CHAPTER 7

GENERAL JUICE PROCESSING PRINCIPLES

7.1 Juice spoilage

Freshly expressed juice, is highly susceptible to spoilage, in fact more so than whole fruit. Unprotected by skin or cell walls, fluid components are thoroughly mixed with air and microorganisms from the environment. Thus, unheated juice is subject to rapid microbial, enzymatic, chemical and physical deterioration. Table 7.1 shows the major categories of juice spoilage. The goal of processing is to minimize these undesirable reactions while still maintaining and in some cases enhancing, the inherent quality of the starting fruit.

Table 7.1: Juice safety and deterioration hazards.

The major concern must be microbial deterioration. The ubiquitous nature of microbes dictates that no matter how sanitary the raw material, juicing equipment and facilities are, the normal fruit microflora represents inevitable spoilage potential. The low pH of most juices favours yeast that rapidly initiates an incipient fermentation. Most civilizations have mastered this type of spoilage and directed it toward the production of alcoholic beverages. However, left uncontrolled it represents spoilage. Furthermore, the presence of aflatoxin producing mould contamination on the surface of fruits generally ends up in the juice.

Certain aflatoxins are potent liver carcinogens (Ashurst, 1995) and commercial standards dictate parts per billion limits on aflatoxin levels, thus restricting the use of mouldy fruit. This is another valid reason for GAPs, sound harvest selection and inspection practices.

In fact, until recently the uncontrolled fermentation of improperly handled low pH juices was wasteful, but not deemed particularly dangerous. However, over the last decade fresh juice has increasingly been the source of serious food poisoning outbreaks and fatalities. Unpasteurized juice has been implicated in outbreaks of Salmonella and emerging pathogens such as E. coli O157 H7 (Table 4.1). These incidents have resulted in far stricter sanitary and labelling requirements for commercial fresh juice producers.

There are several reasons for increased fresh juice food poisoning incidents:

• greater use of manure as fertilizer for fruit crops, a particular problem for organically grown produce,
• increased demand for and consumption of fresh juice as compared to pasteurized and frozen concentrates. The "back to nature" movement promotes a strange philosophy regarding common food processing practices,
• a larger number of individuals are immuno-compromised and quite sensitive to low numbers of pathogens. The very young, very old, pregnant women, transplant patients and those with chronic disease conditions are extremely susceptible to food poisoning, compared to healthy individuals. The consequences of infection are correspondingly more serious,
• the pathogens are also becoming more robust and resistant to preservation techniques,
• efficient manufacturing and distribution systems insure widespread, prompt delivery and rapid consumption of juice products, both safe and tainted,
• detection techniques based on molecular biology can now rapidly identify very low numbers and "fingerprint" specific pathogens or their metabolic products,
• the news media is also more efficient in detecting and publicizing food sanitation failures.

Along with safety considerations, quality factors are also important. Sound fruit, reasonably free from microbial contaminants are subject to biochemical deterioration upon juicing. The mixing of fruit enzymes with substrate and air can rapidly initiate enzymatic browning. Plant phenols, polyphenol oxidase and oxygen react to darken many juices. There are many other enzymes active in juice capable of destroying ascorbic acid, modifying pectin and affecting colour, flavour and texture. So rapid processing and the use of heat or enzyme inhibitors are necessary with some juices.

Even barring microbial and enzymatic changes, other chemical reactions involving oxygen, metal cations and other juice constituents can occur to modify sensitive pigment, taste or aroma substances. There are literally hundreds of reactive compounds in the simplest of juices, so the reaction possibilities are enormous (Tables 3.1, 3.2, 7.1).

Another storage limiting factor affecting most juices is the common sugar-amine or Maillard reaction. Reducing sugars and amines, ubiquitous to plant cells and hence juice, slowly go through a series of steps to form brown pigments. In the baking of bread Maillard products are responsible for the desirable crust colour and flavour, but not in juices. Low temperature greatly retards Maillard browning, but does not stop completely these
undesirable reactions.

Chemical contamination can also occur from the environment. The unauthorized or excessive use of pesticide chemicals is the most common and avoidable source, readily preventable by GAPs. Such contamination is particularly serious in organic juices where no pesticides are permitted. Even trace amounts of innocuous substances present in soil, water, or wind drift can cause rejection. Although the health hazard is trivial, the analytical sensitivity insures detection. Mistakes anywhere in the food chain are more serious. Unlabeled lethal white powders have been mistaken for food ingredients and added to juice, resulting in fatal poisonings.

The use of nonfood grade equipment in the processing line is a relatively minor safety concern that still impacts juice quality. Metals such as copper, bronze, aluminium, iron, galvanized, steel (except stainless) are easily attacked by fruit acids and, in turn contribute metal ions to the juice. These ions can have pro-oxidant properties and adversely affect flavour, colour, clarity and nutritive value. In addition, flexible connections and all food contacting surfaces should be stainless steel or food grade plastic, since off flavours and unauthorized plasticizers could leach into the juice. Certainly, contact with toxic metals such as lead, mercury, cadmium and zinc must be rigorously avoided. Food grade surfaces are significantly less available and more expensive than easily obtained "hardware store" alternatives, but much more durable, safe and ultimately cost-effective.

Physical changes can also take their toll on juice quality. Separation of dispersed particles can help or hinder appearance. Clear juices can turn cloudy or release an unsightly precipitate. Chemically or freeze-thaw induced colloidal reactions can affect juice viscosity to thicken or thin the consistency and influence the taste. In view of the numerous pathways to juice spoilage and delicate nature of juices, it is an impressive testimony to food science and technologies that juice stabilization techniques work as well as they do. Indeed, through most of history, the controlled spoilage (fermentation) to alcoholic beverages such as wine, was the tenuous but only preservation alternative.