Rapid freezing for storage of sheep milk
Authors: Prof Richard Archer, Prof Mohammed Farid, Jolin Morel, Dr Georg Ripberger, FIET
A pdf of the full article, with Figures, is here
Introduction
It has been said that New Zealand’s prosperity was “built off the
sheep’s back”, but in recent years Bovine, ie cow Dairy has become
New Zealand’s largest agricultural earner.
Traditionally, the sheep industry in New Zealand has focused on the
production of meat and wool. In recent years however, a small but
rapidly growing sheep dairy industry has developed. The sheep dairy
industry produces a high value product, with a wide range of possible
applications.
In order to support the growth of this industry, a small team of
researchers from Massey University (MU), The University of Auckland
(UA) and GNS Science (GNS) are working to develop a rapid freezer
suitable for use on sheep farms, to allow raw sheep milk to be
stored for long periods without a loss of product quality. This has
the advantages that milk can be collected from remote locations
(previously deemed uneconomic), and allows small-scale farmers to
sell their milk in batches, as often required by cheese makers. This
gives farmers greater market power and allows them to establish a
profitable sheep-dairy business almost regardless of location.
This work is part of the larger MBIE-funded Food Industry Enabling
Technologies (FIET) research programme, which aims to develop
technologies that will grow export earnings by $250M pa by 2026.
Scope of work
This project is aimed at developing a rapid milk freezer, suitable for
on-farm applications, and taking it to the on-farm prototype stage. This
involves understanding the effects of freezing on milk quality (with a
focus on raw sheep milk in a New Zealand context) and using this
understanding when designing and optimising the on-farm freezer.
This is the focus of the MU and UA team while GNS is developing low
ice adhesion surfaces that will assist operation of the freezer unit and
reduce operational costs.
Sheep milk production
Globally, sheep milk accounts for about 1.3% of total milk production.
Sheep produce much less milk each day than cattle. In New Zealand,
dairy sheep produce approximately 1.5-2L of milk per day, with a total
yearly production between 150-300L of milk. This is significantly lower
than what is achieved in countries with more established industries
such as Israel (up to 750L per season) or Germany (450-550L per
season). The most popular dairy sheep breeds are the East Friesian,
Awassi, and Lacaune. These production systems are much more
intensive than those encountered in New Zealand..
The equipment used in milking, and the design of milking parlours, are
adapted from those used in goat dairy and are widely available.
Sheep milk nutrition
Sheep milk is higher in total solids than both cow and goat milks. This
difference is due mostly to higher levels of fat and proteins in sheep
milk. The lactose level is similar to those in cow and goat milks, and
the ash level is slightly higher. The higher solids content means that
cheese yields are higher with sheep milk.
Sheep milk’s fat is higher in valuable medium-chain triglycerides, and
may be more easily digested than the fat in cow milk due to the lower
diameter of sheep milk fat globules.
Sheep milk is claimed to have a superior amino acid profile when
compared with cow milk. Difference in the proteins between sheep
and cow milk may mean that sheep milk is less allergenic; however
this is still the subject of ongoing research.
Sheep milk products
Traditionally, sheep milk has been used to make cheeses, and this is
still its main use. Sheep milk cheeses include Feta, Pecorino, Manchego
and Roquefort. In New Zealand there are around 18 varieties of sheep
milk cheese being produced by producers ranging from artisan
cheesemakers to large commercial enterprises. Other food products
made from sheep milk in New Zealand include yogurts, ice-creams,
ready to drink products, fermented drinks such as kefir and, of course,
fresh milk .
Sheep milk is also used in the manufacture of whole milk powders
and infant formulas. The different allergenic profile of sheep milk is
helpful to some people who are sensitive to cow milk. Non-traditional
dairy products such as calcium chews, soaps and cosmetics have also
been developed.
Sheep milk products are generally aimed at the premium end of the
market, and towards high-value export markets. As a consequence,
the farm-gate price for sheep milk is significantly higher than cow milk,
on a milk-solids basis.
Sheep dairying in New Zealand
The New Zealand Sheep Dairy industry consists of approximately 16
producers as of 2017, with an annual growth of around 5 farms and
3000 ewes. These producers range in size from artisan cheesemakers
with 40 ewes, to a large, vertically integrated, operation that milks
20,000 ewes and produces milk powder, infant formula, and a range
of cheeses.
As can be seen in the figure on the facing page, the sheep dairy
industry is spread throughout the country. The geographic dispersion
of producers, the seasonal nature of milking, small volumes of daily
production, long distances to processors and intermittent demand
for large volumes of milk (to take advantage of economies of scale in
processing) suggest that a method for long term storage of fresh milk
would be beneficial to the industry.
Freezing— a method for long-term storage
Milk has been frozen on farms previously to overcome the issues of
seasonality, quantity, and market access. A common practice involves
freezing the milk in 2 L bladders, which are then stored and transported
for domestic use or exported to international markets (e.g. yoghurt
and cheese manufacturers). This method however has its drawbacks.
Frozen storage of milk can lead to a decrease in milk quality by several
mechanisms
• The proteins in milk that has been frozen and then thawed can
agglomerate and precipitate, leading to lower product yields and
undesirable textures
• The milk fat globules can be damaged during the freezing
process, leading to oiling-off, oxidation of fats, and off-flavours.
