Harvesting Conditions

The fruit harvesting conditions can also impact the flora present in the juice or must. Once grape surface organisms are brought into contact with the nutrients of the grape, they can accelerate growth and metabolism. Which organisms are favored depends upon the species originally present, how quickly they can adapt to the new growth conditions, how quickly they can grow and produce metabolizing biomass, and how they are able to inhibit other organisms in the environment – more rapid depletion of essential nutrients will favor a competitor, as removal of oxygen will favor fermentative species.  Another factor conferring competitive advantage is the tolerance to grape osmolarity and pH.  Changing conditions favor organisms that are the quickest to adapt and resume growth.  Saccharomyces is rarely found on grape surfaces that are not leaking sugar because this yeast quickly adapts metabolism to sugar consumption and growth.  It rapidly consumes molecular oxygen in this adaptation process, making conditions even more hostile to other organisms present. It also is tolerant of the low pH and high phenolic content of grape exudates and juices, so is not disadvantaged by the presence of these compounds. Although the production of ethanol is often cited as conferring a growth advantage to Saccharomyces, this yeast normally dominates the fermentation long before an inhibitory ethanol concentration is attained.  Breaking of the fruit and release of sugar favors the yeasts, in general, and the lactic and acetic acid bacteria over the other yeasts, bacteria and molds present on the berry.

Fruit Damage

In general, the greater the damage to the fruit during harvest, the higher the microbial populations at the beginning of fermentation. If there is significant damage and release of growth nutrients prior to reception at the winery and use of antimicrobials such as sulfur dioxide, the initial burst of growth will be due to the non-Saccharomyces yeasts of the genera MetschnikowiaHanseniaspora and Candida, depending upon the growing region. The lactic acid bacteria may also be present as they can grow under anaerobic conditions; but which species are present and able to persist depends upon two critical factors: the pH and the temperature. pH values below 3.5 favor a bloom of yeast and low temperatures, below 20 °C also favor yeast over bacteria. Saccharomyces does well at warmer temperatures so very low temperature fruit (below 10-12°C) favors the wild non-Saccharomyces yeasts. If there is significant oxygen exposure to the damaged fruit, blooms of acetic acid bacteria may occur, producing acids that might be inhibitory to initiation of fermentation by Saccharomyces. Sour rots are often dominated by bacterial flora and can lead to fermentation problems. If fruit is held post harvest, significant mold growth can occur. The molds may consume micronutrients needed by the yeast or produce mycotoxins that will inhibit yeast growth.

Damage to the fruit may also serve to activate the enzyme polyphenol oxidase which consumes molecular oxygen. The depletion of oxygen will inhibit the adaptation of fermentative organisms from an environment where oxygen is plentiful to one in which it is restrictive or absent. Adaptation to new conditions typically requires turnover of existing metabolic pathways and the synthesis of new ones, and the more nutrients available at this time, the faster the adaptation. Also, the more gradual the change to the new energy source, the more rapid and complete the adaptation will be.

Cluster Sorting

Cluster sorting can also impact the fermentation in a number of ways. Removal of clusters showing obvious rot will reduce the bioload of potentially inhibitory species and prevent formation of conditions stressful to fermentation initiation or progression. Uneven ripening can impact flora, as well as the yeasts and bacteria of unripe fruit differ from those of ripe fruit. Unripe fruit may contain phenolic species that are more inhibitory to Saccharomyces than mature fruit.  The flora of dehydrated or raisined berries is, likewise, different than the flora of hydrated fruit, but this tends to favor the osmotolerant species which, again, are mostly the yeasts. Very high yeast bioloads can lead to enhanced competition with Saccharomyces.


The temperature of harvesting can also have an impact on the flora present and their abilities to rapidly initiate cell division. Harvesting invariably causes damage to fruit and enhances seepage of nutrients to the surface of the fruit. High temperatures during harvest favor seepage and bacterial growth. Low temperatures generally are inhibitory to microbial activity, but will favor the growth of the non-Saccharomyces yeasts over bacteria and Saccharomyces. Fruit harvested at warmer temperatures will have a higher bioload of organisms upon processing, and these organisms will be primed to grow once the fruit is crushed. Sulfite-addition strategies at the time of harvest and of processing of the fruit can minimize growth of wild organisms, if that is desired.

It is commonly believed that encouragement of grape flora enhances quality, as only the desired flora will bloom post-harvest. All flora will bloom post-harvest, desired or not. If the winery style includes contributions from wild berry organisms, it would be prudent to assess what organisms are present immediately prior to harvest, so that desired populations can be encouraged. This will have to be done on a regular basis as berry flora are not constant and are influenced by a variety of seasonal factors.

Material Other Than Grape

Any surface will support a microbial flora. Microbes are a major component of soil as well. All materials present in the bin at harvest can serve as a source of inoculation of the must or juice. Leaf and bark flora generally are not well-adapted to wine conditions and are frequently neutral. However, there have been reports of bark-associated strains of Brettanomyces that are then transferred to the winery. Soil organisms, similarly, are generally not a problem. But again, there have been reports of soil borne bacteria, bacilli and filamentous bacteria that gain entry into the winery, become residents, and can cause issues of spoilage during bottling or processing.

Summary and Recommendations:

The two main harvesting factors that will affect the flora present prior to and during fermentation and therefore should factor into management strategies are:

  1. Level of damage to the fruit  
  2. Bioload of the fruit, must and juice

The level of damage to the fruit allows proliferation of the berry surface microbes. Which of those microbes are able to dominate depends upon compositional factors such as juice pH, the use of inhibitors such as sulfite, nutrient content, starting bioload, and length of time prior to initiation of fermentation. Acetic acid and spoilage lactics can bloom early in juices that are high in pH, low in sulfite, high in oxygen.

In addition to level of damage of the fruit, other factors also can result in a high bioload at harvest. A high bioload means that Saccharomyces is greatly outnumbered in the must, even following inoculation at typical levels. This establishes a competitive environment for nutrients, but also increases the risk of appearance of a substance inhibitory to the fermentation. The bioload is dependent upon the time of harvest and length of time the clusters have been on the vine (hang time). The longer the hang time of the fruit, the greater the seepage from the berry, and the more likely one or more berries per cluster will be damaged, providing nutrients to the microbial community. Organisms grow faster at higher temperatures, so warmer-region fruit will have higher bioloads than cooler-region fruit. The nature of the organisms present is equally important to the total number. The non-fermentative, aerobic yeasts and bacteria will be unable to persist in grape juice due to the lack of oxygen. High bioloads of these organisms is generally not problematic. It is often difficult to determine if the organisms present are harmless, negative or positive in their impacts from microscopic examinations. Full-blown microbial analysis requires time and is not realistic due to the immediacy of decisions with respect to inoculation, sulfite addition, nutrient supplementation, and must processing that must be made. Exposing a sample of the must to air in the winery overnight may provide a more rapid indication of the quality of the bioflora.