficiently to partially freeze the water contained in grapes before crushing.

These gases are delivered and stored as cryogenic liquids and can be supplied in liquid form through insulated pipes to the press and to other points in the process. Prevention of oxidation is continued throughout the winery by diffusing small bubbles of either nitrogen or a blend of nitrogen with carbon dioxide through the wine or must, as a sparging agent to release dissolved or entrained oxygen. The noble gas argon although more expensive is sometimes preferred for its chemically inert properties.

After the malolactic fermentation stage where bacterial action is used to reduce the acidity of certain wines, minute quantities of oxygen are bubbled through tanks of red wine to stabilise its colour, integrate aromas and ensure softer, richer tannins.

Atmosphere composition in all pipelines and the ullage of storage tanks is monitored and managed to eliminate sources of oxidation, pipelines are purged and even the air in empty bottles waiting to be filled can be removed, by the use of a liquid nitrogen droplet dispenser.

A heady blend for your brew
Traditionally, carbon dioxide was the only gas associated with the brewing and dispensing of beer, because it is produced naturally by yeast during the fermentation process and is responsible for the characteristic effervescence of ales, lagers and pilsners.

Gas available from dissolved carbon dioxide not only expels oxygen from storage barrels, pipes and dispensing systems and so reduces the risk of oxidation and spoilage, but also provides pressure to push the beer up through the tap.

Additional gas is required to tap beer barrels completely, as unfortunately pure carbon dioxide can deliver rather disappointing results. The solubility of carbon dioxide in aqueous mixtures like beer increases with increased pressure and reduced temperature, making the system inherently difficult to balance.

Too little pressure will allow gas to escape from the beer resulting in a ‘flat’ pour, while excess pressure will cause the opposite. Often, residual beer is discarded in the barrel because it is over-gassed and only froth can be poured.

A convenient solution developed by the industrial gas industry and now widely used is to employ a blend of carbon dioxide with inert nitrogen. Relatively inert nitrogen is introduced to boost the gas pressure and being virtually insoluble, this relieves the problem of excess dissolved carbon dioxide.

Experience has shown that a blend of 40% nitrogen with 60% carbon dioxide will maintain the equilibrium in ales and lagers enabling efficient pouring of optimally gassed beers with little residue and wastage.

Stouts may be described as nitrogenous beers and typically they have lower carbon dioxide content, thus demanding lower partial pressure to maintain equilibrium. However, adequate pressure is still required to propel the beer through the dispensing system, presenting an even stronger case for the use of blended gas.

Typically 70% nitrogen with 30% carbon dioxide will produce the thick and tight-knit foam characteristics desired with nitrogenous beers, and the greater available pressure enhances the cascading effect and creamy froth in the glass.