Consequently, the thawed milk becomes unsuitable for uses such as
liquid milk, UHT or drying as powder or formula. Previous research,
and current trials at MU demonstrate that the quality of freeze-thawed
milk is dependent on a number of factors. The most important are the
speed of freezing, the final storage temperature, and the duration of
storage. Broadly speaking, the best quality is achieved when freezing is
conducted as fast as possible, the frozen milk is kept as cold as possible
(below -20ÅãC), and the storage time is minimised.
Figure 4 illustrates the loss in protein stability that occurs during frozen
storage at higher temperatures. Milk samples were stored at -10ÅãC and
-30ÅãC for 9 weeks. After thawing at 20ÅãC, the samples were centrifuged
(3000 g x 60 min), and the amount of precipitate measured. It can
clearly be seen that the proteins in the -10ÅãC samples have precipitated,
whereas they are still in the liquid phase in the -30ÅãC samples.
The current practice of placing bladders or buckets of milk in blast
freezers or refrigerated shipping containers counts as slow freezing
(freezing times on the order of several hours), and is subject to
handling issues (e.g. lots of manhandling and labour required).
Freezing is regarded as processing from a regulatory point of view. Thus,
the unit operation of freezing has to take place in a food processing
environment, and the normal on-farm dairy Risk Management
Programme (RMP) must be extended to embrace processing and the
premises be appropriately registered.
In this project a rapid freezer has been developed that mitigates the
deleterious effects of traditional freezing methods, reduces labour
costs, complies with the regulatory requirements, and enables ease of
further processing.
Rapid freezing
Previous research suggests that the major causes of protein
destabilisation are the physical aggregation of casein micelles after
rejection from growing ice crystals, and the increased concentration of
salts in the unfrozen phase. The damage to fats during freezing occurs
due to physical aggregation of fat globules after similar rejection from
the solid phase.
Rapid freezing, followed by storage at temperatures below -20ÅãC should
counteract these mechanisms. Work reported on freeze concentrators
and falling film crystallisers shows that increasing ice growth rates
increases the partition coefficient of the system, reducing the amount
of solid rejected from the growing solid phase. The amount of a
food product that remains unfrozen is also reduced at lower storage
temperatures. This increases the stability of proteins during storage as
it reduces their aggregation, reduces the amount of unfrozen phase in
which destabilisation can occur, decreases the rates of any reactions
occurring, and increases the viscosity of the remaining unfrozen phase,
thereby further decreasing reaction rates. The damage to fats is also
reduced, as the fat globules are entrapped by the growing ice front
rather than being rejected.
Figure 5 demonstrates the differences between slow and fast freezing.
Sheep milk was sandwiched between 2 sheets of glass in a Hele-Shaw
cell, which was then placed in a Bridgman furnace, and the ice/milk
interface was observed using transmission light microscopy. At low ice
growth rates (0.5 μm/s) the interface is planar, and milk fat globules
are rejected from the growing ice front, leading to an increased
concentration in the unfrozen phase. At high freezing front velocities
(40 μm/s), the growth is columnar with secondary branches growing
at an angle to the main growth direction. At this speed fat globules are
trapped within the advancing ice, and there is no concentration in the
unfrozen phase.
The rapid freezer being developed operates by running a thin film of
liquid milk over a cooled curved surface, onto which the ice freezes.
Once the frozen milk layer has reached the desired thickness, the liquid
flow is stopped and the adhered ice removed. The sheet of ice is then
broken into flakes by an auger and transported to a storage vessel. This
geometry is commonly used for flake ice for fisheries but lends itself
to hygienic design.
In the system adopted the quality benefits of rapid freezing are
combined with ease of handling to reduce labour requirements
and allow for easier regulatory compliance. The thin flakes simplify
subsequent thawing and give a product suitable for freeze-drying,
should a processor desire this.
The technology may also enable processors to make raw milk cheeses,
even if they are not located near sheep milk producers, as milk can be
stored for extended times without thermal treatment.
Low-ice-adhesion surfaces
The key focus of the work being undertaken by GNS is the development
of icephobic surfaces, i.e. surfaces with a low-ice-adhesion. The
surface developed will be both food grade, and able to be applied
economically to large surfaces. Readily available 304 stainless steel is
the base substrate.
An icephobic surface will make it significantly easier to remove frozen
milk from the heat transfer surfaces, with a minimum of heat or force.
This will increase the thermal and mechanical efficiencies of the
system, decrease cycle time and lower ongoing freezing costs.
Other applications
The hygienic flake freezing method and equipment developed can
also be used to preserve in their native state without loss of functional
value, other valuable, perishable liquid raw materials like blood
plasma, whey and colostrum, that arise at times and places remote
from existing central processing facilities. This allows the launch of
new industries, which can secure and harvest their feedstock across
New Zealand. It will also enable the development of new products like
functional beverages (probiotics, ready-to-drink beverages) from niche
raw materials like donkey and red deer milk, which have been shown
to have bioactives.
Current progress and future plans
We have designed, built and instrumented a 50 L/hr pilot-scale unit that
is currently commissioned. Our initial trials are on sheep milk but we
would like to test other products as identified by businesses. Our next
step in the development involves constructing and trialling an on-farm
prototype of perhaps 250 L/hr capacity by mid-2018. We anticipate the
on-farm unit might look like a refrigerated container with freezer, plant,
controls and storage area inside, parked alongside the shed and fed by
a pipe and a power cord. The detailed design is currently being carried
out and we are working closely with MPI to comply with national and
international regulations. And we are currently considering partners
who might manufacture and market the units.
A pdf of the complete article, with figures, is here
